Topic: Carburetor interchange?

[ScottM]

Hi,

This is my first post on here. I searched around but couldn't find a definite answer.  I recently picked up a 1976 cb750f in need of a lot of work. My first order of business is to clean and rebuild the carburetors. When I pulled the float bowls off last night they were packed full of white powder and the aluminum was severely pitted, there were even some holes in a few spots. So now I'm looking for a set on ebay to rebuild for my bike. Do the carburetors I purchase have to be from a 76 cb750f or will any 70-76 carburetor work. Is the main difference between the years and the models (K1, K2, F...etc) the jetting? I know in 77 they went to a different style. Thanks for any and all help. If any one is selling any I'm interested!

Scott

[ekpent]

Your carbs are considered the round top carbs for that year/model. Ideally its always nice to stick with the same year/model parts but you should be able to make 71-76 K model carbs work if setup correctly. Are you running stock exhaust and airbox ?  If not you need to tune anyways for whatever configuration you have..

[TwoTired]

If you are keeping the stock air box and exhaust, you will want the same as what you have.  If you are changing the induction and and exhaust pieces, you will have the re-jet/re-tune whatever you put on.

Your carbs are stamped on the outside with the stock internal set up number. (if you are confident they weren't meddled with since new).  Ideally you would then get the same setup for minimum fiddling with after installation.

According to the chart below, your carbs S/B; 086A.  You could also modify the internals of any casting stamp you wish by fitting the 086A internal bits/settings to them.

http://www.motorcycleproject.com/motorcycle/text/specs.html

[bjbuchanan]

Like ekpent said, any round top will work. Matching numbers is nice but all the important drilled passages are the same for the round top carbs and they all will pop in to place just like your old set. This broadens your choices a lot

Generic sizing for most 750's is 110 main, 40 pilot, middle needle position, 1 turn out for airscrew. That will get you 90% of the way there for probably any of our bikes if you have an airbox and just about any exhaust out there

[ScottM]

The bike has some kind of aftermarket 4 into 1 exhaust. I'm planning to use the stock air box. I purchased the rebuild kit for the 76 cb750f carbs that I plan to use on the carburetors that I purchase. So the main difference it sounds like are the jets, meaning my rebuild kits will work with any carb I get within that year range. Sounds simple enough!

[TwoTired]

The bike has some kind of aftermarket 4 into 1 exhaust. I'm planning to use the stock air box. I purchased the rebuild kit for the 76 cb750f carbs that I plan to use on the carburetors that I purchase. So the main difference it sounds like are the jets, meaning my rebuild kits will work with any carb I get within that year range. Sounds simple enough!

Be warned.  There are many noted discrepancies regarding aftermarket parts for these carbs.  Needle profiles are often different, as well as many machining tolerance allowances.  Usually, unexplained tuning foibles are cured by returning to stock dimension components.

[ScottM]

Hmm, ok. I found a set of 76 cb750f carbs on ebay. I guess those will be the ones I bid on. Thanks.

[harisuluv]

Agreed, be warned again, a lot of the rebuilds that I do involve UN-doing rebuild kits.  After that things work again.  At the very least don't use the needles.  Or the air screws.  And depending on what kit you get... the other components.

[ScottM]

Thanks for the heads up harisuluv. What are the carb rebuild kits good for then. Gaskets only? It would seem that the jets and all of the other components could just be cleaned thoroughly. This is the kit that I bought from vintagecb750.com http://www.vintagecb750.com/parts/480/48-1909.jpg . Part number is 48-1909

[Cqyqte]

I didn't see an indication that you were restoring the bike to period correctness.  If that's the case and if you are planning to run pods and a custom exhaust system different than stock, then you might consider searching for a set of '77 carbs as they are equipped with accelerator pumps which make tuning much easier than the period correct units.

I did just this with my 1975 and have never had an issue with air filter or exhaust when tuning, just my humble opinion though.