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Third pipe not as warm as others

  • Thread starter Thread starter nabrams
  • Start date Start date
N

nabrams

Guest
Hi All,

My 1981 CB750 runs fairly well, but I noticed that the third pipe (from the left) is significantly cooler than the others (but still warm). I know it's firing because when I remove the plug wire, the bike dies.
Does this mean that carb is too rich?
I think I recall somemone saying that it is typical for the third cylinder to take longer to warm up than the others - is this true?
Even though the bike accelerates fairly well to 50 MPH, it seems to accelerate sluggishly above that (in 4th and 5th gears).
The acceleration is a bit better after the bike's been running for a half hour, but that's typical of any bike, and it's not as good as I would think a 750 would be.
I put in larger main jets (115) to accomodate the 4-into-1 pipe and eliminate the backfiring (even with 110 mains I still had some mild backfiring occasionally). The air box and filter are stock. There are no air leaks (I sealed them) or exhaust leaks (sealed those too), which used to contribute to a too-lean condition.
So now I know it's not too lean, but perhaps it's too rich?
 
Have you checked your valve clearances? You may have a tight exhaust valve.
 
Try here for CB750-900-1100 info

Try here for CB750-900-1100 info

Hi there,
Just as this is the formost resource for GS machines this site is the best for the CB750-900-1100's. Great bunch and information out the wazoo on the CB's.
http://cb1100f.net

May be helpfull?
Anthony
 
Since I live in MN and store my bikes for 3 or 4 months each year I usually remove the bank of carbs each spring, remove the float bowl covers, and clean the carbs the best I can with a spray choke cleaner. I also have an external "hanging" gas tank and mercury tube synch tool (both MotionPro) so I can synch the CV carbs. Not fun but it keeps throttle reponse crisp and is worth the effort. One carb should be designated the "reference" carb (number 3 on my GSs) and does not have a "throttle valve adjustment screw". The other 3 carbs are synched to this one. You may also have to check the air/fuel mixture screws - which I only know to turn to factory specs. The center cylinders will heat slower due to more surrounding "mass" but the pipes should heat fairly evenly.
 
What do your plugs say? A quick glance at the ol' electrodes should clear up the mystery.

Personally, I would hazard a guess that something is plugged (possibly something the idle circuit) in the #3 carb. Or at least seriously out of whack. That's just a guess, though.
 
Nabrams, 'jtcrespo' hit the nail on the head. You could very well have a tight exhaust valve, or a burnt exhaust valve(s). CB750/900's were notorious for this. Check your valve clearances first, adjust as necessary, and then do a compression test. If your compression is low after your valve clearances are in spec, do a leak-down test. This will tell you exactly what the deal is.
 
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