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Old 08-08-2010, 09:13 PM   #1
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Default 81 Honda CB 750 not running right.

Picked this bike up and was told that the carbs had been gone through. tank and other parts were off the bike. Used a bottle as a temp gas tank changed the fluids and believe it or not the bike started up but had exhaust leak, fixed that and replaced the regulator rectifier and wanted to take it out for a test run. Bike wont rev over 4500 to 5k and seems to fall flat at that rpm, Replaced all carb insulators same problem. I do get an occasional backfire out the pipes and the idle has been rough to set. Need to get some Idea where to start to diagnose the problem. All help is appreciated thanks. If you need more info let me know.
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Old 08-09-2010, 05:26 AM   #2
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Is the choke opening up fully?

Could that one of the myriad of hoses in the carb system wasn't re-connected properly. Or that the guy who "went through" the carbs screwed something up.
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Old 08-10-2010, 05:20 PM   #3
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Inspect the needles for wear and also check the airfuel mixture screws. Sounds like it's in the carbs.
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Old 08-10-2010, 05:28 PM   #4
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Originally Posted by MOKE1K View Post
Inspect the needles for wear and also check the airfuel mixture screws. Sounds like it's in the carbs.
Not just the needles - look at the part of the Main Jet that the needles glide-through. Now that Moke jogged my memory, a friend that had a similar problem with a VFR 750 had excessive slide-wear and the needle seats were worn into ovals. IIRC he solved it by buying a "new" set of used carbs off fleabay, slapped 'em in, then he sold the bike - as the work and parts to "fix" the original carbs would be about 3X the bike's value.

On some carbs, the needle-guide is non-servicable; i.e. you throw them away when worn too far. I have no idea if this is such on the CB750.
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