Bike surges in RPM randomly and now dies with throttle

[Fishy]

I have a 1979 XS11 that I bought running. It ran pretty well but bogged down at higher RPMs. When I was riding it, it started dying at every stop. I'd have to go to neutral early at a light and rev to keep the bike from stalling. Found out the tank was a rust bucket inside and the filters were shot to hell, so I cleaned the tank, cleaned carbs, and added inline filters.

That seemed to fix the issue, but I noticed the airbox was missing parts and only partially put together. I rode the bike for a few miles and it randomly started running up RPMs and would hold 5-6k for a few seconds before it came down again. Parked the bike and tore down the carbs again and replaced the jets with size 147.5 since I planned to move to pod filters and it has a 4-2 exhaust. It had mismatched jets in the bike originally, so I figured this would help. I also replaced the spark plugs and used an external fuel tank to sync the carbs.

When I started to sync the carbs, the bike started backfiring like crazy from the intake and from the exhaust and I couldn't run the bike long enough to sync the carbs. Since then, the bike sometimes idles poorly with full choke and dies once I pull the throttle. Every few times I start it, it runs great and I can rev the bike properly, but then I'll let it sit overnight and the issue is back.

I've been reading the forums here for a while and I saw one about checking if a timing light would go out on a spark plug when I add throttle. Cylinder 3 has a strong light at idle but dies with the throttle. The other 3 flash at idle and stay on with throttle. I'm thinking my ignition coil is bad, but I don't know if it would cause all the issues I've been having. From what I've seen on these forums, the bike should run on only 3 cylinders.

I checked the valve clearance and it was between 0.04mm and 0.08mm on all but E5 which was 0.13mm. I've never re-shimmed a motor, so I wasn't sure if these were in spec or if I needed to start there. I thought that might have caused the popping, but it seems so intermittent when that occurs that I want to believe it's an electrical issue and not valves.

I've been working slowly on the bike for a few months and I'm at a loss of where to even start. I'll work on something, then the issue goes away, just to come back the next time I look at the bike. Any guidance on where to start or what to check is much appreciated.

[DiverRay]

What model '79? Are the carbs off a '79 or a later year? Do you have a vacuum leak?(probably) These are the first things we need to know in order to help. Upload pictures and we can identify most parts.

[Tonyxs11]

also check the wires at the vacuum advance as they can cause problems.

[LAB3]

^^^^^ What he said, mine was backfiring big time when I discovered that problem which is common to XS's.

[Fishy]

I checked the vacuum advance earlier and no wires were split as other posts said. They seemed solid and had only a little give when pulled.

I pulled off the coils and noticed one looked like it was repaired with some brown junk. It crumbled off easily and the wire was cracked underneath. I just bought a new set of coils, so hopefully that will fix the issue.

[cajun31]

I have a 1979 XS11 that I bought running. It ran pretty well but bogged down at higher RPMs. When I was riding it, it started dying at every stop. I'd have to go to neutral early at a light and rev to keep the bike from stalling. Found out the tank was a rust bucket inside and the filters were shot to hell, so I cleaned the tank, cleaned carbs, and added inline filters.

That seemed to fix the issue, but I noticed the airbox was missing parts and only partially put together. I rode the bike for a few miles and it randomly started running up RPMs and would hold 5-6k for a few seconds before it came down again. Parked the bike and tore down the carbs again and replaced the jets with size 147.5 since I planned to move to pod filters and it has a 4-2 exhaust. It had mismatched jets in the bike originally, so I figured this would help. I also replaced the spark plugs and used an external fuel tank to sync the carbs.

When I started to sync the carbs, the bike started backfiring like crazy from the intake and from the exhaust and I couldn't run the bike long enough to sync the carbs. Since then, the bike sometimes idles poorly with full choke and dies once I pull the throttle. Every few times I start it, it runs great and I can rev the bike properly, but then I'll let it sit overnight and the issue is back.

I've been reading the forums here for a while and I saw one about checking if a timing light would go out on a spark plug when I add throttle. Cylinder 3 has a strong light at idle but dies with the throttle. The other 3 flash at idle and stay on with throttle. I'm thinking my ignition coil is bad, but I don't know if it would cause all the issues I've been having. From what I've seen on these forums, the bike should run on only 3 cylinders.

I checked the valve clearance and it was between 0.04mm and 0.08mm on all but E5 which was 0.13mm. I've never re-shimmed a motor, so I wasn't sure if these were in spec or if I needed to start there. I thought that might have caused the popping, but it seems so intermittent when that occurs that I want to believe it's an electrical issue and not valves.

I've been working slowly on the bike for a few months and I'm at a loss of where to even start. I'll work on something, then the issue goes away, just to come back the next time I look at the bike. Any guidance on where to start or what to check is much appreciated.

You need to verify that the carbs are right before chasing the electrical problems. You need to do a good bench synch before mounting to the bike for final synch. I would go back to original jets too. When you change more then one thing at a time then you start chasing your own tail. Might be creating problems before fixing the original problem. Once you have the bike running properly then you can proceed with changes. If the carbs are not clean then everything you are experiencing is the result. Getting them very close with the bread tie bench synch method will at least allow you the time to properly synch with gauges on the bike. One other thing I always do is to remove the intake manifolds and if present get rid of any gaskets and seal the manifolds with a thin coat of yamabond then remount them. This ensures no leaks. Even the smallest intake leak causes fits with synching.