yamaha C90 rough intermedite speed

[capthobo]

My 1997 C90 is running rough at intermediate speeds between 1400 and 2500 RPM.<br />I have rebuilt the carbs and replaced the spark plugs with no change.<br />It sounds like only two of the cylinders are firing at this RPM range.<br />It idles and starts ok, and top end is fine also.<br />What could be wrong?

 

[jim dozier]

Re: yamaha C90 rough intermedite speed

I have a Suzuki DT85 3 cylinder which had a similar history. My engine seemed to be running rich at low speeds and would conk out in gear at idle. I smelled gas and choking the engine made it worse and disconnecting the fuel line made it better. I first thought the float valve(s) in the carbs were sticking open but after a rebuild the problem was still there. Turned out to be a hole in the inner (engine side) fuel pump diaphragm that was injecting fuel directly into the bottom cylinder and killing off that cylinder. At WOT the engine demand overcame the fuel pump leak an the bottom cylinder would catch and take off. Makes for an interesting day in big seas when you can't go slow.<br /><br />See if you can isolate which cylinder isn't firing by pulling the plugs (best to ground the unattached wire) one at a time. If the offending cylinder is the one your fuel pump is attached to, see above.