My 82 yamaha xs400rj starts and idles perfectly. It runs and pulls up to 5000 rpm perfectly.

http://home.comcast.net/~panofish/WEBLO ... 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.

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?

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?

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!

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

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!

Good catch,
Feels great when you figure out a problem like that yourself...

I was beginning to think nobody cared.
Thanks ZaGhost!!!

hehe,

Here's a funny story.... Just got my Maxim 400 safetied today....
I have a similar issue, when I get to 6000 rpm it seems to spit a bit,
need to look into that. can finally take it on a real run now for diagnostics instead of sneaking around the block (last safety was on 2000, didn't want a fine for no MVI sticker) hehe

I had to swap my carb body, I'm sure I kept the jest right, but maybe I mixed them, will have to double check that (hoping I wouldn't have to pull the carbs yet again....)

Will be checking it tomorrow, raining off and on all day today...wet weather is not the best for troubleshooting

Now that I've been riding it for a while... I've noticed that my seca 400 will have a very minor hiccup occasionally above 6000 rpm. Nothing like before when I had my jets reversed. I think my abnormally high resistance measurements for my spark plug caps is the culprit... I need to replace the caps with new 5k ohm caps. 1 cap reads 11.5k and the other is 8k. If that doesnt fix it, then I only have 2 other guesses... either the petcock vacuum needs replacing (its the original 82) OR my dual carbs are slightly out of sync?

Keep me up to date on your problem and we can share our solutions to help each other out!

THANKS!