604 carb issue???

[fast14]

New to crate racing and trying to pinpoint a problem and find a solution. Running a quickfuel 750cfm carburetor converted for e-85 on gm604. On starts and restarts when I give it serious fuel in falls flat on its face for a second or so. Also when riding around on cautions the car sorta "chugs" or surges. Once we get up to speed on a fast track its okay, but it kills me on a stop and go track. Any ideas would be greatly appreciated. Thanks everyone!

[RacerX10]

LOL mine's doing the exact same thing. See adjacent thread with a few replies

I bet you are over-jetted too. (or will be told you need a carb specific for crate racing)

If you already have a carb specific for crate racing, you should probably call whoever built it.

[Egoracing]

Yup! Check your jets.

[Matt49]

If you're willing to do some testing and tuning, you can probably fix it without replacing the carb.
Off-idle stumbles are usually not at all related to jetting. Usually the problem is in the accelerator pump system. Based on your description of "starving and surging" it sounds like you are all out of fuel before you ever really get on the throttle on your starts/restarts. That being said, the accelerator pump nozzle is probably too big.
The AMOUNT of fuel delivered by the accelerator pump is determined by the size of the pump which you usually won't have to mess with. The timing of when and how fast the fuel is delivered is determined by the size of the nozzle, the cam, and the cam positioning.
Check the little screw that holds the cam and see what position it is in, if it is in position 1, try moving it back to position 2. This will delay the full delivery of the fuel from the accelerator pump. When you move from position 1 to position 2, you put the screw through the corresponding hole on both the arm and the lever. They are both marked.
You could also try a smaller nozzle. The size is stamped on the nozzle. Too big and you're going to dump all of your fuel too early and it will create a dead spot during off-idle acceleration. It sounds like that is what you've got. A smaller nozzle will stretch out the amount of time that it takes to deliver the pump squirt which might fill in your dead spot.
I would try the cam positioning first and if that helps, then try a smaller nozzle on the primary pump nozzle.

[Lazer-10]

Check your plugs if they look OK then go 2 sizes smaller in Primary and 1 or 2 sizes larger in secondary. Also put a slight delay in your secondary to start opening. Had this problem done this and it was OK after that.

[50j]

Call your carb builder. Nobody on this forum or any other knows how it's set up and where it needs to be jetted.

[atilgans1]

nice sharing. success in new jobs.

[atilgans]

Thank you for sharing. success in your studies.

[brad hibbard]

I would first start by making sure you actually have a 750 cfm carb

a 750 by industry standard has a 1.375 venturi size

there are 2 problems if your carb is bigger than a 750

1st) a crate engine does not run enough RPM nor move enough air to justify a carb larger than a 750----the booster signal will be weak with the large venturi and result in flat spots and "falls flat on its face"

2nd) some sanctioning bodies do not allow a carb larger than a 750
FASTRAK is one of those sanctions


I suggest contacting Zach Baker at Quick fuel and he will gladly help you through your dilema.

Zach Baker 270-739-0900 ext223
zach@quickfueltechnology.com

Brad Hibbard
www.race-1.com