new to the forum, looking for some advise\answers about a 1979 cb750


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northern va
afternoon, new to the old school bikes but not to riding. always wanted an old cafe setup so, in short i bought a 1979 cb 750 super sport and am looking to build it as a cafe racer. had to buy a motor for it though. the carb was left open and got water into the engine. valves locked up with rust so i ditched any engine repair and bought a 1980 cb 750 motor (one of the guys labeled as gurus said its an even swap) so now the question is will a 1979 cb 750 carb work fine with a 1980 cb750 motor? ive read some into it that there may be some differences between these two years in relation to the carbs (rebuild kits come with extra parts depending on design) and im wondering if i need to buy a matching year carb set or can stick with\rebuild what i have. the carbs off the 79 motor are in good shape. little help? Also im probably going to have a bunch of questions later so please be patient. thanks for any advice
The motors are identical. Any slight carb changes are due to exhaust system differences. If still using the F type 4-2 exhaust they will work perfectly and even without if you change maybe a couple parts.

CB750 K, C, L, F, and SC all use the same engine and the carbs interchange too with only slight jetting changes of maybe a couple numbers.
you were the one i was talking about. thank you, ive read a bit of your comments, very informative stuff. it came with a 4-1 exhaust which needs replace. when you say change of a couple numbers what do you mean? ive only ever had fuel injected yet i have the carb vacuum gauge tool on order
What brand of 4-1? I ask because a lot of them are eye candy only and do not work that well and if well baffled to be pretty quiet. A few headers can flow better and help the engine enough to need to jet them up a bit. The max I've seen stock engine is 75-115 but that is on a very tight engine with no leaks at the valves and a good header and pods. The valves tend to leak if not set and the carbs being CV type then open slower and the more jet you add the worse the response due to that. You can't overemphasize how much good engine condition has to do with the carb jetting. If the header is quiet you generally don't need any jet changes at all, Cv carbs tend to self jet a bit on their own.

The OEM carbs often move the second main jet around a couple numbers or so say 98 to 100 and seen some 102. The primary is usually 68. Primary is front jet and secondary is the jet in back. The numbers above are 75 primary and 115 secondary.
i apologize for my delay. working like crazy in the midst of this quarantine. the pipes that came off have a vance hines badge on the muffler. whether thats a full system or a welded on muffler im not too sure. in fact thinking about the condition they were in i dont know if i can reuse them with some thorough cleaning. the newer motor i had sent was reported to have good compression (150-160 over the 4 cylinders, and i intend to have it looked at when the build is complete) as far as carbs go ive read in previous posts two things. 1) stock air box and OEM setup on carb jets. 2) pods with different carb jets to adjust for the higher volume of air flow, the latter being the bane of existence and frowned upon by traditionalists (to which i fully understand) so advice on how to go about both options is appreciated.
at the moment im stuck on my seat. ive stripped the bike down to bits (all labeled for easy reassemble of course) and im at the part were if im to do any minor frame changes nows the time before i paint. (all the paint has been removed and thats taken alot longer than i anticipated) i wanted originally a brat seat. nice and flat in relation to the tank. but im looking at the shock mounts to the frame and to make it flat im going to have to relocate these mounts. not doing that. instead ive changed my mind on the seat to a more traditional cafe seat, with the hump in the back. so the question is who to buy a seat and frame hoop from to make this an easy part to tackle?
Once again, I thank you for your time, stay healthy out there.
If asking me then the wrong guy, I don't mod frames much and if I did I would do the work myself. I tend to lean on the engines a lot more than frames, I get the frame I want to begin with. It usually has the relevant parts with it.
Thanks for this thread, it is very helpful. I am in a similar predicament. I have a 79 cb750k that I bought with a cracked cylinder head (didn't realize this till I rebuilt the carbs and got it running and found oil pouring out). Came across an engine from a 1980 cb750c without carbs and was wondering about compatibility (79 carbs to 80 engine, 80 engine to 79 frame). Are there any ignition or charging system differences to be aware of? Or is this truly a bolt-on plug-n-play swap?