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Old 06-08-2010, 09:04 AM   #1
Kevin McNeil
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Default help on 1981 Honda CB900 Custom

I have been restoring a 1981 CB900 Custom. I have gone through the entire bike with the help of an original Honda shop manual specifically written for this bike. I rebuilt the motor, new rings, valve stem seals, ground and lapped the valces, adjusted the valves, etc. I also have rebuilt the carbs, replacing all pertinent parts. I might have made the mistake of removing the limiters while rebuilding the carbs. I have set them 2 turns out, synchronized the carbs. The bike starts with some difficulty, but once sdtarted seems to run fine, for above five minutes until warmed up. Then it starts to smoke, runs rough, and when riding it, while slowing down, under compression, a couple small backfires. I am unsure where to start as far as troubleshooting this problem. Does this sound like a carburator or motor problem? Anyone have any experience in this type of problem? Or ideas as to where to start? I thank you in advance for your help.
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Old 06-08-2010, 10:06 AM   #2
seruzawa
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Black smoke? You are running rich. You may have a plugged passage in one or more of the CV carbs. Or perhaps the choke system isn't completely opening. The rich condition allows the bike to start but bogs it down when it warms.

By limiters do you mean the air screws? Start with the air screws 2 1/2 turns out.
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Old 06-08-2010, 10:29 AM   #3
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Thanks Seruzawa. I'm at work so I'm unable to make any adjustments in the air screw right now, looking forward to rushing home and giving it a try. Question, isn't opening the screws 1/2 turn further out enrichening the blend? From your response I would guess it's giving it more air...Thanks
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Old 06-08-2010, 10:35 AM   #4
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Opening the air screws should lean the low rpm mixture if I recall correctly. I had a 1981 CB900C at one time, but never messed with the carbs. Nice running bike. I hope you get yours fixed up.
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Old 06-08-2010, 10:17 PM   #5
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One Rule of Thumb is any screw that's upstream of the centerline of the carb-needle is fuel - going out richens. Any screw that's downstream of the centerline is an air-bleed - going out leans.

This isn't a 100% Rule, BTW.
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Old 06-10-2010, 12:23 PM   #6
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I thought it was exactly the opposite.

Really.
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Old 06-10-2010, 01:21 PM   #7
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Quote:
Originally Posted by pplassm View Post
I thought it was exactly the opposite.

Really.
Perhaps I'm mis-reading the the chicken-scratches I penned onto my thumb. I DO know that of the carb'd bikes I have currently in my stable - all of them have mixture-screws ahead of the centerline (i.e. toward the airbox), and those all control fuel-flow.

This is from mem'ry - as I haven't had to diddle with a carb for a couple of years, I could be mis-remembering the rule. A small amount of experimentation (or perusing the bike's Factory Manual) will confirm what function your particular metering-screw performs.
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Last edited by The_AirHawk : 06-10-2010 at 01:26 PM.
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