Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

[drchpete]

Yamaha C40 TLRY (2000 40 Hp 2-stroke pre mix) misfires at low RPM in or out of gear. Sounds like "run-run-run-run-PUTT-run-run-run-run-PUTT-run-run-run-run-PUTT". The engine has been running just fine for 2 months and ran fine earlier in the day for an hour or so, then this pattern just started. Give it some fuel and it smooths right out and runs normally. Any suggestions on what I can check or do in relatively simple terms?

The fuel is only 3 weeks old but I did notice there was no "hiss" when I opened the vent the last 2 times and was wondering if there's a small leak in the vent allowing moisture in. I do have a water separating filter attached but it looks OK.

 

[99yam40]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

Sounds like a lean sneeze from pilot (idle)jets/passages plugging

 

[drchpete]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

Sorry, but I am an amateur at this.

Does that mean the carb(s) are getting plugged up? What can/should I do about this? Spray some Carb cleaner or do these have to be taken apart and cleaned professionally? I do have the service manual and am not afraid to give most things a try.

 

[renegade15]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

I second the lean sneeze, however is it doing it in the water AND out of the water on a flushing attachment? Since these motors are tuned to run properly in the water sometimes they will act differently out of the water with no back pressure on the exhaust. Just something to check. However if it is doing it IN the water then a carb clean would seem a good start finished up with a link n sync. Just cleaner sprayed in them probably won't help, a full dissassembly and thorough cleaning, maybe even rebuild depending on how the needle and seat are? If you don't have access to compressed air to blow through the passages you can sometimes get by with a can of the compressed electronics cleaner air...
Good Luck!

 

[drchpete]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

I got lucky. After talking to a local Yamaha mechanic he agreed with the diagnosis of lean sneeze and told me to take it out and run it hard and I might get lucky and avoid a carb cleaning. I took it out and ran it hard at different RPM's then held it WOT for a few minutes and believe it or not the sneeze is gone. Took it out again the next day and ran it WOT for a few more minutes and let her run at idle for a while and again no sneeze. Probably unusual to work like this but I'll take it. Thanks for the help guys.

 

[99yam40]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

You lucked out, maybe it was some water in the bowl and you finally sucked it on through.

Bad part of trying that is if it is running too lean because of plugging you can toast a piston by running it to see if it will get better
Be careful

 

[drchpete]

Re: Mis-Fires at low RPM / idle - Yamaha 40 2-stroke

If it happenes again I'll just clean the carbs as suggested. I won't risk it again.

Thanks for the tip about toasting the piston.