Good day,
I have a Briggs & Stratton 'Champion 35' petrol mower. I bought it second hand, and it has always been hard to start. Now it's impossible to start! I downloaded the B&S owner's operating & Maintenance manual hoping to find some detailed instructions but there was little in there of any help.

What are some things I can check? I tried to remove the carb but it doesn't want to come completely free even with the screws removed. It seems to be catching on something. I was trying to get to the jets to see if they are blocked. I noticed that the rubber right-angled sleeve that joins two metal pipes at the carb is not very airtight, and nor is the joint between the carb and the engine. I can see the rubber O-ring in intact, but the joint still felt a bit loose when I jiggled it.

Any suggestions as to what I can try to get it to start? It's full of petrol and the air cleaner is A-OK.

Many thanks!

Al

wrote:
Good day,
I have a Briggs & Stratton 'Champion 35' petrol mower. I bought it second
hand, and it has always been hard to start. Now it's impossible to start!
I downloaded the B&S owner's operating & Maintenance manual hoping to
find some detailed instructions but there was little in there of any help.

What are some things I can check? I tried to remove the carb but it
doesn't want to come completely free even with the screws removed. It
seems to be catching on something. I was trying to get to the jets to see
if they are blocked. I noticed that the rubber right-angled sleeve that
joins two metal pipes at the carb is not very airtight, and nor is the
joint between the carb and the engine. I can see the rubber O-ring in
intact, but the joint still felt a bit loose when I jiggled it.

Any suggestions as to what I can try to get it to start? It's full of
petrol and the air cleaner is A-OK.

Many thanks!

Al


Fixing the carb securely to the engine seems the obvious first port of
call. Any air leak here will weaken the mixture which will make cold
starting very hard.

Lots of other things in *could* be, but you're going to have to fix this
anyway so get it sorted.

Tim

--
--
Trolls and troll feeders go in my killfile

On 26/04/2016 12:35, wrote:
Good day,
I have a Briggs& Stratton 'Champion 35' petrol mower. I bought it second hand, and it has always been hard to start. Now it's impossible to start! I downloaded the B&S owner's operating& Maintenance manual hoping to find some detailed instructions but there was little in there of any help.

What are some things I can check? I tried to remove the carb but it doesn't want to come completely free even with the screws removed. It seems to be catching on something. I was trying to get to the jets to see if they are blocked. I noticed that the rubber right-angled sleeve that joins two metal pipes at the carb is not very airtight, and nor is the joint between the carb and the engine. I can see the rubber O-ring in intact, but the joint still felt a bit loose when I jiggled it.

Any suggestions as to what I can try to get it to start? It's full of petrol and the air cleaner is A-OK.

Many thanks!

Al

Replace the spark plug and the diaphragm in the carburettor. Take the
top cover off and check that it's not full of debris which would prevent
the governor vane from working properly.
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

On Tue, 26 Apr 2016 04:35:01 -0700 (PDT), wrote:

Any suggestions as to what I can try to get it to start? It's full of petrol and the air cleaner is A-OK.

Go he

http://www.repairfaq.org/sam/lmfaq.htm

The short version: FAST -- fuel air spark timing...

I.e. is the fuel fresh? etc.


Thomas Prufer

Years ago, I appropriated my dad's old B&S-engined mower on which the engine would not start. I eventually discovered that the exhaust valve seat insert had detached itself from the head and was preventing the exhaust valve sealing properly. I imagine that I discovered this with a compression test, but I might just have stripped the thing down for the fun of it and then happened upon the problem (it was a while ago).

A compression test is easy and will allow you to rule in/out the problem I had.

I simply fixed the valve seat insert back in place with exhaust paste and the application of a blow torch, to cure the paste. It ran fine when put back together.

Regards.

Terry.

[We've had this discussion before btw.]

My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well. It's a bit of a fiddle taking the
air filter off then screwing it back on before pulling the cord -- but I
know it works, so that's what I do.

I never have the time, nor the inclination, to take it apart and try and
diagnose the problem further: my aim is to cut grass, and this is the
quickest route to that goal.

If the mower were newer I might be more concerned to get the problem
fixed.

Mowers seem to be an assembly of hard-rock components that will take
piles of punishment .... combined with little itsy-bitsy delicate things
like springs that have to be *just so*: nightmare.

John

On 27/04/16 09:36, Another John wrote:
[We've had this discussion before btw.]

My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well. It's a bit of a fiddle taking the
air filter off then screwing it back on before pulling the cord -- but I
know it works, so that's what I do.


I had a B & S like this,. I got the valves reground and it was if
anything worse.

Then I noticed the choke wasn't coming on - the throttle cable that
pulled the choke on past 'maximum rabbit' had stretched.. A simple two
screws, slide the cable up a bit and re-tighten had it as new.





--
All political activity makes complete sense once the proposition that
all government is basically a self-legalising protection racket, is
fully understood.

The Natural Philosopher wrote:
On 27/04/16 09:36, Another John wrote:
[We've had this discussion before btw.]

My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well. It's a bit of a fiddle taking the
air filter off then screwing it back on before pulling the cord -- but I
know it works, so that's what I do.


I had a B & S like this,. I got the valves reground and it was if
anything worse.

Then I noticed the choke wasn't coming on -

I was about to suggest this too. A few minutes checking this would be
easier than fannying around with sprays etc.

Tim

--
Trolls AND TROLL FEEDERS all go in my kill file

On Wed, 27 Apr 2016 09:36:32 +0100, Another John wrote:

Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well. It's a bit of a fiddle taking the
air filter off then screwing it back on before pulling the cord -- but I
know it works, so that's what I do.

I never have the time, nor the inclination, to take it apart and try and
diagnose the problem further: my aim is to cut grass, and this is the
quickest route to that goal.

An engineers solution to the problem.

Commendable.

But: why bother with taking the air filter off?

"Sure Start" in the air filter works, too. I sort of point it in an accessible
air hole in the air filter cover, give it a spray which presumably ends up that
foam thing, and done.


Thomas Prufer

In article ,
Thomas Prufer wrote:

On Wed, 27 Apr 2016 09:36:32 +0100, Another John wrote:

Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well. It's a bit of a fiddle taking the
air filter off then screwing it back on before pulling the cord -- but I
know it works, so that's what I do.
...
But: why bother with taking the air filter off?

"Sure Start" in the air filter works, too. I sort of point it in an
accessible air hole in the air filter cover, give it a spray which presumably ends up
that foam thing, and done.

Ooh thanks Thomas - I'll try that! :-)

J.

Another John wrote:


Mowers seem to be an assembly of hard-rock components that will take
piles of punishment ....

Our 20 year old 3.5hp B&S lawnmower engine "stopped" last year due to lack
of oil. A few squirts of 3inOne down the plug hole and a bit of jiggling
freed off the seized piston. As a special treat I put some new oil in the
sump. ;-)

I had expected to have to replace the mower soon after but it's been a year
now and it's running perfectly with no smoke. Started on the second pull
with old fuel after its winter hibernation.

Tim

--
Trolls AND TROLL FEEDERS all go in my kill file

On Wednesday, 27 April 2016 13:05:12 UTC+1, Tim+ wrote:
Another John wrote:


Mowers seem to be an assembly of hard-rock components that will take
piles of punishment ....

Our 20 year old 3.5hp B&S lawnmower engine "stopped" last year due to lack
of oil. A few squirts of 3inOne down the plug hole and a bit of jiggling
freed off the seized piston. As a special treat I put some new oil in the
sump. ;-)

I had expected to have to replace the mower soon after but it's been a year
now and it's running perfectly with no smoke. Started on the second pull
with old fuel after its winter hibernation.

Let the grass grow under it, do you mean?

replying to Tim+, Hulltiger wrote:
How did you know it was a seized piston
Did you try pulling the starter cord first

Iam having a similar prob starting mine
I have done all the obvious i e plug ,oil , x filter but still know joy (full
with petrol )

--
for full context, visit https://www.homeownershub.com/uk-diy...r-1125213-.htm

On 28/08/2019 12:44, Hulltiger wrote:
replying to Tim+, Hulltiger wrote:
How did you know it was a seizedÂ* piston
Did you try pulling the starter cord first

Iam having a similarÂ* prob starting mine
I have done all the obvious i e plug ,oil , x filter but still know joy
(full with petrol )

Obvious question is does the spark plug actually visibly spark?
Can you smell petrol when you try to start it and fail?

Worth having a look online for the full service manual. That is what I
did for mine when it's self propelling drive stopped working.

I even discovered from that document that the official correct way to
upend it was not what I had previously been doing.

BTW I presume you have resurrected some prehistoric thread on the
parasitic mirror of uk.d-i-y - why don't you use the newsgroup?

--
Regards,
Martin Brown

Hulltiger m wrote:
replying to Tim+, Hulltiger wrote:
How did you know it was a seized piston

Its so obvious that if I need to explain how I suspect you wouldnt
understand. Not that it matters what I say because as usual, rather than
start a new thread youve tagged on to an old one and unless you search old
messages, you wont see this.

Feel free to prove me wrong by replying to this.

Tim

--
Please don't feed the trolls

On 28/08/2019 12:44, Hulltiger wrote:
replying to Tim+, Hulltiger wrote:
How did you know it was a seizedÂ* piston
Did you try pulling the starter cord first

Iam having a similarÂ* prob starting mine
I have done all the obvious i e plug ,oil , x filter but still know joy
(full
with petrol )


Drain petrol into a glass jar, let it settle for half and hour. If you
see it separate into two parts then likely you are trying to start it
with water! Also check any petrol you have in a can. Shake it up first
and pour off a glass jars worth.

Also don't use any old (6 to 12months) petrol unless perhaps you have
added a fuel stabiliser additive


--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:


My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well.

This great tip got me our of trouble - excellent tip. Luckily I had a so-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds. It was fairly easy to prop this up against the underside of the air cleaner while holding the throttle open and pulling the rope (without needing three arms)..

I still wish I could cure the starting problem though. I bought a brand new carburettor, complete with diaphragm, gasket and o-rings etc, but it didn't make a ha'p'orth of difference. So I now suspect the ignition system. I cleaned up the spark plug but that didn't help. I pinched the plug gripper together slightly inside the suppressor cap, but that didn't help. I guess I could try buying a new spark plug and see if that helps, otherwise, I don't know what to try... A new coil perhaps? I don't really understand how the spark is generated on these motors. Clearly there is no contact breaker (unless it's hidden somewhere). Clearly the spark must be occurring, or it wouldn't run at all. I tried shortening the throttle cable to that it really does open to the max when starting. The carb doesn't have a separate choke or limiter cable. The motor does pulsate a bit rather than running dead evenly. I also notice a little bit of smoke coming from around the silencer-thingie after I shut the engine off.

Al

On Sat, 30 Apr 2016 07:30:23 -0700, wasaol wrote:

On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:


My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit,
obviously!)
into the carb works just as well.

This great tip got me our of trouble - excellent tip. Luckily I had a
so-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds.
It was fairly easy to prop this up against the underside of the air
cleaner while holding the throttle open and pulling the rope (without
needing three arms).

I still wish I could cure the starting problem though. I bought a brand
new carburettor, complete with diaphragm, gasket and o-rings etc, but it
didn't make a ha'p'orth of difference. So I now suspect the ignition
system. I cleaned up the spark plug but that didn't help. I pinched the
plug gripper together slightly inside the suppressor cap, but that
didn't help. I guess I could try buying a new spark plug and see if that
helps, otherwise, I don't know what to try... A new coil perhaps? I
don't really understand how the spark is generated on these motors.

So, seemingly not a fuelling issue (assuming no unwanted air leaks past
the sealing rings or gaskets where the carb is bolted onto the inlet
manifold or anywhere else between the inlet port and the carb body).

Ignition problems can be the result of several possible deficiencies or
defects which can be aggravated by fuelling issues. In two stroke
engines, plugs can become fouled up with a carbonised oil film on the
insulator nose which provides a leakage path which can dissipate enough
of the spark energy to prevent the inductive back emf mechanism in the
traditional CB/spark quench capacitor magneto ignition system from
generating a high enough voltage to jump the spark plug gap (and
carbonised whiskers on the central electrode which shortens the effective
spark length and hence its efficacy as an ignition source).

In four stroke engines, spark plugs can also suffer similar electrical
leakage from extended running with an over-rich mixture (sooting up).
And, finally, plugs can develop faults which mimic all of the above so
investing in a spare plug is highly recommended.

Also, the cylinder pressure at the top of the compression stroke which
depends on throttle opening, will vary the breakdown voltage across the
plug gap, the higher the pressure (larger throttle opening), the higher
the voltage required to create the spark which is why it's not a good
idea to have the throttle wide open at cranking speeds (also, wide open
throttle compromises the quality of the fuel/air mixture ratio at
cranking speeds with simple carburettors where the slide is directly
controlled from a throttle lever rather than indirectly via a vacuum
controlled diaphragm).

Clearly there is no contact breaker (unless it's hidden somewhere).

In the classic flywheel housed magneto system, it's hidden under/behind
the flywheel itself. The flywheel in this case contains magnets embedded
into the outer rim of the flywheel which pass within a millimetre or less
from an inner laminated ferrous stator with both low and high tension
windings with a contact breaker in the LT windings, timed to interrupt
the induced current at just before top dead centre (in a simple single
cylinder engine, on both the end of the compression and exhaust strokes),
the flywheel is keyed to synchronise the ac waveform generated in the
magneto windings to achieve maximum current at the point when the contact
breaker opens to generate the spark.

The nice thing about magneto ignition is its almost zero maintenance
requirements in regard of 'points adjustment' compared to the battery
powered Kettering system and a spark energy output proportional to demand
(rather than inversely proportional as is the case with the traditional
battery coil ignition system).

The battery powered Kettering ignition system has to be designed to
provide enough spark energy at the minimum engine rpm limit and
consequently is over-specified for startup and tick-over where, despite
the CB points spark quenching capacitor, most of the surplus to
requirements energy lands up counter-productively eroding the CB points
in rather short order necessitating frequent filing and burnishing of the
contacts themselves, along with a gap adjustment to recalibrate the
ignition timing[1].

Incidentally, it's worth noting that the HT overwind on an ignition coil
isn't designed to step up a mere 12 (or 6 or 24) volt pulse to the 20 to
30 Kilovolt level but rather the three to four hundred volt pulse
generated across the contact breaker points when they open to interrupt
the 3 or 4 amp current flow (typical 12v system) to generate a back emf
voltage. If it weren't for the CB points capacitor, most of this energy
would be wasted in arcing across the points. It's also worth noting that
the same considerations apply to a magneto CB points setup (but here, at
tickover speeds, there's a lot less 'surplus spark energy' to be handled
as unwanted CB points arcing energy)

The capacitor value (and voltage rating) is optimised to moderate the
voltage rise such that the effective breakdown voltage of the air gap
being opened up across the points increases faster than this inductive
voltage rise for moderate to maximum engine rpm. Even so, at tick-over
rpm, there will still be a good portion of the spark energy being
dissipated in arcing at the CB points.

In both the magneto and Kettering ignition systems, it's important not
to introduce any 'electrical damping' (eg electrical leakage in the HT
circuit - fouled plug or damp getting into the HT circuit) since it robs
energy from the high voltage spike being generated at the points
sufficiently to stop the HT voltage spike from attaining enough potential
to break down the spark plug gap insulation (which depends on the
pressure within the cylinder).

Clearly the spark must be occurring, or it wouldn't run at all. I tried
shortening the throttle cable to that it really does open to the max
when starting. The carb doesn't have a separate choke or limiter cable.
The motor does pulsate a bit rather than running dead evenly. I also
notice a little bit of smoke coming from around the silencer-thingie
after I shut the engine off.

This last could be oil or excess fuel vaporising. I'm not familiar with
the fuelling system on this mower but, according to others', along with
your mention of 'pulsation' (or 'hunting') it does seem as though it's
using a slightly more sophisticated diaphragm controlled slide carburetter
with some sort of airflow vane control over the engine rpms.

It's surprising that there isn't some sort of fuel enrichment device to
aid starting such as a choke or a float depression button (or both) to
raise the float chamber fuel level. Also, 'pulsation' can also be a
symptom of an overly rich fuel mixture.

Examination of the spark plug insulator nose can be very revealing of
this condition. Indeed, you can find any chart of spark plug conditions
which provide diagnostic information about the condition of both two and
four stroke engines so if you don't have the owner's guide to hand, you
should be able to track down one for any petrol engined machine on the
internet to aid your diagnosis. Probably your best source would be a
spark plug manufacturer's web site where you'll find a comprehensive
chart covering both types of engine.

[1] When I "Transistorised" the twin coil ignition on my Triumph
Bonneville T120V with a homebuilt custom designed Capacitor Discharge
ignition module (way back in the mid 70s), driven from the original CB
points, the maintenance period on re-gapping the points went from "Every
3000 miles", in the owner's manual, to "never needed" simply by removing
the 3 or 4 amp inductive load and replacing it with a purely resistive
100mA load.

Also, another handy benefit was reduced electrical erosion of the spark
plug electrodes (20,000 miles versus 5 to 10 thousand miles regapping/
replacement schedule) along with immunity against leakage effects
allowing for a grade harder plug to be used without the higher level of
plug fouling that results under stop start urban riding conditions
presenting any issues whilst permitting a leaner mid range mixture and
the use of a 40 thou gap instead of the original 25 thou gap
specification which permitted an even leaner idle mixture setting.

Ton Up motorway cruising speed still remained at the 45mpg mark but
50/60mph "A road" crusing speeds enjoyed a 100mpg fuel economy versus the
almost consistent 50mpg I had previously been suffering almost regardless
of road conditions and whether solo or two up with a pillion passenger.

--
Johnny B Good

On Sat, 30 Apr 2016 18:34:59 GMT, Johnny B Good
wrote:

snip

In the classic flywheel housed magneto system, it's hidden under/behind
the flywheel itself. The flywheel in this case contains magnets embedded
into the outer rim of the flywheel which pass within a millimetre or less
from an inner laminated ferrous stator with both low and high tension
windings with a contact breaker in the LT windings,

snip

I'm not sure how many of these basic / stationary engines have points
and so normally just use the close proximity of the rotor to the
stator to generate the spark and determine the ignition timing?

snip

Cheers, T i m

On Saturday, April 30, 2016 at 7:35:03 PM UTC+1, Johnny B Good wrote:
On Sat, 30 Apr 2016 07:30:23 -0700, wasaol wrote:

On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:


My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit,
obviously!)
into the carb works just as well.

This great tip got me our of trouble - excellent tip. Luckily I had a
so-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds..
It was fairly easy to prop this up against the underside of the air
cleaner while holding the throttle open and pulling the rope (without
needing three arms).

I still wish I could cure the starting problem though. I bought a brand
new carburettor, complete with diaphragm, gasket and o-rings etc, but it
didn't make a ha'p'orth of difference. So I now suspect the ignition
system. I cleaned up the spark plug but that didn't help. I pinched the
plug gripper together slightly inside the suppressor cap, but that
didn't help. I guess I could try buying a new spark plug and see if that
helps, otherwise, I don't know what to try... A new coil perhaps? I
don't really understand how the spark is generated on these motors.

So, seemingly not a fuelling issue (assuming no unwanted air leaks past
the sealing rings or gaskets where the carb is bolted onto the inlet
manifold or anywhere else between the inlet port and the carb body).

Ignition problems can be the result of several possible deficiencies or
defects which can be aggravated by fuelling issues. In two stroke
engines, plugs can become fouled up with a carbonised oil film on the
insulator nose which provides a leakage path which can dissipate enough
of the spark energy to prevent the inductive back emf mechanism in the
traditional CB/spark quench capacitor magneto ignition system from
generating a high enough voltage to jump the spark plug gap (and
carbonised whiskers on the central electrode which shortens the effective
spark length and hence its efficacy as an ignition source).

In four stroke engines, spark plugs can also suffer similar electrical
leakage from extended running with an over-rich mixture (sooting up).
And, finally, plugs can develop faults which mimic all of the above so
investing in a spare plug is highly recommended.

Also, the cylinder pressure at the top of the compression stroke which
depends on throttle opening, will vary the breakdown voltage across the
plug gap, the higher the pressure (larger throttle opening), the higher
the voltage required to create the spark which is why it's not a good
idea to have the throttle wide open at cranking speeds (also, wide open
throttle compromises the quality of the fuel/air mixture ratio at
cranking speeds with simple carburettors where the slide is directly
controlled from a throttle lever rather than indirectly via a vacuum
controlled diaphragm).

Clearly there is no contact breaker (unless it's hidden somewhere).

In the classic flywheel housed magneto system, it's hidden under/behind
the flywheel itself. The flywheel in this case contains magnets embedded
into the outer rim of the flywheel which pass within a millimetre or less
from an inner laminated ferrous stator with both low and high tension
windings with a contact breaker in the LT windings, timed to interrupt
the induced current at just before top dead centre (in a simple single
cylinder engine, on both the end of the compression and exhaust strokes),
the flywheel is keyed to synchronise the ac waveform generated in the
magneto windings to achieve maximum current at the point when the contact
breaker opens to generate the spark.

The nice thing about magneto ignition is its almost zero maintenance
requirements in regard of 'points adjustment' compared to the battery
powered Kettering system and a spark energy output proportional to demand
(rather than inversely proportional as is the case with the traditional
battery coil ignition system).

The battery powered Kettering ignition system has to be designed to
provide enough spark energy at the minimum engine rpm limit and
consequently is over-specified for startup and tick-over where, despite
the CB points spark quenching capacitor, most of the surplus to
requirements energy lands up counter-productively eroding the CB points
in rather short order necessitating frequent filing and burnishing of the
contacts themselves, along with a gap adjustment to recalibrate the
ignition timing[1].

Incidentally, it's worth noting that the HT overwind on an ignition coil
isn't designed to step up a mere 12 (or 6 or 24) volt pulse to the 20 to
30 Kilovolt level but rather the three to four hundred volt pulse
generated across the contact breaker points when they open to interrupt
the 3 or 4 amp current flow (typical 12v system) to generate a back emf
voltage. If it weren't for the CB points capacitor, most of this energy
would be wasted in arcing across the points. It's also worth noting that
the same considerations apply to a magneto CB points setup (but here, at
tickover speeds, there's a lot less 'surplus spark energy' to be handled
as unwanted CB points arcing energy)

The capacitor value (and voltage rating) is optimised to moderate the
voltage rise such that the effective breakdown voltage of the air gap
being opened up across the points increases faster than this inductive
voltage rise for moderate to maximum engine rpm. Even so, at tick-over
rpm, there will still be a good portion of the spark energy being
dissipated in arcing at the CB points.

In both the magneto and Kettering ignition systems, it's important not
to introduce any 'electrical damping' (eg electrical leakage in the HT
circuit - fouled plug or damp getting into the HT circuit) since it robs
energy from the high voltage spike being generated at the points
sufficiently to stop the HT voltage spike from attaining enough potential
to break down the spark plug gap insulation (which depends on the
pressure within the cylinder).

Clearly the spark must be occurring, or it wouldn't run at all. I tried
shortening the throttle cable to that it really does open to the max
when starting. The carb doesn't have a separate choke or limiter cable.
The motor does pulsate a bit rather than running dead evenly. I also
notice a little bit of smoke coming from around the silencer-thingie
after I shut the engine off.

This last could be oil or excess fuel vaporising. I'm not familiar with
the fuelling system on this mower but, according to others', along with
your mention of 'pulsation' (or 'hunting') it does seem as though it's
using a slightly more sophisticated diaphragm controlled slide carburetter
with some sort of airflow vane control over the engine rpms.

It's surprising that there isn't some sort of fuel enrichment device to
aid starting such as a choke or a float depression button (or both) to
raise the float chamber fuel level. Also, 'pulsation' can also be a
symptom of an overly rich fuel mixture.

Examination of the spark plug insulator nose can be very revealing of
this condition. Indeed, you can find any chart of spark plug conditions
which provide diagnostic information about the condition of both two and
four stroke engines so if you don't have the owner's guide to hand, you
should be able to track down one for any petrol engined machine on the
internet to aid your diagnosis. Probably your best source would be a
spark plug manufacturer's web site where you'll find a comprehensive
chart covering both types of engine.

[1] When I "Transistorised" the twin coil ignition on my Triumph
Bonneville T120V with a homebuilt custom designed Capacitor Discharge
ignition module (way back in the mid 70s), driven from the original CB
points, the maintenance period on re-gapping the points went from "Every
3000 miles", in the owner's manual, to "never needed" simply by removing
the 3 or 4 amp inductive load and replacing it with a purely resistive
100mA load.

Also, another handy benefit was reduced electrical erosion of the spark
plug electrodes (20,000 miles versus 5 to 10 thousand miles regapping/
replacement schedule) along with immunity against leakage effects
allowing for a grade harder plug to be used without the higher level of
plug fouling that results under stop start urban riding conditions
presenting any issues whilst permitting a leaner mid range mixture and
the use of a 40 thou gap instead of the original 25 thou gap
specification which permitted an even leaner idle mixture setting.

Ton Up motorway cruising speed still remained at the 45mpg mark but
50/60mph "A road" crusing speeds enjoyed a 100mpg fuel economy versus the
almost consistent 50mpg I had previously been suffering almost regardless
of road conditions and whether solo or two up with a pillion passenger.

--
Johnny B Good

Thanks for the informative reply. You gave some useful info that hadn't occurred to me concerning cylinder pressure affecting things. As a fellow ex-tinkerer and owner of several British twins (1953 BSA Golden Flash, highly customised, plus a couple of Royal Enfield twins and one 350cc Triumph twin) I rebuilt engines on three occasions, so I expected that a simple little petrol mower would be a piece of cake to undestand, but I am baffled by bot the way the carb works and also the ignition system. I can see that the main flywheel-like thing has a smooth shiny surface that passes within 1mm of something covered in plastic that is about 2" diameter and an inch thiock from which the HT lead goes to the spark plug. That must be the coil, is it? I presume that there is no contact-breaker. And this would also mean there is no condenser, yes?

The carb has no air leaks. It has a rubber bulbous primer that I am instructed to push three times. I *think* this sucks petrol up out of the tank sufficiently to fill what appears to be a sort of floatless float chamber. Its' hard to see how this carb works by looking at it. All my bikes had Amal monoblock carbs AFAICR, and I could see exactly how they worked. Anyway, this is a moot point since by replacing the carb entirely, with no improvement, I think it's safe to assume it's not a carb or fuel problem.

I will try cranking it over with the plug out in the dark and see how bright the spark looks within the next 24 hours. If said spark looks feeble, I will try buying a new plug. If that makes no diff I will try buying a new coil (or at least, the thing I suspect is the coil)! It doesn't remotely resemble a motorcycle coil that I've seen (However, my motorcycle familiarity is limited to 1950-1070 British bikes and 1990-2006 Harley Davidsons. Oh, and more recently, a Honda 125 scooter, which was IMO, the most practical machine of the lot!!

Al

On Sat, 30 Apr 2016 07:30:23 -0700 (PDT), wrote:

On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:


My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well.

This great tip got me our of trouble - excellent tip. Luckily I had a so-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds. It was fairly easy to prop this up against the underside of the air cleaner while holding the throttle open and pulling the rope (without needing three arms).

I still wish I could cure the starting problem though. I bought a brand new carburettor, complete with diaphragm, gasket and o-rings etc, but it didn't make a ha'p'orth of difference.

If it ran well when it finally started and presumably started
eventually then a new carb probably wouldn't make much difference.

So I now suspect the ignition system. I cleaned up the spark plug but that didn't help.

Have you tried pulling the starter with the plug out but in the cap
and laying on the engine somehow? You should see / hear a nice healthy
spark across the gap.

I pinched the plug gripper together slightly inside the suppressor cap, but that didn't help.

The thing there is you are playing with high voltage and not 'current'
as such so even a small cap between the cap contact and the plug top
wouldn't make a huge difference, unless ignition was marginal etc.

I guess I could try buying a new spark plug and see if that helps,

Definitely.

otherwise, I don't know what to try...

Ok ...

A new coil perhaps?

Not unknown to make an engine more difficult to start.

I don't really understand how the spark is generated on these motors. Clearly there is no contact breaker (unless it's hidden somewhere). Clearly the spark must be occurring, or it wouldn't run at all.

Basically it's a large coil of copper wire wrapped around an iron core
that sits just outside (normally) the flywheel / ignition rotor. This
has a magnet inside it and when the magnet passes over the poles of
the coil, the coil generates a high voltage spark.

I tried shortening the throttle cable to that it really does open to the max when starting.

Hmm, do you mean you try to start it on full throttle? You don't
normally I don't think.

The carb doesn't have a separate choke or limiter cable.

Does it have a primer?

The motor does pulsate a bit rather than running dead evenly.

They often do because they have a flap sitting in the cooling air that
acts as a speed regulator (normally at tickover / lower speeds). When
the air pushing against the plate slows and the pressure drops, a fine
spring increases the throttle to speed it up again and stop it
stalling.

I also notice a little bit of smoke coming from around the silencer-thingie after I shut the
engine off.

That 'could' just be residual smoke from the heat of the exhaust /
silencer that you can't normally see when it's running?

Cheers, T i m

On Saturday, April 30, 2016 at 7:35:32 PM UTC+1, T i m wrote:
On Sat, 30 Apr 2016 07:30:23 -0700 (PDT), wrote:

On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:


My old B&S-powered mower never starts. Not, that is, unless I spray a
couple of bursts of "Sure Start" into the carb. Then it starts first
time, and always starts after that, once it's hot. (Another regular
mentioned that spraying gas from a blow torch (i.e. not lit, obviously!)
into the carb works just as well.

This great tip got me our of trouble - excellent tip. Luckily I had a so-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds. It was fairly easy to prop this up against the underside of the air cleaner while holding the throttle open and pulling the rope (without needing three arms).

I still wish I could cure the starting problem though. I bought a brand new carburettor, complete with diaphragm, gasket and o-rings etc, but it didn't make a ha'p'orth of difference.

If it ran well when it finally started and presumably started
eventually then a new carb probably wouldn't make much difference.

So I now suspect the ignition system. I cleaned up the spark plug but that didn't help.

Have you tried pulling the starter with the plug out but in the cap
and laying on the engine somehow? You should see / hear a nice healthy
spark across the gap.

I pinched the plug gripper together slightly inside the suppressor cap, but that didn't help.

The thing there is you are playing with high voltage and not 'current'
as such so even a small cap between the cap contact and the plug top
wouldn't make a huge difference, unless ignition was marginal etc.

I guess I could try buying a new spark plug and see if that helps,

Definitely.

otherwise, I don't know what to try...

Ok ...

A new coil perhaps?

Not unknown to make an engine more difficult to start.

I don't really understand how the spark is generated on these motors. Clearly there is no contact breaker (unless it's hidden somewhere). Clearly the spark must be occurring, or it wouldn't run at all.

Basically it's a large coil of copper wire wrapped around an iron core
that sits just outside (normally) the flywheel / ignition rotor. This
has a magnet inside it and when the magnet passes over the poles of
the coil, the coil generates a high voltage spark.

I tried shortening the throttle cable to that it really does open to the max when starting.

Hmm, do you mean you try to start it on full throttle? You don't
normally I don't think.

The carb doesn't have a separate choke or limiter cable.

Does it have a primer?

The motor does pulsate a bit rather than running dead evenly.

They often do because they have a flap sitting in the cooling air that
acts as a speed regulator (normally at tickover / lower speeds). When
the air pushing against the plate slows and the pressure drops, a fine
spring increases the throttle to speed it up again and stop it
stalling.

I also notice a little bit of smoke coming from around the silencer-thingie after I shut the
engine off.

That 'could' just be residual smoke from the heat of the exhaust /
silencer that you can't normally see when it's running?

Cheers, T i m

Thanks... The instructions tell me to hold the throttle fully open while startung the engine. Yes, it has a carb primer: a rubber bulb thing that you push three times. I *think* this sucks petrol up up out of the tank enough to fill a little resevoir that feeds the jet, as far as I can make out. I'll try cranking it over with the plug out, but touching the engine and see if the spark is bright. If not I will buy a new plug. If that doesn't help, I'll perhaps try a new coil. If that doesn't work, I may just modify the air cleaner with a gas blow lamp permanently attached to it so I can implement Another John's lifesaving tip each time I start it - without needing to grow a third arm!

Al

Al

On 26/04/2016 12:35, wrote:
Good day,
I have a Briggs & Stratton 'Champion 35' petrol mower. I bought it second hand, and it has always been hard to start. Now it's impossible to start! I downloaded the B&S owner's operating & Maintenance manual hoping to find some detailed instructions but there was little in there of any help.

What are some things I can check? I tried to remove the carb but it doesn't want to come completely free even with the screws removed. It seems to be catching on something. I was trying to get to the jets to see if they are blocked. I noticed that the rubber right-angled sleeve that joins two metal pipes at the carb is not very airtight, and nor is the joint between the carb and the engine. I can see the rubber O-ring in intact, but the joint still felt a bit loose when I jiggled it.

Any suggestions as to what I can try to get it to start? It's full of petrol and the air cleaner is A-OK.

Many thanks!

Woodruff Key that holds the fly-wheel in place.
They're made of a soft(ish) alloy and can become partially sheared
through rotor blade impact.
This throws out the timing with regards to peak voltage generation.

You'll need to make up a DIY flywheel puller with some steel plate with
holes drilled to align withthe holes on the flywheel which may require a
thread tapping into them if it's never been removed before.

Think I still have the puller I knocked up in my apprenticeship years.
If you need to see a picture I'll try and find it.

You can't get the flywheel off without one if the key is damaged and
typically the vast majority I ever had dealings with were.

It's a common fault and the B&S service kit had a replacement with the
points and condenser.

Mind you, it's 30 years ago since I was doing that stuff so I forgive
any errors in my advice.

replying to wasaol, Roj wrote:
Hi where can I get the manual from? When I search the Briggs and Stratton
website for my model number it just draws a blank. Cheers

--
for full context, visit https://www.homeownershub.com/uk-diy...r-1125213-.htm

Roj m wrote:
replying to wasaol, Roj wrote:
Hi where can I get the manual from? When I search the Briggs and Stratton
website for my model number it just draws a blank. Cheers

My experience when looking for information on the B&S web site was
that you have to be quite 'inventive' with the model number to find
the relevant information. If I remember right for my B&S engine one
had to prefix the model number (as shown on the engine) with a zero.

However the B&S engine 'manuals' are fairly minimal and are really
only tell you how to do fairly trivial things like change oil, lay up
for winter, etc.

--
Chris Green
·

On 28/04/2019 22:44, Roj wrote:
replying to wasaol, Roj wrote:
Hi where can I get the manual from? When I search the Briggs and Stratton
website for my model number it just draws a blank. Cheers


You first need to find the real model number.

https://www.briggsandstratton.com/eu...r-engines.html

--
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