Hi guys, I have a 4779 750cfm holley thats off a wild BB. I de-tuned it as much as I could but its still too fat at idle for my car. The idle air ports etc have all been hogged out and the throttle blades drilled. I have a 4777 650cfm here that I think is just a little too small for my car. Can I swap the body/throttle plate and end up with the 700cfm? What makes the 4778 700cfm?? it seems like they just mixed 750 and 650 stuff to get the 700, but what did they use?
Thanks for any input guys, if nothing else I will just try the 650 as is but im almost positive it wont be enough.
CoryM

 

You need to take that to a good tuner. There could have been several modifications done that you are unaware of. Many times it is necessary to enlarge the air bleeds to accomodate a BB. You should be able to darn near shut the fuel supply off with the idle adjustment screws. If you can not do that then fuel is perhaps entering the stream through a blown power valve. Or the butterflies are open enough to access the enrichment circuit. Many things to look for. The bottom line is, either pick up a good Holley book and do lots of reading, or take it to a tuner. The carb is not too big, just not tuned properly.

 

Hey Cory.

I talked to a friend of mine who is very good with holleys and had him reference his holley books. Here's what he came up with:

-600s and 650s have the same main bodies and different bases.
-700s and 750s have similar main bodies...the secondaries are the same but the primaries are smaller on the 700s.

These are the base plate bore sizes:
4777-1 1/4"
4778-1 15/16"
4779-1 3/8"

If you wanted to put a 650 main body on a 750 base you would have to flare the bottom of the 650 main body to fit the base plate.Not worth it IMO.

I would use the 650 on the street for now.It will give you better throttle response and driveability and be easier to tune on the street.Then slap on the 750 when you go racing on the dyno.but ultimately you won't notice too much difference until you really wind it up.

 

Great, thanks. Thats the info I was looking for. Seems like no one has holley books anymore . Guess I will try the 650 and hope it feeds it well enough.
Thanks again.
CoryM