Runs great on one cylinder, surges on two

JJs80XS

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Hi xs650 geniuses,

1980 xs650SG. It's been surging randomly and also won't come down from high rpms. Something interesting I've noticed is that when I pull either the left or the right sparkplug cap and make the bike run on only one cylinder it will idle nicely without surging, and as soon as both sparkplug caps are on (i.e., it is running on both cylinders) it surges, even after I turn the idle screw out. Same with the hanging high rpms: it won't hang high if it's only running on one cylinder, but as soon as both are going, it won't return to idle.

Thoughts? Thanks!
 
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Each cylinder runs nicely and idles at the same rpm when run individually. Does that mean they are “synced” or not necessarily? The manual says never pull spark plug caps while running the engine or you can fry the coil…oops. Seems ok still.
 
The cylinders have to run nicely in conjunction with each other.
Do you have to adjust the idle screw to get either individual cylinder to run i.e. the left cylinder runs fine at this idle screw setting, the right cylinder runs fine at this different idle setting.
If that's the case then they aren't synced.

Lots of info on carb syncing.
Sitting on the bike, the left idle screw setting moves both carbs butterfly's. The setting between the carbs just adjusts the right carbs butterfly.
The last set of carbs I synced I used straight pins, one for each butterfly.
With the carbs off and in hand I placed a straight pin under the left carb butterfly and adjusted the left idle screw to where the butterfly just gripped the pin.
Then going to the right butterfly I set a pin under that and used the center screw setting to achieve the same grip as on the left carb. Remove the pins when done.
Putting the carbs back on the bike you may have to adjust the left carb idle screw but only the left, don't mess with the screw in the middle.
Now, get the bike running and warmed to operating temperature, once warmed adjust the left idle screw to where the bike idles at 1200 rpm. Yeah, you may have to throttle the engine when cold BUT if you set the idle @1200 rpm on a cold engine once it warms up it may begin to rev on its own......something about advance in the TCI or sumpin'.
Other opinions may vary.
 
I have an '80SG. Build yourself a manometer and sync them using the vacuum barbs on your intake manifolds. It's super easy.
 
I built and hooked up the manometer yesterday and carbs were nicely balanced. Next thought is that I might just have multiple air leaks. I feel like it's impossible on these old bikes to eliminate all leaks, but I'm getting revving when I spray carb cleaner on the carb boots, and when I spray it on the top of the carbs, and if I spray around where the throttle cable connects. Maybe I'm just leaking really bad...
 
I replaced everything multiple times except the butterfly seals. Will try that next. It stops surging as long as the idle (when warmed up) is around 900. And as long as the idle is this low it'll also rev down from higher rpms, albeit slowly. Gotta keep a hand on the throttle at stoplights though or else it sometimes dies. Maybe this is just the way it is haha. I mean...it's old.
 
Yes, you need to set the idle at 1100-1200 RPMs or it will just up and die sometimes at idle. Slow return to idle or hanging idle speeds are both lean indicators, and air leaks would do that. The way the speed increased when you sprayed around the throttle cable points to the butterfly shaft seal. One of them is located right there. That one you could replace without removing the butterfly plate and just by removing the cable arm. You could do the outside one on the other carb like that too. Maybe replacing those two might improve things.
 
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