yamaha mid range stumble

[flashback]

Hi folks, I generally hang out in the I/O forum and know almost nothing about outboards.. My brother in law has about a 10 year old yamaha 115 that will stall around 3000 rpm. if you start out from idle and slowly advance the throttle, the engine starts to stumble around 3000 rpm if you then advance to WOT it will recover and run great. slowly backing off from Wot to idle does not cause the stumble. its just while accelerating that it occurs we,re stumped.....you guys have any ideas?......

 

[jim dozier]

Re: yamaha mid range stumble

You need to check 2 things. First check to see if the ignition timing is advancing properly (does it have a mechanical advance or electronic?). It must be properly synchronized with the throttle advance on the carburetor. If that is OK then I would suspect a clogged high speed jet and/or low speed jet. The idle circuit needs to be right for the transition from idle to mid range and the high speed needs to be right because everything is connected to it. A complete carb rebuild is probably necessary. All of these checks require a manual.

 

[flashback]

Re: yamaha mid range stumble

Thanks Jimd,