750cmf Carter AFB & Edelbrock Perf. carburator tune up

[Finn Riv]

What size jets and rods is best for stock 401? I just chance used 750cmf carb to my Riviera, old was only 600cmf.

Edited May 5, 2023 by Finn Riv (see edit history)

[TexRiv_63]

600 cfm probably better for a stock 401.

 

[Finn Riv]

Original Q-Jet is 725cfm? I read from nailheadbuick.com that nailheads like big carburators, there came that idea to try bigger.

 

[carbking]

The original Carter AFB used on the 401 in the early 1960's was approximately 650 CFM. Probably more than necessary, but due to the air valve design, a little over-carburetion is possible ON THE SECONDARY SIDE without giving issues.

 

The air valve in the Buick AFB's is significantly different than the Carter aftermarket high performance carbs.

 

If the 401 is basically stock, the original Carter will probably run BETTER than an aftermarket 750.

 

If you have a 400 new enough to have came with a Q-Jet, it would have been 750 CFM, BUT THAT IS MISLEADING!

 

The Q-Jet 750 CFM is 150 fixed CFM (primary side), with a variable up to 600 CFM on the secondary side, depending on how much the engine wanted. Rarely, if ever, would a basically stock 400/401 use the full 750 CFM of the Q-Jet.

 

At 5000 RPM, the engine at a VE of 100 percent (very doubtful on a street engine) would require 580 CFM.

 

If you really want to try the oversize carb, would suggest running 3 timed 0-60 runs, and compute an average. After installing the larger carb, do the same thing, and compare. You may get a surprise!

 

As far as calibrations are concerned: Unless you can find some one with the EXACT same tune of engine and the exact same carburetor that has been professionally tuned to the engine, ALWAYS build the aftermarket carb to the stock specifications for the CARB. By doing this, you have a repeatable base-line if things don't play the way you anticipate. Once running, you can attempt to tune the carb.

 

EDIT - since you are talking 750 CFM Carter and imitation, and Q-Jet, remember: the Q-Jet is a spread-bore, the Carter AFB is a square-bore. ALWAYS USE THE CORRECT MANIFOLD DESIGN FOR THE TYPE OF CARBURETOR YOU WILL BE RUNNING. Carburetor adapters are for snaaaaaakkkkkeee-oil salesmen

 

Jon.

Edited May 4, 2023 by carbking (see edit history)

[RivNut]

You’ll need a 66 Riviera only intake manifold to run the 750 Qjet. Adapters narrow down the venturi size so  you negate the effects of a larger CFM carburetor

[BulldogDriver]

Also don’t try to use an adapter for the original air cleaner. There is almost a 50% reduction in area between the 5 1/8” Edelbrock and the original carburetor cover/top. The adapter will interfere with the Air Fuel Ratios. I am tuning my installation using an AFR gauge and can vouch that it caused a significant lean condition at the circuit transition.
 

I modified my original air cleaner to cit the new carb and the lean problem has improved significantly. 

 

I had a metal shop roll a 1/8” thick piece of steel to 5 1/8” ID, weld it to the air cleaner and cut the opening to the new size. The new piece had to be 1 1/2” high to clear the carb and keep the correct height as originally on the car.
 

I also added 2 oval openings in the rear of the cleaner to let the carb breathe better. If you look at the snorkel on the cleaner it is somewhat restricted. 
 

Ray

[Finn Riv]

Thanks for replies. Both carbs are Edelbrocks performer series (old name Carter AFB), first one was 1405 600cmf and this 750cmf is 1407. These carbs don’t need adapters.

[drhach]

I installed an O2 sensor for tuning my aftermarket (Edelbrock) carbs. It saved a ton of guess work. 

[Finn Riv]

There is big difference between those carbs. Now my Riv burn rubber when push pedal to floor😀

[XframeFX]

On 5/4/2023 at 5:27 PM, BulldogDriver said:

I modified my original air cleaner to cit the new carb and the lean problem has improved significantly. 

aaah, finally see the modified pancake air cleaner! Your image explains a lot. 1.5" riser eh? What about on a Rochester Q-Jet?

 

On 5/4/2023 at 5:27 PM, BulldogDriver said:

There is almost a 50% reduction in area between the 5 1/8” Edelbrock and the original carburetor cover/top.

I've abandoned the idea of switching to the smaller airhorn on an Edelbrock 1407 750 CFM to fit the pancake air cleaner. I'm holding out for a spreadbore setup topped with Ray's modified air cleaner.

 

19 hours ago, Finn Riv said:

There is big difference between those carbs. Now my Riv burn rubber when push pedal to floor

Good to share "Big Difference" going to a 750 CFM carb and stating the displacement of your nailhead as a 401. TX!

 

[BulldogDriver]

If you are determined in using an Edelbrock look at the Martin’s website for more info on using them. 
http://centervilleautorepair.com
 

I don’t think the difference between a Quad and AFB would warrant the extra expense. I had a chance to get a Quad intake but passed. The height difference between the 2 carbs would have to be considered for modifying the air cleaner. I originally thought 3/4” was enough to do the job but clearances on the Edelbrock dictated the 1 1/2”. The air cleaner is now just below the original height.

 

Now if some is determined to use the adapter for the original hole size on the air cleaner, I will give it to you so you don’t spend 50 bucks for one. Just pay the shipping.

 

Ray