Topic: '76 CB750F vacuum leak problem or low compression?

[operasinger750]

Hey all!  I'm reasoning my way through an issue and I have a couple hypotheses, just looking for any other thoughts...  My bike was running great until one day when I discovered too late that one of the cables had come loose from the coil and the bike ran really rough on 3 cylinders.  I limped home convinced that I had water in the airbox from the heavy rain I had been stuck in the night before.  Just previous to that, I had tuned up the tappets, synced the carbs with great success, new plugs, oil, etc.  The rough running seemed to knock the tappets out of adjustment, so I tuned those up, but the bike still ran rough, so I tried to sync it again.  This time, i noticed that no matter how i adjusted the #2 slide, i couldn't generate the vacuum pressure of the other three.  I've read from many of you that it doesn't matter so much the level of the vacuum, just that the needles move at the same time when you twist the throttle.  They do, but vacuum on #2 is definitely an issue.  At idle, it will read about 1/2 of the others, when I twist the throttle, it will continue to be low, then as it gets up around 5K rpm, it somtimes jumps up to the same level as the others, sometimes, not.  I'm only guessing that it's either that the incresased rpm has either sucked the intake rubber tight against the carbs and engine intakes or the increased oil pressure has mitigated a lack of seal from the rings.

Am I on the right track?  Am I right to think that the next step is a vacuum test with brake cleaner around the intake rubber and then a compression test?  Just looking for the next step in diagnosis..  Thanks all.

[KRONUS0100]

are you maikng sure engine is at operating temps before you adjust carbs?
double check and make sure all sealing surfaces are clean, cables lubed.  then try the ether test at the intake boots.  if you are still running the spark caps, make sure they ohm out at close to 5k ohms, and make sure they connect properly to the plug cables.  then warm up the engine and try to synch them again.


you can cheat and remove carbs, and using a 1/16th drill bit, bench synch the carbs from the head side.  this will get you real close.

[phil71]

If you want to test for vacuum leaks, spray some wd 40 around the boots while running. If the idle changes at all (Up or down), you have an air leak somewhere.
  If that doesn't show any results, the next thing would be to recheck your valves. If a valve is set too tight it will hang open by very very small amounts, and that will make you lose vacuum/compression .

[operasinger750]

Hi Kronus.  Yes, the engine is warm.  I bench synced the carbs as you described in August with no issues, so this is a new thing.  Phil, I'm planning to do that, just wanted to get opinions on the predicament before I futz any further.  I've done valves so many times, I'm 95% certain that I have that correct.  Does anyone out there suppose that it could be that the ring on #2 is no longer fitting tightly with the cylinder wall?

[phil71]

barring some oil problem, rings tend to go gradually. Unfortunately, as you described, you did a bunch of little things at the same time right before this started happening, and that makes troubleshooting annoying.
Troubleshoot easiest to hardest, outside to in, which means don't start talking about rings valves or pistons till you've truly eliminated every other potential culprit.
     I realize you rode for a bit with one cylinder not sparking, and it's true that the unburned fuel will be kind of bad for that cylinder, but you'd have to have really logged some miles on it to wipe out rings.