i got a question i have a holley 750 singel pumper vaccum secondarys.i wanna down size the cfm a littel with out buying a new carb. now what whould i be abel to do to decrease the cfm rating on the carb thank you  

I think you're taking the wrong approach if you think this carb is too large for your engine. The flow rating has nothing to do with physical size of the opening available to the engine has nothing to do with the velocity of the air entering the engine, these are the important aspects to fuel 'quality,' and fuel quality is all that matters to an engine.

Lowering the CFM rating would entail RESTRICTING the carb in some way that is unnecessary. When the better way would be improving fuel atomization so the same 750 CFM carb can be utilized by the engine well. Then, the extra CFM that some people may wrongly think that would make the engine seem "over-carbed" would actually be working to your advantage by adding top-end power by posing as less of a restriction to the induction tract.

But, this is a tricky business, improving the booster signal and actually measuring the improvements. I suggest studying this...
"Booster Science" by David Vizard

This article explains some modifications that can be done to improve booster signal that won't require scientific equipment to measure the gain.

The fact that you're dealing with a vacuum secondary carb is also working in favor as the engine is given what it needs, so to speak. Specifications of the engine and more specific details about the carburetor may help rectify the situation.  

GibTG had an excellent response and I'll back it up. One thing you may try as a fast and simple benifit would be to get a spring kit for the vac. secondary. If this was a used carb there is no telling what spring is in it. If the spring is to light for your motor the secondarys will come on to soon and fast. This will give you the feel of a to large carb and more times than not when you floor it smoke will come out the tailpipe. Start with the heavy spring first and work down to a lighter one until you find the right one. Spring kits cost about $15. Remember Holley carbs on oem cars and trucks had a very stiff spring from the factory. Most vac. Holleys on factory cars never fully got the secondaires open all the way.