QuestionMaybe you can give me some advice on my problem.
My 82 yamaha 400 starts and idles perfectly. It runs and pulls up to 5000 rpm perfectly.
http://home.comcast.net/~panofish/WEBLOG/2007-08-25_1982_Yamaha_Seca/index.html
If I push it beyond 6000 rpm ... the engine hesitates randomly (as if it is misfiring or fuel trouble).
My current best guess is the left cylinder is running lean.
What I have done so far...
I checked the petcock.
Replaced the spark plugs.
The right spark cap is 8.5k and the left cap is 11.8k.
The coils measured identically and withing specs.
Spark plug wires are good.
The left spark plug looked super clean ... the right plug had some black sooting.. http://home.comcast.net/~panofish/plugs.jpg
I swapped the coils and the caps and the bike ran the same and the plugs appeared the same.
I checked and cleaned the fuel cap vent holes.
I have thoroughly cleaned the dual carbs twice, but I think I missed something.
Fresh tank of gas and a long ride made no improvement.
Today I ran the bike with the choke pulled slightly and the bike had more power and the hesitation above 5000 rpm appeared to go away.
This also makes me think the problem is still inside my carbs... the bike was sitting for the last 3 years.
The needle diaphragm is like new and no holes. I blew out all of the carb jets and holes.
I have not yet checked the petcock vacuum diaphragm... thats next before I pull the carbs again.
Any other ideas and suggestions would be greatly appreciated.
AnswerAlan, you have done a lot of good troubleshooting there. Usually spark plug caps are around 5k ohms, when new and your seem to be getting out of range.
Pulling on the choke and having the performance improve indicates lean running condition, as you have probably guessed. I went online at bikebandit.com and noticed that on the carb list the left and right side carb jets are different calibrations, if I read the information correctly.
If that is the case, then make sure that the correct jets are on the correct carbs. Listed are a "right" jet at #115 and another jet at #125. It is hard to tell if there are just two different jetting options for both sides or if one side is jetted differently than the other side, which is pretty unusual for a parallel twin. Usually different main jets are seen in the front/back cylinders of V-twins or V-4 engines.
Your mention of a vacuum diaphragm in the petcock is definitely a clue, especially if the vacuum line is going to the left side. Petcock diaphragms are prone to die, especially at this age, so I wouldn't be surprised to find that this is the actual culprit, causing your performance problem. A bad diaphragm creates fuel flow restriction and a vacuum leak at the same time.
Bill Silver
