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