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1981 Yamaha 400xs carb adjustment

3K views 5 replies 3 participants last post by  dale847 
#1 ·
Hi, new to the forum, trying to help my son get his bike running. General question on carb fuel adjustment which is accessible from under the tank. Do mechanics remove the tank to adjust or does someone with tiny hands and a stubby screwdriver access from the side? From what I read, you normally start at around 2 turns. Trying to avoid taking off the carb just yet. Thanks.
 
#2 ·
It would be nice and appreciated if you would go to the New Members forum and introduce yourself and tell us a little about yourself. Folks tend to open up more if they feel they know you just a little bit.

 
#3 ·
The idle adjustment for rich lean, can be in different places on the 400. First clarify if it is a single or dual overhead cam. Big difference in horsepower. If the adjustment screws are on top and forward of the carbs, remove the tank. Might have to remove the seat and side covers too. I use a plastic gas can with a vent. Drilled out the vent hole to accept a fuel line. Does not leak if it is a tight fit. I have a pic somewhere. Some of the bikes have different size main jets due to the air box shape. I use foam filters, but have found the right hand carb likes a bigger jet. I have info on the carbs if it is a DOHC. The SOHC uses similar carbs and are a bit smaller I think. UK
 
#5 ·
One and three quarters is a common place to start. Screw them in and the engine should run sweet, then stall. screw them out, it should stay running, but not as smooth. Why is the plug fouled? Bung in a new one or two. Idle settings are just that. Idle. As soon as you move the throttle you are above idle, at that is what you need to go places.
The tuning guide, available on line, and racers, start at the other end. WOT, because you are most often running on the main jet on the street. Mid range in the parking lots.
A piece of paper dangled over the end of each pipe will also give you a sign of how well that cylinder is firing. Exhaust heat is another indicator. You did not answer about which engine you have.
If you are going to run it for long periods at idle speeds, you will probably foul more plugs, especially if the compression is low. Use hotter plugs. What is the compression? UK
 
#6 ·
Hi UK,

This is the bike, assume it's SOHC.
Not sure why the plug fouled, just replaced them both and it started right up. The bike seemed to be sitting a lot and the previous owner may have tinkered quite a bit with it.
I don't have a way to check compression today, but can work on that in the future. It's starting and running consistently so now I'll have a better opportunity to mess with the adjustment screws.
Thanks for all of your wisdom, you pushed me along the way for sure.
Dale
 
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