Topic: Suggestions on rejetting 75 CB750

[stickman]

Good Morning All,  I recently picked up a 1975 CB750 stock bike that was running poorly because the carbs needed cleaning and possibly rejetting after the PO had installed 4-2 chrome turn outs.  So right now, I have cleaned the carbs and the mains are 120's.  With the stock air box, I am suspicious that a 120 may be too large for these 4-2 turn outs. 

Is there anyone on this forum who has gone thru this rejetting and can tell me what main jet size I should be running with a stock set up, stock air box and 4-2 turn outs added?

Its been some time since I have done this but going from stock 105 mains to 120's seems excessive.  Can you get me close? 

Also, where can I buy drill bits these days small enough to drill out stock 105 jets to increase them to 110, 115, etc?  Any assistance in pointing me in the right direction will be much appreciated.  Thanks, Ed

[stickman]

Sorry guys, should have stated this is a CB750 K5. 

[Johnny5]

Have you ran the bike after cleaning the carbs? Has it had a full tuneup? If so and it still runs poorly, you can always try a 110 main.

[stickman]

yes, after cleaning the carbs, they ran lean with the 105 jets installed.  But rather than try other jet sizes incrementally, I was hoping to find someone who had the same set up and could share what yet size worked for him.  I am also in the process of adjusting valves, cleaning and setting points and checking timing as I feel I cannot accurately rejet the carbs without knowing all else is right.  Thanks for the input.  Ed

[Johnny5]

Is it running lean wide open? Or do you have flat spots in the middle of your rpms? Wide open means a main change, middle - the needle.

Usually a pipe change wont effect too much if the airbox is all stock. Bumping up to 110 should help with that. I'd try your needle in the middle slot first, then 2nd from the richest.

[stickman]

Thanks Johnny5,  I will give it a try.  Ed