panofish Posted August 26, 2007 Share Posted August 26, 2007 My 82 yamaha xs400rj starts and idles perfectly. It runs and pulls up to 5000 rpm perfectly. http://home.comcast.net/~panofish/WEBLOG/2...Seca/index.html If I push it beyond 6000 rpm ... the engine hesitates randomly (as if it is misfiring or fuel trouble). 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, I'm meticulous, but not an expert, so I could have missed something. Fresh tank of gas and a long ride made no improvement. My best guess is that everything points to the right carb as being to rich? Is this a good guess? How can I lean it out a little to test? Any other ideas and suggestions would be greatly appreciated. Link to comment Share on other sites More sharing options...
panofish Posted August 26, 2007 Author Share Posted August 26, 2007 After further research on the web... I came across this post... The bad things are as follows: Petcock failure from an inverted diaphragm, which could be caused by running the motor with the petcock in the off position. Petcock failure due to a vacuum leak. And of course fuel flow, at high rpm, around 5,000 rpm we have found the vacuum petcock does not pull enough vacuum to supply the fuel the motor needs to operate. Recently we had a 1200 Sportster that would not pull past 4700 rpm, we changed to a non vacuum operated petcock while still using the same 5/16 fuel line, and were able to pull straight to red line, if we wanted to. If I set the petcock to PRI... wouldn't that prove if I have a problem with my petcock diaphragm? Link to comment Share on other sites More sharing options...
panofish Posted August 26, 2007 Author Share Posted August 26, 2007 I found a suggestion to pull the choke a little while running the bike... the hesitation problem above 5000 rpm appeared to go away and there was noticeably more power. This make me think I'm still missing something that is gummed up inside the carbs. (The bike has been sitting still for the last 3 years) The needle diaphragm is like new and no holes. I blew out all of the carb jets and holes, but I didn't play with the factory set needle that adjusts the mixtures in each carb. I'm sure it doesn't need adjustment, but could there still be some gummyness in that area that could cause my trouble? Any ideas? Link to comment Share on other sites More sharing options...
panofish Posted August 27, 2007 Author Share Posted August 27, 2007 I think I've got it figured out.... When I took off the carbs for cleaning (after 3 years sitting idle)... I took out all the jets and bits to clean carefully. The dual carbs appear to be exactly the same as each other, so I assumed that all internal parts were interchangeable. Checking my service manual, I see that the left main jet is bigger than my right main jet. I must have swapped the main jets by accident. Since the main jets are responsible for 5k and up power, and my left spark plug looked lean and my right plug looked rich.... this explains my symptoms exactly!!! I still have to pull the carbs and swap the jets back to confirm, but I am certain this will fix my high rpm hesitation problem! Link to comment Share on other sites More sharing options...
panofish Posted August 27, 2007 Author Share Posted August 27, 2007 I just pulled the carbs and sure enough.... the right jet was larger than the left jet. However, they should be in reverse. Here is a photo of the jets... http://home.comcast.net/~panofish/carbjets.jpg Link to comment Share on other sites More sharing options...
panofish Posted August 27, 2007 Author Share Posted August 27, 2007 Went for a test ride after swapping the jets. My bike is running perfect!!! Good as new!!! Not bad for an 82. Pulls hard all the way through the rpms. I hope my experience helps someone. Good Luck! Link to comment Share on other sites More sharing options...
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