C55 unburned fuel....What am I overlooking?...Resolved!

[Privateer]

My motor is a 1994 C55 and runs great. The issue is copious amounts of black crud coming from the exhaust ports. I searched the forums to determine it is likely caused by unburned fuel. I rebuilt the carbs with the Yamaha kit and set everything to spec and also checked the reeds. That did not correct the problem. Next, I checked the thermostat and it was stuck open. So, I replaced it, the pressure relief valve grommet and the water pump while I was at it. Lastly, I replaced the fuel pump and I still have the exhaust issue. I am not ready to accept this as “normal” for a two-stroke. I have owned this motor since 1996 and have only been having the problem this year. Any suggestions?

 

[bigbrownbuku]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

the 'washing' from raw fuel will not go away immediately. sounds like you have covered most bases. give it an hour or 2 running then check again.

 

[Privateer]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

Thanks for your input. I have put around ten hours on the motor since the repairs have been made, but I haven’t noticed any real improvement. I will continue to monitor. <br /><br />When I removed the reeds for inspection, there was a puddle of fuel in front of the reeds. Is this normal?

 

[bigbrownbuku]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

the puddle of fuel isnt normal. you likely still have a carb problem or your choke is getting stuck closed? remove the carbs again but keep the gas line connected. prime the fuel slightly and check for any fuel leaks aft of the throttle butterfly.

 

[Privateer]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

Thank you BenC. I did not find any obvious leaks, however I have found a bad recirc valve. Used the alcohol and syringe method to determine the check valve is not working. It free flows in both directions. This motor is supposed to have two recirc valves, but I can only find one and it is at the bottom of the intake. The part # is 650-11370-01-00 and the parts breakdown shows two, one at the bottom and one at the top. Anyway, gonna replace the one that I have tested to see if that helps. Did not mention that the motor was not used for 9 months due to hull repairs. It was not until after this that I had the fuel problem. Maybe the check valve gummed up?

 

[Privateer]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

Checked with a local dealer this morning. He couldn't tell me if there were two recirc valves on this motor. Does anyone know if there were production changes on this motor or perhaps the Yamaha parts breakdown is just wrong. The part(s) have to be ordered.

 

[Privateer]

Re: C55 unburned fuel....What am I overlooking?...Resolved!

Both recirc valves were stuck open. I have replaced them and the problem is gone. Thanks BenC, Rodbolt and Ray. You guys all played a part in me getting this thing resolved. Happy Holidays!