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