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Carburetors New member query

Old Shep

CB750 Member
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Derby England
I have never rebuilt a four cylinder bike before, I have only done singles and twins, so I thought it was time for a four.
I have just purchased a 1980 CB750K dohc, i say its a 1980 but the engine number has been messed about above with so could be later, its had 22 owners so is in a bad state with bodges all over it.
I have done some work and restoration and it runs well until its gets hot then the tick over is erratic and then cuts out, also the revs are slow to run down.
All the books and manuals say it should have VB42A carburetors this has VB52A(no accelerator pump) with 68 & 102 jets, would welcome comments and thoughts on these carburetors being fitted
 
Are there any tips for fixing erratic tick over, the revs bonce around and cut out when the engine is hot and when you rev up are slow to run down.
 
Compression test to make sure you aren't wasting your time. Nobody ever sets the valves and then they burn, that will never idle correctly.

Sync the carbs, if idles are set too far up on one or two the idle hangs high to drop slowly. Same with a vacuum leak at the boots ahead of carbs, they typically crack with heat to leak. Leaking aircuts are vac leaks too.

Mixture screws need to set all the same, if they won't then carbs have internal clogging issues.
 
The Engine as been re-bored and new pistons and rings fitted. The carburetor boots are new, sprayed them with carburetor cleaner and all seem to be leak free.
I have only bench sync the carburetors using drills in the butterfly's

What i have found - The air valve cover on the side of number 1 carburetor is that the very small hole at the bottom of the hole were the O-ring fits appears blocked with metal. I have ordered a new valve kit. The price in England from David Silver spares is £119 pounds which seems ridiculously high, ordered a set from Taiwan at £12 but can take 3 weeks for delivery. I do not if this is cause until i fit the new parts.

I will then sync the carburetors with the vacuum gauges. Thanks for the information its much appreciated.
 
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