Topic: 750 carbs on 550

[cwchan]

Hi,

I read that there is some performance increase to putting 750 carbs on a 550.  Is there a particular year that is best?  are there fitment issues (with 550 airbox or with uni-pods)?

can anyone attest or make comments?

[CBGhia]

Unless you have some serious internal mods, I think it would be pointless. 
If you are running an open exhaust and a free flowing air filter you may have to bump your stock main jets up a bit, but that should be it. 
Do you have anything else done internally (605cc kit, head work, cam)?

[cwchan]

The engine is currently stock but i purchased a 650 cam and plan on installing it this winter.   Currently has pods on it.

[TwoTired]

The venturi size would be 28mm for 750 carb vs. the cb550's 22mm.
In theory, it should give a a bit of performance increase at or above red line RPMs.  For normal street speeds, the throttle response will be poor and the carb will have poor atomization do to low throat velocities.
Of course, operating at or above Red line on a consistent basis will shorten engine life.  But, why put on bigger carbs if you aren't going to use them to best advantage?

The carbs are physically bigger, so none of the fore (or aft) couplers will work.  The 550 intake runner and head ports are about 27mm. So, that and the valve dimensions will be the new choke points for induction.  A head porting job should help.  Still, you might get 11000-12000 RPM out of the induction set up if you stiffen the valve springs.

I've no idea what to tell you about carb jetting, except you will have to find new settings for the slide needles (possibly a custom needle profile) and jets for each operating range of the engine.

In theory, the throttle valve cutaway may also need changing to make transition from the pilot system to the throttle valve system give predictable response.  In this regard, maybe the CB750's PD carbs (77-78) with the accelerator pumps would make thing a bit easier.

Sounds like quite project to learn from.

Cheers,