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1981 CB750K Carb? Problems... Classic bikes discussion group:
1981 CB750K Carb? Problems...

1981 CB750K Carb? Problems... 
Justin Brown said 2007-03-27 03:30:28 
I recently purchased a 1981 Honda CB750K about a month ago, and ever since, it just hasn\\\t been running right. When I start the bike, there is an obvious miss, and the #1 pipe doesn\\\t heat up at idle. When I give it gas to take off, it breaks up until around 3000 - 3500rpm, at which point I can feel the #1 cylinder open up and when I check, the #1 pipe is hot. It doesn\\\t accelerate smoothly until around 4000rpm, and it backfires occassionally throughout throttle positions. It seems to run the best when down shifting to a stop, with only a backfire here and there.

After varifying air and blue spark, my next instinct was the carbs, so I took them off and cleaned them up pretty good, taking care to replace all fuel lines and the aftermarket inline filter, cleaning the tank, and filling it with 93 octane as I had since I bought the bike. I noticed that the choke position when the lever was fully disengage was about 1/4 on, so I adjusted the cable to allow the butterflies to be completly open. Questioning compression, I comp. tested it and all four cylinders came out at 155psi @full throttle. Next, I took the bike to a mechanic to have the carbs synched, and he synched them, but told me that the #1 carb wasn\\\t responding to the pilot screw, and that he thought the circuit was fouled. Also, the throttle linkage between the #1 and #2 carbs was messed up. So I ripped the carbs off again and carb cleanered the hell out of the #1 carb, ensuring that all passageways were getting cleaner through them, and fixed the linkage, bench synching as best as I could.

I take the bike back to the shop to get the carbs synched again (by the way, it accelerated/ran smoother than it had previously) and the mechanic says the #1 pilot screw STILL isn\\\t responding, though he synched them and they had been a little out of synch. He also said that with a little starting fluid, the #1 cylinder fires right up. When I pick the bike back up again, its back to it\\\s original tricks. Bottom line, the bike breaks up something fierce off of idle - missing, backfiring, etc. I am desperate here for any insight or advice, as this is a daily rider for me and I don\\\t have tons of cash. What could the problem be? The mechanic says pilot circuit, but I\\\ve cleaned the hell out of it um-teen-times, I\\\m starting to think the problem is elsewhere. Any suggestions would be greatly appreciated, I\\\m really wracking my brain.

By the way, the bike is entirely stock with the exception of an aftermarket Mac, 4 into 2 exhaust.
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