I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

spongehead wrote:
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring
on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.


Spongehead--

Recommend cleaning the air intake sponge first then let us know if the
problem still exists. I assume you have the 99 dollar Walmart mower
with no variable throttle.

Tut

I agree that cleaning the air intake and replacing the filter may help. I'd
also suggest making sure the carburetor intake is clean and the mixture
adjustment is set properly.

Also make sure all the linkage spring are in place too.

If I had to guess, I'd say that the mixture may be too rich (or lean) and
the engine's governor is keeping it alive.

Good luck!

Bob

"cnctut" wrote in message
oups.com...

spongehead wrote:
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring
on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.


Spongehead--

Recommend cleaning the air intake sponge first then let us know if the
problem still exists. I assume you have the 99 dollar Walmart mower
with no variable throttle.

Tut

I would guess that the carby needs cleaning, or the fuel filter is clogged

David

Bob Shuman wrote:

I agree that cleaning the air intake and replacing the filter may help. I'd
also suggest making sure the carburetor intake is clean and the mixture
adjustment is set properly.

Also make sure all the linkage spring are in place too.

If I had to guess, I'd say that the mixture may be too rich (or lean) and
the engine's governor is keeping it alive.

Good luck!

Bob

"cnctut" wrote in message
oups.com...

spongehead wrote:
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring
on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.


Spongehead--

Recommend cleaning the air intake sponge first then let us know if the
problem still exists. I assume you have the 99 dollar Walmart mower
with no variable throttle.

Tut

"cnctut" bravely wrote to "All" (11 May 05 10:52:14)
--- on the heady topic of " Lawnmower idle question"

From the small motor faq:

" Air vane governor. The air flow provided by the
flywheel/blower passes by a plate which is coupled to the
carburetor throttle plate acting against a spring force.

Flyweight governor. A spinning gear assembly with a pair of
weights is driven by the camshaft. As the centrifugal force
of the weights increases, they move a sleeve which presses
against a lever whose shaft passes through the crankcase.
This lever then operates the carburetor throttle plate
against a spring force.
With both types, increasing the spring force will increase the
engine speed.
"

Hunting is often caused by a defect in the governor system. Check the
blade (plastic?) of the governor paddle is free to move and that the
cam to which it connects has the proper spring tension. As the motor
speeds up the flywheel impellor blows air against the blade and cuts
the trottle down. When the motor slows down the blade drops back and
the spring returns the throttle higher.

Often grass goop gets into the governor paddle axle and may stick or
seize. You might need to remove the pull cord/motor cover and perhaps
the carb/fuel-tank to get at it. Thankfully you won't need a puller to
get the flywheel off. If you ever do, inspect the key pin in the
shaft, as it must be snug, not worn. This could also cause hunting.

OTOH, if instead of the fan your motor has a governor linkage coming
out the bottom of the motor, then the linkage may have slipped.
Hunting means in this case it may be opening the throttle too much.
There is also a spring arrangement in this type to the trottle cam.

A*s*i*m*o*v


cn From: "cnctut"
cn Xref: aeinews sci.electronics.repair:47853

cn spongehead wrote:
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring
on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.


cn Spongehead--

cn Recommend cleaning the air intake sponge first then let us know if the
cn problem still exists. I assume you have the 99 dollar Walmart mower
cn with no variable throttle.

cn Tut


.... The first rule of intelligent tinkering is to save all the parts.

I replaced the air filter and even let it run without it, but still
revvs up and down. Next will remove the carb and I actually have
another carb from my older lawnmower which I'll swap.

spongehead--

A carb swap may solve your problem if off a similair 3.5 engine--too
much work though. You mentioned your throttle adjustment was
frozen--you might try advancing the control wire (pliers might help)
toward a faster run postion to get away from slow idle.

If this doesn't work--carb cleaner would be my next attempt at solving
the problem. Squirt it every where (in and around the carb.)

Lastly, as your time and parts cost creep higher and higher--don't
forget a new 3.5HP mower is only $99. ;-))

Good Luck

Tut

"cnctut" writes:

spongehead--

A carb swap may solve your problem if off a similair 3.5 engine--too
much work though. You mentioned your throttle adjustment was
frozen--you might try advancing the control wire (pliers might help)
toward a faster run postion to get away from slow idle.

If this doesn't work--carb cleaner would be my next attempt at solving
the problem. Squirt it every where (in and around the carb.)

Lastly, as your time and parts cost creep higher and higher--don't
forget a new 3.5HP mower is only $99. ;-))

What fun is that? ;-)

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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spongehead--

When you get it fixed--would appreciate an update on what was the
problem.

Thanks

Tut

spongehead--

One last thought--check the plug for soot (running rich)--clogged air
filter causes lower Air/Fuel ratios--leads to poor burn, soot and
suttle misfires--leading to rpm searching.

Good Luck

Tut

I took the carb off and found that rubber O-ring was broken. I assume
that was the culpret. I hosed down the carb with wd-40 and she started
up with no idle problem. However now the idle is too low. I probably
stretched the spring or something but atleast it mows the lawn! Thanks
for all the help!

Asimov

Not sure what you meant by "XX bravely wrote" but--Tut "bravely writes"
more again.

Briggs makes a 3.5 HP engine matted to a 20 Murray mower sold by
Walmart for $99 US. My experience is that if the air cleaner (which
uses a sponge filter) is dirty--which happens frequently--the engine
rpm will search for neutral idle. Perhaps because the mixture is over
rich and the engine Air Fuel ratio changes on the "fish hook" pattern
as engine speed changes--or perhaps because manifold vacuum fluxuates
excessively causing irratic flapper valve overshoot action--not sure.
Cleaning the sponge is quick, easy, cheap--and requires no $$.

The 3.5 Briggs engine is also sold in a throttle free
configuration--meaning no throttle control for the operator. In this
case, a small thin bar toward the front of the engine holds the idle
springs at the proper tension thus preventing the engine from
unneccessary RPM searching. The bars location is just at the proper
height to be bent rearward when the mower is pushed under low
brush--this reduces the idle spring tension and the mower will RPM
search.

Tuts suggestions/comments.

1. Clean the filter sponge and let us know if the problem exists.
2. Does your mower have an adjustable throttle control on the handle?

The answer may not be here--but a great starting place--short of carb
teardown or flywheel removal--wouldn't you agree?

Tut

"Asimov" writes:

"cnctut" bravely wrote to "All" (11 May 05 10:52:14)
--- on the heady topic of " Lawnmower idle question"

From the small motor faq:

" Air vane governor. The air flow provided by the
flywheel/blower passes by a plate which is coupled to the
carburetor throttle plate acting against a spring force.

Flyweight governor. A spinning gear assembly with a pair of
weights is driven by the camshaft. As the centrifugal force
of the weights increases, they move a sleeve which presses
against a lever whose shaft passes through the crankcase.
This lever then operates the carburetor throttle plate
against a spring force.
With both types, increasing the spring force will increase the
engine speed.
"

Hunting is often caused by a defect in the governor system. Check the
blade (plastic?) of the governor paddle is free to move and that the
cam to which it connects has the proper spring tension. As the motor
speeds up the flywheel impellor blows air against the blade and cuts
the trottle down. When the motor slows down the blade drops back and
the spring returns the throttle higher.

One note though: He has an idle speed problem where the governor is
likely not involved unless it is way misadjuated.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

First you want to make sure all the phase alignment coils are peaked and the transratiometric detector is properly balanced.
Then inject a circular wave about 1A P-P into TP31C and listen for feedback around the modulator. If any is found, replace
F4.

Eric Law

"spongehead" wrote in message oups.com...
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

"EL" bravely wrote to "All" (11 May 05 13:52:03)
--- on the heady topic of " Lawnmower idle question"

Eric,

Don't quit your day job to become a stand up comedian.

A*s*i*m*o*v


EL From: "EL"
EL www.newshosting.com Xref: aeinews sci.electronics.repair:47856

EL First you want to make sure all the phase alignment coils are peaked
EL and the transratiometric detector is properly balanced. Then inject a
EL circular wave about 1A P-P into TP31C and listen for feedback around
EL the modulator. If any is found, replace F4.

.... Don't mess with a man that has a rubber chicken.....

spongehead wrote:

I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

Hi...

Happens that I have one; as well as still have the manual...
hard to believe 'cause it's about 15 years old

Anyway, still runs perfectly, so I'd be happy to take a couple
of pics of the springs and carb linkage etc, and mail them to
you if you like?

Could also scan the applicable pages of the manual if you
think it would be helpful.

Take care.

Ken

Carb KIT needed....

"spongehead" wrote in message
oups.com...
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

"spongehead" writes:

I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

Depending on the model, there may be an idle adjustment, but as
others have said, the carb either may need cleaning or a refurb.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
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In article .com,
says...
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

The speed control servo on these does not have much phase
margin, and may even have a pole in the right half plane.
Fuel mixture errors can easily add enough loop delay to
push the system into oscillation. Accumulation of gum and
varnish and the tendency of the carburetors's soft metal
to flow with age and develop leaky gaskets all contribute
to long-term performance drift.

In layman's terms; adjust, repair, or replace the carburetor
as needed. A professional who works on these often enough
to be familiar should be able to diagnose your specific
needs in very short order, but will probably expect to be
paid for his expertise.

Thanks for all the replies, I will be taking the carb off for a good
cleaning and replace the air filter. I'll let you know how it goes.
Thanks again!!

Change the needle valve..
Get a Carb KIT.


yikes

"spongehead" wrote in message
ups.com...
Thanks for all the replies, I will be taking the carb off for a good
cleaning and replace the air filter. I'll let you know how it goes.
Thanks again!!

"kip" writes:

Change the needle valve..
Get a Carb KIT.

Unless he tightened the needle valve with a pair of Vice-Grips, needle
valves don't go bad, but may get dirty.

Clean the carb first.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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Yes But its just easier to put a New one in
cause the chances of him ruining the valve is pretty good.
We are not talking big coin here ..
Just done my Lawnboy ..
cheers
kip
"Sam Goldwasser" wrote in message
...
"kip" writes:

Change the needle valve..
Get a Carb KIT.

Unless he tightened the needle valve with a pair of Vice-Grips, needle
valves don't go bad, but may get dirty.

Clean the carb first.

--- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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Note: These links are hopefully temporary until we can sort out the
excessive
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Important: Anything sent to the email address in the message header above
is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

spongehead--

Take off the air cleaner and run the engine--if the problem corrects
itself--clean or replace the old foam filter. Make sure the carb is
securely mounted to the engine.

Does your mower have an adjustable throttle control on the handle or
not?

Yeah, it has a lever which is never used because its stuck, but can
adjust it right on the carb itself.

But dont forget to check and clean/replace the fuel filter while you are
doing the job

David

spongehead wrote:

Thanks for all the replies, I will be taking the carb off for a good
cleaning and replace the air filter. I'll let you know how it goes.
Thanks again!!

On 11 May 2005 10:40:01 -0700, "spongehead"
wrote:

I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

It's the carbuerator diaphram, worn out most likely.
Yours is the plastic bodied carburator with the black or red primer
bulb yes?
If so you'll need to purchase a new diaphram and gasket for the carb,
about 4 bucks or so at a lawnmower parts shop.
The carb is secured with 5 philllips head screws. Take those out and
gently pry the plastic carb off the fuel tank.
Take care not to lose the diaphram spring( it'll be on the bottom side
of the carb.) or the mesh filter screen on the main fuel intake port.
Note whether the fiber gasket is on the carb side or the tank side(
varies with each model.) of the rubber diaphram.
Replace new ones in that order. Take care not to crease or wrinkle the
rubber diaphram whille tightening the screws down and you should be
good as new.
While apart use a good aerosol carb spray to clean out the carb and
tank resevoirs.(just about any brand but Gumout will do, I use Super
Tech brand found at Wal-mart. Autozone brand of carb spray is good as
well.)
Spray through the intake port(under mesh screen) to back flush the
supply pipe and the main port in the carb throat as well.
blow off any excess cleaner or allow to dry before replacing rubber
diaphram.(cleaner will attack the rubber shortening it's life.)
Any questions in particular just post back.
One of my many skills is small engine mechanic and I have years of
experience with lawnmowers.