1984 Yamaha 115. Sporadically Missing

SAH

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Jun 14, 2012
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I've got an 84 Yamaha 115 that runs like a top other than the fact that it will sporadically start missing, usually starts at high RPMs. You can back off the throttle, bring it back up and it will run fine for a little while and then start up again. Carbs have been rebuilt, new gas lines, new primer bulb, new fuel pump, gas tank is vented properly. I'd like to say that we have ruled out a fuel problem. Starting to think its electrical, possibly charge coils? Any ideas would be greatly appriciated!
 

99yam40

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Sep 7, 2008
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Re: 1984 Yamaha 115. Sporadically Missing

All you can do is test things like the manual says while problem is there to see.
Loss of spark should be evident with a timing light.
If there is good spark when problem is happening then too much fuel is just as bad as too little to let cylinder fire properly
 

SAH

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Re: 1984 Yamaha 115. Sporadically Missing

Thanks for the feedback. What would cause too much fuel?
 

99yam40

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Sep 7, 2008
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8,851
Re: 1984 Yamaha 115. Sporadically Missing

Usually the fuel pumps dumping into the crankcase pulse port, but you have replaced the pumps.
Wrong jets on carbs maybe.

A previous owner of my C40TLRX drilled out the main jets trying to make a 50 (I guess) and it did the same thing until I replaced the jets. Took me close to a year to find the problem.

check the spark at the problem RPM first to make sure it is not dropping spark, or measure peak voltages at problem RPM, but that is not as easy as the timing light way.

Test everything you can to make sure everything is in spec.
then you are left with fuel/air ratio is all else is good
 
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