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« Last post by Grabcon on July 27, 2025, 09:00:00 AM »
Okay I wanted to stay out of this but here goes since you mentioned you were pulling the carbs again. I have owned a half a dozen or so of these CB750 SOHC bikes. Ethanol fuel and the pot metal of these carbs don't work well if sitting for long periods, the carbs will corrode.
First the number 1 carb always has issues if gas is left in the bowl. It is the left carb sitting on the bike. I would always run my carbs dry when I knew it was going to sit for any time.
When pulling the carbs apart lay the parts out in order for each carb.
Make sure you pull and clean the emulsion tubes. Welding tip cleaners work well but be careful as they are files and can remove matierial and make the holes larger.
Make sure all main jets are the same size. This is something I have found is that sometime folks get things mixed up.
If possible I would go back to standard OEM jetting, most likely 110 or 115. Also set the needles back to OEM original position. It is hard to chase a problem without a baseline.
If doing parts replacement only use keihin carb parts. No aftermarket stuff.
I know this is a custom bike but those types of air filters are known to be Performace inhibitors, the factory air box is best.
If you set the needles back to OEM spec you will need to sync the carbs and will need a 4 gauge setup. In those carbs there is no baseline carb so syncing takes time. Set all air screws the same number of turns out to factory spec. and then adjust the slides first. It is a therapy session.
And the comments about a single carb forget it. One would need a massive carb for the airflow requirements on that engine.
That is my 2 cents. You may want to do a valve check, points and timing check. If a aftermarket points plate was used there Amy not be enough movement to rotate for proper timing. A static timing setup is fine to get running, once running use a timing light.
Note: I was doing some digging and this was my carb setup for a CB750 with an 836cc big bore and a Web performance cam. (Set for 6500’ elevation 105 main jets, 38 slow jets and
needle valves set to second ring from top)
Best of luck
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