1999 C40 TLRX, Quit Idling w/ Prime Start in Normal Position

[a387673]

Over the course of about 30 minutes run time, my engine's idle went from normal to very rough/stalling. WOT is still fine. Cranks fine when cold, but when RPMs drop after a little warm up, engine runs rough, stalls at idle. I can manually turn the Prime Start "emergency switch" to Open and it will idle and run fine. I suspected Prime Start "thermo-valve" problem, but wonder why engine RPMs rise as normal at cold start, then fall as normal after warm-up if the Prime Start is the problem.

Is there a particular problem indicated by this symptom: Idles fine with Prime Start emergency switch in "Open" position, but won't idle when warm with switch set to normal? Also, is Prime Start normally the only mechanism which influences engine speed at cold start? Thanks for any feedback you may have.

[99yam40]

The prime start system advances the timing until the temp switch tells it the motor is warm or the throttle is advanced. It also allows extra fuel to flow into the motor until the heater closes the needle. this is heated from 12v coming from the alternator while the motor is running.

Sounds like you prim start is working but your carbs idle(pilot) jets/ passages are plugged. need to pull and clean the carbs

[a387673]

I am probably still at the same point, needing to clean carbs, but I noticed another symptom..... The number 2 plug is much darker (dry, dark deposits) than the other two. Could be from fouling due to carbs??? or with it being on # 2, where the prime start resides, could there be another cause/effect scenario? Thanks again for any feedback.

[a387673]

Based on another thread, I may have another symptom, which I took somewhat for granted. Fuel comes out the carb throats when tilting the motor up for trailering. I thought this might be somewhat normal, but the other thread mentioned points again to carbs (floats, seats, etc.) needing attention...... as always, thanks for any feedback.

[99yam40]

When you take apart carbs to clean look at the diaphragm on the prime start system and make sure it is not torn or have a hole in it.

My C40 does not leak fuel out when tilted, check floats and needles