[JohnGree]

Have finally got the 39 unstyled AR back together, but for some reason cannot get her to start. Hand crank of course. Redid the engine (bored to +.090, new pistons, rings, new head [that's another story]), rebuilt the carb (totally disassembeld, made sure all drill holes etc. were clear), carb fuel level at 3/4" below top of bowl (meassued with a clear hose from the drain hole), fuel flows freely, idle needle at 1 1/2 turns out, load 1 turn out, linkage seems ok, new plugs (Autolite 3076), spark, #1 cylinder impluses when it is suppose to at the flywheel Impulse mark, valves set at .020". She will fire when hand creanking failry easy, but will not continue to run. Occasional "weak" backfire. I suspect either the operator (being me) does not know how to start it, or something is amiss. Before the teardown she was an easy starter (the teardown was mearly to repair the intake manifold and one thing lead to another).

So, any hints on troubleshooting the problem would be appreciated. I have not tried to pull start her (in my opinion, if she is properly tuned etc, she should start easily by hand). I have tried starting her so many times I've had to add oil to the cylinders as they were getting dry (flushed by gas I assume) and "sticking".

 

[johndeerefan]

It could be the plugs. Even though they are new, it happens sometimes. Same thing happened to me, turned out to be bad new plugs.

 

[Lovesthedrive]

Is your timing correct? Sounds like the engine is timed until it starts turning. Then the mechanical advance puts the engine out of time (like it is out of time by one tooth).

 

[JohnGree]

I changed plugs to the old Champion W18 (used) ... she started (yea, thanks Johndeerefan) but seems to be running on only one cylinder. Had to keep her on a fast idle or she stalls. Tried shorting the plugs with a screwdriver ... with number 2 shorted no change, with number one shorted she quits (that tells me something). So I guess I'll check the mag.

Lovesthedrive - how would I tell if she is out of time by "one tooth"?

 

[Lovesthedrive]

It was an assumption based only that it would run briefly at startup.

Usually mags are set up with a retarded spark (10 degree after tdc) for hand cranking. When it starts the advance will bring it back to 10 degrees before top dead center. If the mag was off by a tooth, you may have been starting it on the 10 degree before tdc, so when it was running it would result in a 20 degree before tdc stopping the engine.
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Since you ruled it was the plugs. What happens if you swap plugs with the good side? If you want to clean the plugs, put the tiny end into a vise (the end the wire connects to). Then take a propane torch and cook the plug till it glows red. Let it air cool (cold when you can touch with your fingers) and blow off the carbon dust with compressed air.

 

[JohnGree]

Here is a thought ... somewhere, I don't know where (and I'm looking through the literasture I have now), I thought I had read that #1 impulses (at the flywheel mark), then 1/2 a flywheel turn later #2 impuses. On mine, # 1 impulses and then 1 1/2 flywheel turns later #2 impulses. Is it possible I've timed the machine to the wrong impulse?

 

[Lovesthedrive]

Yup good probability. 90 degrees out will do it.

 

[JohnGree]

Yes, looks like my timing is out. Firing cycle is #1 at 0 degrees, #2 at 180 degrees, then "coast" for 540 degrees. Should find time today to correct the error!

 

[JohnGree]

Up and running! The timing was the problem. Now I just need to clean her up, put the decals on, and do some fine tuning. Try to post a picture or two in the next day or so.

 

[Lovesthedrive]

Congratulations!