idle problems with 115 Yamaha

[pontoon chief]

I am working on a 115 twostroke motor that will not idle correctly.

C115TLRY

The motor appears to have a mind of it's own.When I start it,it will idle at around 850 RPM and then for no reason it will jump to 2000 RPM and then down to 400 RPM.

It bounces around for no reason,or at least no reason that I am aware.There does not appear to be anything loose that would cause this condition.

I could better understand it if the motor would just run at the incorrect RPM,however the bouncing RPM has got me stumped.

Any idea what is wrong ?

 

[WillyBWright]

Re: idle problems with 115 Yamaha

Probably an ignition problem, spark cutting in and out. If you have a timing light, you can see it misfire. Just move the pick-up from cylinder to cylinder as it's running and look for steady flashing.

 

[Ray Neudecker]

Re: idle problems with 115 Yamaha

Very likely a fuel restriction problem. First step would be to replace the fuel pump and rebuild the carbs.