Spitting back through the carb

[Richard Pitman]

What causes this ?

is it an ignition issue or a carburation issue ? Reason I ask is that the CD175 head engine that I'm currently playing with occasionally does this when I snap the throttle open. No air leaks on induction or exhaust. Wondering if weak springs in the advance mechanism would cause this, or is it an incorrectly set idle circuit - wrong size idle jet / mixture screw position ?

 

[EzPete]

What causes this ?

is it an ignition issue or a carburation issue ? Reason I ask is that the CD175 head engine that I'm currently playing with occasionally does this when I snap the throttle open. No air leaks on induction or exhaust. Wondering if weak springs in the advance mechanism would cause this, or is it an incorrectly set idle circuit - wrong size idle jet / mixture screw position ?

I usually immediately jump to spark issue if it’s unburnt fuel, timing issue otherwise.

It takes some change in vacuum to create that dynamic, so a faulty advance makes that very viable. If the engine is compressing well ahead of the spark, when the intake reopens on the following revolution that fuel/air mix has to go somewhere and would compress at a different rate to exhaust - so back through the carb.

 

[mike in idaho]

Lotsa times, off-idle spitting back thru the air cleaners is a sign of too lean an idle mixture. A change in the idle mix adjustment or a different slide cutaway may be needed.

 

[bilbikek411]

No carbon on the intake valve seats ?
You have the cold valve clearances within spec.

 

[Richard Pitman]

No carbon on the intake valve seats ?

No, it's a rebuilt engine, new exhaust valves, intake valves in good order. New rings, honed bores.

I'm inclined towards the weak idle mix. I don't remember it doing this when I just bolted on the CL72 PW22 carb and ran it on my test rig, using the jets that the carb came with. I think it may have started when I fitted the 'correct' CD175 idle jet, so yesterday afternoon I refitted the original pilot jet. Engine started and ran, but bad light stopped play before I could really test it.

I really ought to write down all these changes, I am getting to the point that I can't keep track of what is going on in these various 175 engines. I did do a compression test on this rebuilt engine the other day, when it was warm, rather than fully hot, and got readings of 125 psi on each side. Sounds low, but this was with a cheapy Amazon tester, my ancient Gunson tester having ceased to work for some reason. Just based on 'feel', this engine has the same, or better, compression than the other two engines that are currently in use.