750 Holley too big? will less CFM be better

Myles

Myles

Apprentice
Jun 17, 2011
75
8
Hi Guys,

I want to ask if you guys have lost significant mph by trying a smaller cfm carburettor.

you guys have already helped me out, but i think my car doesnt like this carb.

my car will still hesitate, launching or in nutral, the holley 750 is full mechanical, iv tried every combo with squirters, nozzels, pump cams, etc.
best but not perfect with massive 42cc squirters, but then fouling plugs. carb now set back to stock. its almost new. same problem.

I have a mis matched set up, small cam, low compression but massive heads because i got them for almost nothing.

750 non vaccum, i feel like my combo doesnt want to pull or suck the air in hard enough

i want to bolt on other carbs just to see if smaller cfm helps, more importantly, maybe a vaccum carburettor.

theres some dirt cheap 450cfms for sale, i have the rebuild kits, vaccum, let me know what you guys think.

i believe a smaller, but more imporatly, a VACCUM carb could help both tuneup and my 60ft a touch, but i dont want to loose like 3 or 4+mph.


my other malibu had this issue with the 750 Q jet, 383cu.in, i tightened the rear flaps on the back (allenkey adjustment)

and car leaves crisp and hard (for whats in it). that ones 3000stall 4.11 26" tire.

thanks guys!

set up im trying to tune:

81 malibu
355 dish piston
dart iron eagel 64cc 215 runners
106lobe flat tappet 510lift 242/[email protected]
open plenum offenhouser old intake
(not very big)
1 inch carb spacer
vaccum is 10

750cfm Hp holley full mechanical
no accesories or belts
electric water and fan

8" 5000 stall
4.56 gears
28" slick

shift and trap around 6800rpm

other than that issue car is fun runs great

canada october cool
11.67 @ 113mph
1.57 60ft (best one)
 
Last edited:
39F

39F

Master Mechanic
Sep 9, 2019
273
43
Pittsburgh PA
I'd try a mechanical 650 Holley get rid of the carb spacer , definitely way too big of squirters ,having that big of a head is your biggest handicap , you'd be better off getting vortecs or double humps cheap and put them 215s on a 10-1 400 sbc ,what kind of timing you running?
 
superbon54

superbon54

Master Mechanic
Apr 15, 2014
330
93
Based on the gearing, the revs should come up quick, but the 750 still is large for what you’re running. 650 to 700 would be better, and mechanical secondaries are fine. How is it jetted? You could be masking improper jetting with the accelerator pump, especially with the mechanical secondaries. Always always always start with proper jetting when carb tuning. If jetting is off, nothing else will make it right. I learned that the hard way. Wide band AFR gauges make quick work of tuning too.
 
Myles

Myles

Apprentice
Jun 17, 2011
75
8
thank you all,

to answer everything, no i dont have a AFR gauge, timing is 36 total and runs best there. iv tried a new locked out HEI, and now new stock hei, all the same. not really affected by gears or stall, same response issue in nutreal

i know 42 squirters are ridiculous, its was a silly experiment but it gave a more positive result. it was the only thing that made it better. now back to stock (28 and 32 i think)

jetting is stock what it came with, a few old timer die hards at the track say my plugs look A1.


my Q jet car had the same same problem, i tighted those rear flaps to solve it. -not the best technical answer but 2 ways to look at that is i'm cutting the air, and/or making it richer in a funny way.

i feel like the extreme squirters on my holley (even though they are way wrong) had the same effect. i couldnt minimize air on a mechanical carb, so i mimicked it by adding more fuel and it got better.

low vaccum, low compression, big heads etc, a pro tuner in florida i know said my car isn't pulling the air well enough at low rpm. a smaller carb would be better.


not ruling anything out but nothing to loose by trying some other carbs.

2 jegs guys say 600 or 650 vac. but thats another 700$ to the door to throw at a problem.
some 450's here for sale cheap.. but those are really small?

it was just going to be a street car, but those huge heads are 8-10mph in the 1/4 more than the stuff i have laying around.

--have you guys lost or gained any numbers in you ET's or 60fts by trying smaller carbs that you've run before?

thanks again all
 
Last edited:
39F

39F

Master Mechanic
Sep 9, 2019
273
43
Pittsburgh PA
thank you all,

to answer, no i dont have a AFR gauge, timing is 36 total and runs best there. iv tried a new locked out HEI, and now new stock hei, all the same.

i know 42 squirters are ridiculous, its was a silly experiment but it gave a more positive result. it was the only thing that made it better. now back to stock (28 and 32 i think)

jetting is stock what it came with, a few old timer die hards at the track say my plugs look A1.


my Q jet car had the same same problem, i tighted those rear flaps to solve it. -not the best technical answer but 2 ways to look at that is i'm cutting the air, and/or making it richer in a funny way.

i feel like the extreme squirters on my holley (even though they are way wrong) had the same effect. i couldnt minimize air on a mechanical carb, so i mimicked it by adding more fuel and it got better.

low vaccum, low compression, big heads etc, a pro tuner in florida i know said my car isn't pulling the air well enough at low rpm. a smaller carb would be better.


anyways, nothing to loose by trying carb some carbs.
2 jegs guys say 600 or 650 vac. some 450's here for sale cheap.. but those are really small?


have you guys lost any numbers in you ET's by trying smaller carbs that you've run before?

thanks again all!
Start with checking the transfer slots on your 750 they could be getting drained that causes a nasty bog also 020-.060 open no more ,check your timing could want more
 
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fleming442

fleming442

Geezer
Dec 26, 2013
9,301
113
Start with checking the transfer slots on your 750 they could be getting drained that causes a nasty bog also 020-.060 open no more ,check your timing could want more
Good point! If the blades are cracked at idle, you're creating an over lean condition. Pull the carb, set the transfer slots, and start over with a vacuum gauge.
 
Myles

Myles

Apprentice
Jun 17, 2011
75
8
thank you guys,

blades are probably cracked because i lean into it. its brutally worse at of idle,

new set up and car/build. iv run it as much as 42 degrees, back safe at 36 because it wasnt faster or slower. had same issue.

this is the only clean pass, because i floored it slowly:

 

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