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Carb Adjustment


CB750 Member
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Hudson, New York
1980 CB750F
Idle with choke engaged sits kinda high at around 3000rpm. Disengage the choke and the bike just cuts out.... so do I lower the fast idle speed or do I simply increase the idle? And in increasing the idle, do I adjust the pilot screws or just the black knob beneath the carbs?

If it won't idle but revs high with the choke, you likely have a pilot jet issue.
Brett is correct. The passages that go from the air screws to the throttle body are super small. They get gummed up just a little no gas get thru, no idle.

That being said I’d start with the black knob. Turn it in maybe 1/8 turn see if it doesn’t cut out. If it does repeat and restart till you’ve gone maybe half a turn in, in 1/8 a turn increments.

If no change, go back to where knob was when you started. The 1/2 turn.
Then try turning the knob opposite direction 1/8 increments, restarting each time if it dies.

No change, pull the carbs, guitar string and clean passages that lead from air screw to throttle body. Make sure to count turns in to bottom out air screws so you know how far they go back in when reassembly.