Re: Johnson 5hp TD20 ignition timing facts & figures?

[legendre]

Howdy Frank,

quote FrankR:

The timing is fixed by design, and cannot be changed. At least not if you have all the right parts and assembled correctly. So think of something else being your problem.

Now that I’ve said that, there is another look-alike magnet that may have gotten put in there. Or maybe the magnet is upside down?

A magnet issue wouldn’t alter the timing, it would only impact the quality of the spark. On this motor, the spark timing cam is an integral part of the parent metal of the crankshaft; or at least it seems to be. And as I said, spark quality seems excellent – and if it can be easily pull-started (it can) then there’s plenty of spark at very low RPM.

quote :

Have you got the slow speed needle set at 5/8 turn open?

Yes, very close to that for best running.

quote :

Perhaps the Y fitting is destroyed?

Externally, it shows no evidence of abuse or damage, and the valve action works just as expected. Likewise the jet of fuel from the primer can be shut off or made quite fat by changing the needle setting. I’ve been all through the insides, and there are no blockages in the crank passages or the case that I could find..

quote :

Perhaps the piston skirts are scored? That is a third port engine with a rotary valve, and everything has to be in good condition if you expect it to run properly. Compression is only half the story.

The pistons were in reasonable shape, not perfect but not ruined by any stretch.

But now I’m curious – assuming compression is OK, how would scored pistons prevent this motor running at low throttle settings? Below half-throttle, the ‘main’ carb is completely shut-off, and all fuel & air are delivered via the rotary valve / low-speed carb system. Which does raise a question.. if the air / fuel settings are fixed at these low speeds, is engine speed controlled entirely by spark timing?

Think about it – you have no control over throttle position, the low-speed system is always ‘full open’ – so there’s nothing except spark timing to control speed, right? This is why I’m looking to verify ignition timing, as I don’t know what else can be so strongly influencing speed at these low throttle settings.

quote :

Also: It has to be cooling properly. Because the carburetor is part of the exhaust manifold, it gets hot as the motor heats up. Too hot and the gas vaporizes inside the carburetor and it won’t run good at all.

Sure – but it behaves as it does from cold start.. it’s not like it’s faltering once things get hot. Also the manifold has a new gasket + sealer, and the water pump works great.

So.. what about a head gasket? It’s not leaking externally, but it could be leaking internally.. how would that affect this?