Running Lean...

marioc89

marioc89

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Well first off this is my time to this site and I hope y'all could help me on a few issues. I have a 1981 XS400 and it runs great other than sometimes when after coming to a stop after doing some riding, it tends to idle high sometimes and at times it doesn't. I'm trying to figure out why is it doing that and what can I do about it. Does that mean I may be getting too much air in the intake? :confused:
 
tighten those clamps, but more likely, the carb boots are cracked, and occasionally open up. or the butterfly shafts on the carbs are letting by unmetered air. hope its just the boots, bud.
If it usually does it once its warmed up, its prolly the boots. when they get hot, they get soft.
 
One more thing. What if one of the exhaust pipes have a small hole in it around the clamps, do you think that'll do it too?
 
After making sure all the holes are sealed, you may need to adjust the idle mixture. The settings suggested by the manual (something like 1.25 turns out) is very lean. These bike run better if the idle mixture is richened. I would start at 3 turns out and adjust up or down in reference to the spark plugs.
 
On most carbs I've worked on, anything much over 3 turns out on the mixture screws is as far as you want to go before you have change other things. Do you see any changes with adjustments over 3 turns?
 
As mentioned, check the carb boots. If you haven't already cleaned the carbs I would do that too. You could very well have a clogged, or partially clogged pilot jet. Besides, if it hasn't been done it's probably due for a good cleaning. Also check to make sure the cable isn't the problem. I've had throttle cables that will stick sporadically.

Once you've got the boots taken care of, and the carbs thoroughly cleaned you might want to look into buying a colortune. The colortune plug makes tuning the mixture screw easy, and accurate.
 
Forgot about those colortune things. A couple of years ago I read a little about them and planned to buy one but never did. Do they really work that well?
 
Yeah, if you have everything close, meaning you've got it jetted right, no air leaks etc. Then you can really dial in the mixture screw.

If you don't have it jetted right you'll see that you're running rich, or lean on the pilot, but not much else. I suppose if you're really into revving the hell out of it in neutral you could use it to gauge the main jets, but I'll pass on that. It's mainly for setting the low speed mixture, and does that very well.
 
I've tried color tune, and didn't get on with it (couldn't see a thing!)

used the dead cylinder method and dialed it in that way!
 
if it is idling high or tends to rev on its own it could be a hole in the diaphragm or cracks in the seal of it. they do go bad after a while and are not cheap to replace $125 each from yamaha.
 
main jets only help you at high rpms say about 5k and up. pilot jets are for low rpm and your needle setting is for the middle. hope this helps a little.
 
just bought an engine off ebay the engine number is *2a2-101305* can anyone tell me which model this is from and possibly a year thanks
 

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