Yamaha F100A misfire - a challenge to say the least

[Herman Melville]

I am sorry to ask - I have been trying to fix this for weeks.

This engine starts first time. Idles sweetly. But sneezes at 2500 rpm (with no load on the muffs)

I have done:
New plugs (putting the old plugs back in also made no difference)
New fuel filter.
Cleaned and dried the fuel tank.
Sonic carb cleaning.
New carbs kits fitted.
Checked the float height and reset idel screws to 2 turns (stock).
Put in brand new fuel. Made sure that new fuel is in the carbs not the old stuff.
Tested the primary and secondary coil resistance - in spec.
Tested the HT lead resistance - in spec.
Tested the spark gap - jumps the specified 11mm no problem.
New CAM belt - it made no differnce (just needed doing)
Triple checked that after the new CAM belt is on that all the marks are where they should be.
Balanced the carbs - this would really affect idle so i did it mostly to make sure there was no vacuum leak after the carbs.
I have also kept the bulb pumped hard while runing to make sure the carbs are full of fuel - no difference
I have bent and poked and moved and prodded every lead in the ignition while it is running to see if I can close or open an internittment bad connection.

Compression test gives 175psi+ on all four pots.

There is not a lot left to try...


Any clue would be appreciated.

 

[Herman Melville]

A clue? Maybe ?

I was checking for air leaks due to the prime starters - were the orings in place etc..

I saw what I thought was a crack in the prime start body. Pretty easily a piece came off - it had been glued / bodged.

The piece missing is right from the bottom - I think it takes fuel in from the bowl and uses it to enrich the mixture at the start.

Anyway maybe air has been getting in there.... ?

I don't see how this will solve the prob because it didn't show on the vaccuum balance and also when the prime starter is warmed up it expands the plunger and blocks all this up....

In other words when the engine is warm and missing like the video then this hole is plugged anyway...

But it is so far the only component that I can find that has any defect so it's the best lead I have got at the moment.

£266 plus shipping ouch.