carb size something new.

slowsuki · #1 (**permalink**) 07-02-2008, 10:08 PM

have a 400 ford that is mild built was running a carter afb 750 but it was to big so i got a carter afb 625 cfm. what i want to know is i only turn 4500 rpms wich by the formula says a 500 cfm carb is all i need. if this is true what would happen if i closed my choke part way and then retuned it by changing jets and rods as needed. would this be the same as buying a 500 cfm carb or am i thinking to far out of the box. i do know with out flowing carb it will be hard to know exact cfm but i can experment with that. if it will work i was going to by cal kit for 500 cfm carb and set it up as close as i can to stock specs and then play with the choke. reason for this is i have talked to couple people that said they realy liked the small carb on there engines.

compressorguy · #2 (**permalink**) 07-03-2008, 01:13 AM

If you close the choke, you are putting more vacuum in the venturi and it will pull more fuel out of the low speed circuit in the carb. The 625 should be just fine for a 400. Just be sure it is jetted right. Otherwise I'd say you are better off getting a smaller carb.

monsterbaby · #3 (**permalink**) 07-03-2008, 01:25 AM

remember the secondaries on an AFB are vacuum operated so the carb automatically adjusts to a certain extent by not opening the secondary carbs as much. It can't compensate for massive over carb but it does take care of minor differences. This is about the same as you closing the choke to restrict the air flow except the carb is actually setup for this way.

slowsuki · #4 (**permalink**) 07-03-2008, 01:35 AM

the secondaries are manual on afb carbs at least the ones i have used. the avs have a valve that can control when the sceondaries open but no easy way on the afb series carbs. i think i am going to try just to see what happens.

monsterbaby · #5 (**permalink**) 07-03-2008, 01:43 AM

actually I mispoke the actual secondary butterflies are mechanical but they use an "air valve" type of secondary (the plates above the secondaries) that work like your thinking the choke will. And those ARE vacuum operated.

skunktruck · #6 (**permalink**) 08-20-2008, 03:18 PM

ok so i have to step in on this and ask. if I have a 351M edelbrock intake headers, RV cam would a 470 holley truck avenger be to small or would it be good. I think jumping up to a 670 CFM may be to much.
James

slowsuki · #7 (**permalink**) 08-20-2008, 04:19 PM

it all depends on what rpm you want to turn. my truck shifts at 4500 rpm and 500 cfm should be plenty. 351m should go to 5000 rpm with 500 cfm. 470 is not far so say 4800. i have read bad things about truck avenger carbs.if i was going to buy a holley style carb i would take a good look at barry grant carbs. they make a 525 cfm carb road demon jr. i do not or have i ever used eather carb just what i have read doing lots of reserch.