1980 XS400 reving problems

[Aleks]

Hi

I have some problems with my XS400. I bought it last year and it was running fine, took it totally apart, painted the frame and did some rebuilding over the winter and now finished it. I got it running pretty good right after the rebuild, was revving great, idling great, but then I discovered a charging problem. While messing with the charging and trying to figure out the problem, the bike started to run more and more rough. Now I got it idling almost fine, but when giving some throttle the revs will stay up for a while and come down really slow, cable is not sticking, usually it means that its running lean. But when I checked the spark plugs they were black, which usually means that it's running rich. Is it possible that its running lean but the spark plugs are turning black, because the ignition is off and not burning gas properly?

Also checked the things that are moving up and down in carbs, they are moving freely and the rubber things are in great condition. Floats measured ~27.5mm, not sure if it matters for that

Im pretty new to motorcycles and would appreciate to get some advice.

 

[xschris]

What year is the bike and post pics so we can see what you have. I would do 26mm on the float height and pilot mix screws at 2.5-3 turns out then sync the carbs with a manometer with the bike fully warn and at 1200 rpm's. What size jets are you running. Make sure the charging system is working correctly. All the specs are in the manual.

 

[Aleks]

Its 1980. Bike originally looked like that, but had some mods done to it and was cosmetically in bad condition when I bought it (had some bad paintjobs done to it, but mechanically felt good): 2560px-Yamaha_XS-400.JPG (2560×1862) (wikimedia.org)
I'll take some photos and do the floats to 26mm next time I go to garage.
Engine compression was over 140psi. (have not touched the engine, only changed the cluch cover and alternator cover gasket)

It had cheap pods on the original H-pipe. I installed foam pods. I have considered buying original airfilters if I can find some from Europe, but as much as I have read, foam pods should work ok.
Main jets are 137.5, but I have new 132.5s also, have not used them cause I thought bike is running lean. Should I change them?

Charging system is working correctly now, it had a aftermarket reg/rectifier, which was not working. I bought a new good quality one from UK (Im from Europe).

I'll try to do carb syncing next. Never done it, I hope youtube will help out

 

[JPaganel]

Also, make sure the vacuum pistons in the carbs aren't sticking. It doesn't take that much dirt to do it.

 

[Aleks]

Few days ago went to garage and found out that one cylinder is actually not firing, don't know how I missed it last time. Bought new spark plugs and got it working. Now the right cylinder (the one that was not working) backfires to the carbs. Felt like the warmer the engine gets the more it backfires to the carbs.
Did some research and I'll check for vacuum leaks tomorrow. If theres no leaks then I should adjust the ignition timing and then valve timing?

 

[Aleks]

SOLVED!

Finally actually found the problem. One ignition point was not opening and closing fast enough when the engine was running. The small shaft what the point moves on had really small amount of rust. Opened and closed correctly when just turning the engine by hand, but not fast enough when engine was running. Cleaned the small shaft and now everything is running perfect. No throttle sticking, no misfire etc.
So small problem, but really hard to find, also read a lot about xs400 ignition problems, but never saw anyone mentioning that