7448 Stumble Questions

[Farva]

I have a '77 FJ40 and the engine was just rebuilt, 4.3L I6. The previous owner replaced the carb with a Holley 7448. I also rebuilt the carb with new pump and gaskets.
It idles fine, but when pressing the pedal it wants to die/stumble. I checked the pump arm adjustment per Holley instructions and still having the problem. I now am assuming it is the accelerator pump nozzle size but not sure to order a larger or smaller size.

When it warms up to temp it doesn't seem to stumble at all just when it is cold. Gas is flowing through the nozzle fine. The engine was bored out because of a bad piston does it needs more gas now from the accelerator pump?

Is there any other test I can do to determine the sizing I need (larger or smaller) before I start buying random nozzles I don't need? Thanks for any help.

[Gaz64]

If is OK when hot, and stumbles cold, it is lean.
Are you using a choke mechanism, manual or auto?
What color pump cam do you have? Factory is orange.
What pump nozzle? Factory is 31. Gary

[Farva]

@Gaz64: It's a manual choke which I never use and is always open. I'll have to check the cam later today. I'm guessing factory though. Nozzle is a 31. Maybe going to a 35 then would fix it? Been looking at the nozzles and none I've seen say they are for a 7448. Are they all interchangeable? Thanks for your reply!

[Gaz64]

A list 7448 is a model 2300, same as the primary side of a 4 barrel model 4160/4150.
All tuning parts interchange between a 2300 and a 4160/4150.
So a pump nozzle is only marked with it's size, e.g. 31.
You can buy a pump cam assortment:
https://www.holley.com/products/fuel...ts/parts/20-12

You may want a faster cam, a higher lift cam, and/or a larger nozzle.
35 is a big jump, I would number drill a 31 out to 32, then 33, etc.
When you find the size that causes a rich stumble, come back one size.

[Farva]

What's a good indication of a rich stumble?
Also, where is the best place to find the drill bits that small?

[Gaz64]

What's a good indication of a rich stumble?

A stumble or flat spot can be lean or rich.
A puff of black smoke from the exhaust is a rich stumble, no smoke points to a lean stumble.
Backfire or sneeze through the carb can also be lean. Gary

[Farva]

I installed a 35. The stumble isn't as bad and goes away before it's halfway warmed up. Fully warmed up it runs great. Only notice a little black smoke when revving it. Also smells a little rich. Would this indicate changing the cam or going to an even larger nozzle?

[Gaz64]

Depends on the rate of throttle opening. Is this stumble on gentle acceleration off idle, or a stab to say 1/3 throttle? Gary

[Farva]

More of a stab.

[Gaz64]

I'd come back to a 31, and a blue pump cam to start with.
Then possibly a 33 (with number drill 66).
Then up to 35, and possibly even 37.
Bear in mind any increase in pump circuit will decrease fuel economy.

Page 7 & 8 may be of help: https://documents.holley.com/199r7849-2rev1.pdf

Have you had the carb apart recently, or do you have a log of parts fitted?
Main jet, PVCR, PV, etc. Gary