Timing?? 81 XS650SH

[Tim Meaders]

I've gone through the carbs thoroughly and balanced the float levels. It won't run without back firing
and surging. When I attempt the dead cylinder set up to adjust the air mixture it run's on that cylinder
just fine, no backfire. I won't mention how many times I've pulled the carbs and disassembled them for cleaning but I've gotten very proficient in this step anyway. I'm starting to wonder if when I put the top
end back together I offset the timing chain?? Is it possible that the engine will run if your off 1 tooth?

Thanks

 

[xjwmx]

I think on one end of the camshaft that sticks out there's a hole. Put the crank at TDC and the hole should be nearly vertical. Find out how many teeth are on the camshaft then divide 360 by that and the hole should be less than that number away from vertical.

 

[Jim]

When I attempt the dead cylinder set up to adjust the air mixture it run's on that cylinder
just fine, no backfire.

Think about that.... you didn't change the timing, yet the problem went away. If cam timing was the problem, most likely it would still backfire. Not sayin' you're not off a tooth one way or the other, you could still be, but in all likelihood... that's not causing the backfiring. Cam timing being off by one tooth,usually causes a lack of power and/or hard starting, not backfiring. I would still check the cam timing if you're not sure and check spark timing for good measure... but it sounds to me like an induction leak.

 

[Tim Meaders]

I did have air induction on one of the carbs so I replaced both intake manifolds with new one's. I also changed the screws on the band clamps
that hold the carb to the manifold so I could tighten them all the way. I guess I need to look for another air leak elsewhere, any suggestions
are appreciated.

xjwmx & JimD54, thank you for your responses.

 

[Jim]

so I replaced both intake manifolds with new one's.

Here's one way to look for a leak. Another way is to move something around the carb that makes a fair amount of smoke and look for it getting sucked in somewhere.

I'd be suspicious of aftermarket intakes. I have no personal experience but I've read stories of them leaking because of poor/loose fit. Did you replace the butterfly shaft seals?
Good luck.

 

[xjwmx]

If the carbs are the stock ones, bs34, I'd just turn them out 3 turns and probably leave them there. There's no real peak, just very wide peak. Then see how it does. If still a problem then make sure the float height is right and that the float needles aren't swollen and sticking. Make sure 42.5 is stamped on the pilot jet. I've had lots of minor-ish air leaks and none of them caused as big a problem as you're describing. Lots of popping can be caused by super lean, including running out of fuel, like a petcock messed up some way, so make sure gas flows freely/

 

[robinc]

Carb cleaner will find leaks for you quickly but overuse can cause damage to rubber parts. DAMHIKT Be judicious if you're using it.

Another idea I think from 5twins is to use an unlit propane torch. Idle will change at the leak source and no damage done.

 

[bikefreak69]

Yes,the unlit propane torch trick finds air leaks very quickly and cleanly.

 

[TwoManyXS1Bs]

Your symptom sounds like some of the "weak TCI magnet" stories.
Try a forum search on "rare earth magnet"...