original main jet size?


CB750 Enthusiast
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Chattanooga Tennessee
I have recently installed a 4 into 1 exhaust and pod filters on my 1972 CB750 project and i have read in several different places (including Carpy's site) that with those mods you need to go up two jet sizes. My question is, how do i tell what the original jet size is or was? i pulled out the old jets that have obviously been tampered with and they are 135's! is there any way this is the stock size? they seem too big for me, but ive been wrong before. are all the close years the same main jets? or did the sizes vary alot year to year?

The reason im concerned is there is a relatively good chance these carbs are not from a 1972 and I cant find any way to identify what year they are or what the correct "two sizes up" jets would be for my 72.

Thanks for any help



Staff member
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Black Hawk SD
You still need to jet so it runs good with your modifications. There are too many variables to say "you need this size" type of jetting. Different pod filters will require different jetting. Exhaust type and construction has a large affect on jetting. Two different brand of 4 to 1 exhaust could take radically different jets. two sizes will probably get you close but you will still need to check. You will need to check you pilot jets and more than likely also change the needle position. Jetting changed in different years as the exhaust got more restricted every year. Is your motor stock? I run 135 in my 836 with ported head, 63b cam and 4 to 1 exhaust. The only time I had to run 130 in a stock motor was with short drag pipes (drag pipes on a stock 750 are fairly inefficient and you actually loose power over stock). I would start with a 120 and see how it runs then go from there but then again I have a set of all the sizes available for the 750 for tuning purposes.