Stock carbs are 28mm. The basically come in three varieties: The very early carbs which had four individual cables and probably were a nightmare to synchronize. The majority (K1-K6, F1) were linked and had the choke lever at the carbs.; Latest carbs were jetted leaner (to comply with emission regulations) and had a accelerator pump to make up for the leaner mixture. Rumor has it that the late K6 models had 26mm carbs but I never was able to verify that.
For performance applications, I found the K1-K6, F1 carbs the easiest to work with, if you want to stay with stock. Some people bored them out to 30mm. The late carbs have pressed in pilot jets which cannot easily be changed.
If you are looking for more power, there are aftermarket carb banks from Keihin, Mikuni and Dell'Orto which fit the 750.
Depending on the engine's displacement, and the application here is a rule to the carb size:
|900cc or larger||33-34mm|
For roadracing (if you are ready to sacrifice bottom end for top power) you may use one notch larger carbs.
Dell'Orto of Italy made aftermarket carbs for the 750 Honda:
Dell'Orto PHBs were available in 30mm, 32mm and 34mm, but they are no longer produced.
The most prominent aftermarket carburetors are Keihin's CR. The older version had no idle stop as they were only designed for racing:
Old Keihin CR
These were mostly in 31mm.
New Keihin CR
These are the new (and current) CR carburetors:
These are available in 29mm, 31mm and 33mm. The rubber mounts from the '78 F and K models fit perfectly in case you have trouble finding the original Keihin rubbers.
Unfortunately, they have no adapters for vacuum gauges, so you have to help yourself:
Sandcast Keihin CR
And these are the carbs that started the whole CR family: Sandcast CRs, as they were included in the factory CR kits!
Mikuni SR Smoothbore
The latest development in carburetor technology, Mikuni's TMR32 also fit the 750:
Weber DCOE dual throat carburators, well known from cars, were also used on the CB 750 SOHC/4. Mats Larson of Sweden has adapted a set of those to his CB 750:
Solex PHH carbs, much like the Webers, were also available for the 750:
For carbs with velocity stacks or individual pod filters, 125 mains seem to be a good starting point.
Read the spark plugs
The air/fuel ratio is just right if the spark plugs have a nice light brown. To read the plugs, you kill the engine usind the kill switch after running a while with full open throttle. Then pull the plugs and loook at the color the insulator has.
The key to optimum performance is to get the air to fuel ratio right. A Lambda probe or Lambda tester measures the amount of oxygen in the exhaust gas and so indicates whether the mixture is too lean, too rich or just about right.
Why not go ahead and dump the whole carbs? The throttle valves and fuel injectors from a current 400cc four cylinder bike might have the right size for the 750, and an oxygen sensor is easily installed in the exhaust.
Porting the head
Please do not forget that larger carbs require porting the head. There is no use in putting big carbs on small ports.