Your Yamaha XS250/360/400 doesn't run right

[JimR]

If you have an XS250/360/400 sohc that doesn't run right .. bad starting revs hanging at 2~4k .... engine idle dipping when you put the lights on ?
does this sound like your bike ?
The answer is simple ! the bike needs TLC !
Do the following :-
Change the oil & Filter
Set the Valve clearances to 0.1 mm intake and 0.15 mm exhaust
Change or clean the air Filters
Gap the spark plugs or fit new ones (fitting new ones is the best idea)
Run up the motor and set the dwell angle to 22.5% or if you don't have a dwell meter 15 thou at tdc on the fire stroke will do.
Adjust the ignition timing l/h cylinder first to the mark LF and RF for the right cylinder.
Recheck the dwell angle or regap the points (this bit may take a few times to get it right !)
When you are happy with the dwell/point gap and timing move on to the carbs !
If you have time remove the carbs & clean everything out (if you can have them sonic cleaned) while your there (XS400) check that the main jet rubber plugs are ok & sealing.
Remove the Mixture screws, check that the tapered points of the screws are not broken off (big probs ! as the bikes are now old) clean them off and reinstall and set them to 1.5 turns out from home ( a good place to start).
Restart the motor with vac gauges connected and an hours time to spare !
Balance the carbs with the screw situated between the carbs, when you are happy this is correct switch the lights on (I know some of you cannot do this) if the idle speed drops the mixture stength is incorrect. (for those who cannot switch the lights on or off rig up an on/off switch on the light fuse).
Adjust both mixture screws to the point that the motor idles correctly and responds to the grip as it should.
I know this is a difficult subject so post replies as to which stage your at so we can get that old bike running fine !
Regards Jim