Topic: 77 CB750 three-and-a-half F2 - diagnosis help?

[capoeirabike]

Hey folks,

Been a while since I've talked to you guys.  I'm liking the new digs, though!

Some of you may remember my email from the old board (mike?).  Anyhow, I've got a 1977 750F2 that's been through some mild modification since my purchase near three years ago, and now I've detected a problem that either wasn't there before or that I was simply too ignorant to notice earlier.  The issue is such:

Since fitting a set of aftermarket carburetors (29mm keihin CRS roundslides - The problem could very well have been there before), I've had intermittent problems with a backfire into the #2 intake (numbered from rider's left, correct?).  I recently decided to attempt a more thorough diagnosis of the issue and found that the cylinder seems to be pumping un-combusted charge back through the intake at idle.  However, it IS firing somewhat, as the exhaust pipe is only slightly cooler than the other three.  Here's the kicker:  When I cover the carb throat at idle, the bike continues to run "normally."  When I cover any OTHER carb, it'll begin to die out.  I know this is bad news but am unsure where to begin chasing the problem.

I don't have access to a compression tester, so I haven't done THAT check yet.  I'm assuming a problem with an intake valve, correct?  Should I expect it to be chewed up, bent, stuck?  Is this going to be an engine-out procedure?  Seat-of-the-pants dyno runs suggest a weaker running condition than before my last issue, which was a pocket of water in the fuel line, causing starvation, after a switch to reserve one morning - probably unrelated, though.

Thanks in advance for your help!
Charlie
Flagstaff, AZ

[MRieck]

Charlie... funny I was just thinking of you the other day as I installed my synch tubes in new intake boots!! I hope everything is going well for sir and you must be finished with school ( I think ). Check your float heights on those carbs- it should be 14mm, the tabs just resting on the needles. Those floats are often misadjusted from the factory. #4 on my new CR-31's was off a couple of mm which made the plugs foul at idle and run like crap. Those carbs can be a #$%*. Your float needles could be crapping the bed too. Did you set timing with a light?? Vacuum leaks ?