81H timing and vac advance question

[GLoweVA]

Okay, I'm an 80 owner, so I don't know about the differences in the 81.
I'm trying to help Nightengale out on his 81 project and his XJ carbs.
So, He's got an 81, so it has the different TCI and no mechanical advance and the 'mech advance' is programmed into the TCI.
And right now, he doesn't have his vac advance hooked up.
So currently with out a timing light, I'm not sure when he's firing. I think it might be slightly retarded (like 5deg or so, but again no way to verify).
Some of the symptoms are: ocasional to frequent backfiring out the exhaust and out the carbs.
He has no mufflers on right now.
I provided a suction to the vac advance (don't ask me how) and his idle went up nicely (a little high) but smooth with little to no back firing.
We also tried putting his mufflers on to see if back pressure helped.

I'm just trying to figure out where to go from here.
We're thinking of riging up some kind of temp vac source. (like a turkey baster)
This will 'advance' the timing so that he won't back fire much if at all.
He could have a lean condition too, so we might back out the idle mix screws a turn or 2.
I'm going to have a look under the timing cover and see what's what.

I guess maybe I'm getting ahead of myself. Maybe we've got to fix his charging problem before we can dial in the engine. It's just a PITA trying to troubleshoot the electricals with the bike running when it's acting so erratically.

Anyone with any ideas? (Setup: 81H with XJ carbs & Pods, no vac advance port (yet) and no mufflers)(guy who set up the carbs says they're ready for pods and straight pipes.)

[crazy steve]

You should have no vacuum advance at idle. The vacuum port (on the stock carbs) is located in such a way that it's not open to engine vacuum until the throttle is opened; even then, the amount of throttle opening will affect how much vacuum you do get, with 'full' vacuum not coming in until the throttle plates fully expose the port. This is why carb swaps are such a challenge on these, drilling that port into the right location isn't easy.

Are you sure the timing is right? Yes, pulling in more advance by applying vacuum will smooth/speed up the idle, but that's just masking the problem. The bike should idle properly with the advance disconnected. Checking the timing assembly is a good idea, make sure it's free to move with vacuum but shouldn't have any 'play' in it either. And in spite of the fact that the '81 has 'non-adjustable' timing, you should still check to make sure it's right. It might also be a good idea to check the cam timing; if one or both cams are off by one tooth, the bike will still run, but not right.

((guy who set up the carbs says they're ready for pods and straight pipes.)

That statement right there says to me the carbs aren't right, as these motors won't run right with straight pipes...

[motoman]

You should have no vacuum advance at idle. The vacuum port (on the stock carbs) is located in such a way that it's not open to engine vacuum until the throttle is opened; even then, the amount of throttle opening will affect how much vacuum you do get, with 'full' vacuum not coming in until the throttle plates fully expose the port. This is why carb swaps are such a challenge on these, drilling that port into the right location isn't easy.

Are you sure the timing is right? Yes, pulling in more advance by applying vacuum will smooth/speed up the idle, but that's just masking the problem. The bike should idle properly with the advance disconnected. Checking the timing assembly is a good idea, make sure it's free to move with vacuum but shouldn't have any 'play' in it either. And in spite of the fact that the '81 has 'non-adjustable' timing, you should still check to make sure it's right. It might also be a good idea to check the cam timing; if one or both cams are off by one tooth, the bike will still run, but not right.



That statement right there says to me the carbs aren't right, as these motors won't run right with straight pipes...

+1 what Steve said! He's no real carb 'wrench' by that kinda statement. Gonna be an issue with the XJ Mikunis with no port for ported vacuum. Least he can do is get some mufflers on it......for a start or your pi$$in in the wind no matter what you do. BTW, static timing is NOT the issue, although the vacuum advance plays into the correct firing time as engine speed increases. Being so lean at this point, wouldn't ride it much as it stands a chance of doing piston damage.

[Nightengale]

electrical

well george came over today and we were able to get all my electrical issues worked out. one main one was that the rectifier was shot, but he had one off a bike he had and it worked great. after we got all the electrical issues worked out i got the bike put back together, and decided to start it up and run it a little to make sure that the battery had a full charge before i put it up for the night. it started up fine ran it on choke for a little then let it idle there was some backfiring for about two minuites then it seemed to idle great. so i then tried to rev it up to get the battery charging, this is when it would hesitate and back fire alot and almost cut off at times, but not this time it reved fine and and seemed to run 100% better before we started the electrical. so i decided to take it down the street and through my neighborhood. it ran great no sputter, no backfiring and went thru the gears with no hesitation what so ever. so would the bad rectifier have been the problem with the engine not running like it should. ( i know i need mufflers and the vac advance for it to run its best)
any input or ideas would be great

[cywelchjr]

well george came over today and we were able to get all my electrical issues worked out. one main one was that the rectifier was shot, but he had one off a bike he had and it worked great. after we got all the electrical issues worked out i got the bike put back together, and decided to start it up and run it a little to make sure that the battery had a full charge before i put it up for the night. it started up fine ran it on choke for a little then let it idle there was some backfiring for about two minuites then it seemed to idle great. so i then tried to rev it up to get the battery charging, this is when it would hesitate and back fire alot and almost cut off at times, but not this time it reved fine and and seemed to run 100% better before we started the electrical. so i decided to take it down the street and through my neighborhood. it ran great no sputter, no backfiring and went thru the gears with no hesitation what so ever. so would the bad rectifier have been the problem with the engine not running like it should. ( i know i need mufflers and the vac advance for it to run its best)
any input or ideas would be great

Yes, the charging system not working can make the ignition system not work correctly and make the bike not run well. It still won't run as it can with straight pipes, at least nobody on here has been able to get that to happen, and if it's actually been tuned to work that way, putting mufflers on WILL mess it up bad and require re-jetting and re tuning of the bike and carbs.

[GLoweVA]

He's been riding it around the neighborhood and he says it runs just fine and no backfiring, so I guess the carbs are jetted right. Sometime soon, I'll get over there and put a sync rig on it to dial the butterflies in.