Could use some help with a launch problem

I have a 350 Chevy with approx. 370hp, a 350 turbo trans. with a 3000 stall conv., and a 3.90 rear. It's ready for the track except for one problem. When I floor it off the line, the engine hesitates or bogs once, then picks up and runs fine. I don't have a lot of experience diagnosing engine problems, so I'd like to have a good idea of what's causing the problem before I start changing things. Like I said, it runs fine except for the one hesitation when I floor it at launch, so any opinions as to the cause and cure for the problem would be appreciated.

 

What size carb and brand? Also cam specs and Ignition system you are running.

 

It sounds like the accelerator pump (s) may not be putting out enough of a shot when you punch it.

What are you running for a carb or carbs?

 

accelerator pump?

I was typing too slow. Check the linkage on the accelerator pump, and work it by hand to see it's functioning.

 

why not? What carb? Q-jet? They don't call them Quadra-bogs for nothing. How about cam and intake? They all kind of need to be matched up to work well together. If you have a big cam and a dual plane intake that may be a part of it.

 

We had that happen once. Too big a carb for the engine.
Easy cure- launch at 5,000 rpm, no bog.

 

I have a 670 Holly carb, Vortec heads w/5.2 roller rocker arms, Edelbrock Air gap intake, and a 460 lift hydro. cam.

 

Tire size?

 

well are you leaving at idle, foot brake, trans brake, line lock, if your leaving off idle e.pump problems ,not enough or too much to fast, if your leaving the other ways (foot trans lock) you should be leaving in the power range of your engine thats the theory behind that let me know a little more so i can help you

 

I have 28.7"/8.5"/15" Cheater Slicks. I've been using my foot brake and launching at about 2000 rpm.

 

Vacuum or mecanical secondary? May need a stiffer spring in the secondary or the check ball may be missing if it's vacuum. May be timing related as well. Try moving the advance over to manifold vacuum if it's not there already.
Jeff

 

Depending on what you are stalling up to, it sounds like your running alittle on the lean side, If you have a va*** secondary you need to check what spring you have. Also check your advance on the distributor you might have to strong of a spring in their, you might not be pulling enough va***

 

I've been using the medium stiffness (silver) secondary spring in the carb. I have a Mallory Unilite distributor.

 

You need to change your engine to Pontiac!! LOL! kidding.
http://www.youtube.com/watch?v=Xuq-3a0AHEU&feature=channel_page
My guess is lack of fuel.

 

Try holding say 2200rpm with the trans in neutral. Is the engine running smoothly?

 

A .460 lift is the duration around 220 @.050, do you know the specs of the cam? you might not be pulling enough va***, you would have to put a softer spring for the secondary and again what RPM do you stall up to?

 

IF you are running a Holley double pumper, it sounds as if the accelerator pump is not timed or adjusted properly. The holley carb book can explain the adjustment in details--- the pump needs to deliver a good shot of gas that is delivered when the you stomp the gas. The adjustment is made by changing pump cams and adjusting the pump arm adjustment. Holley sells the pump cams. They are easy to install; only 1 little screw holds them in place on the outside of the carb. They are hard to see if you don't know where to look. They are made of plastic and coded by being different colors and have numbers on them. The cams are shaped differently so that the pump shot is delivered at different rpms. IF you are running a carb with vacuum secondaries, you probably need a heavier spring in the vacuum diaphram. I highly recommend that you purchase a good carb book and study it.

 

Thanks for the reply. I have a Holly 670 carb with a va*** secondary. I've been using the medium stiffness secondary spring, but I have the heavier stiffness (black) spring that I will try. I haven't tried the heavier spring because I didn't think it would have an effect until I got to higher speeds, not off the line, but it sounds like I may be wrong because several people in this thread have mentioned trying the heavier spring. I do have a Holly carb manual and from reading it, I think my 670 is the right size for my setup, but I didn't find anything that addressed the problem I'm having. Their was a question about what the spects were on the cam I'm using. It's a Hydraulic flat tappet, Lift .450"I, .460"E - Duration @0.050": 222 deg. I, 222 deg. E. I have a B&M 2800 to 3200 stall converter, I'm not sure what my exact stall speed is. Since I live in a residential area, most of my testing will have to be done at the track, which is only a few miles from my home.

 

I wonder that your vacuum would be holding the brake at 2000 rpm. The hesitation may be the power valve. You are at a steady 2000 rpm the vacuum is high and the power valve would be closed. Going to a higher number power valve may help fill the hole. Put a vacuum gauge on and see what it reads when it stumbles, it may help. You go to high it will open at idle and make it run to rich. Get 2 or three and work your way up until it works.

What gears you running in the rear end?

 

I'm running 3.90 gears. I have 28 3/4"X 8 1/2" cheater slicks on 15" rims, and the car weighs 3700 lbs.

 

Where are you at in Md ?

 

try this and it's free borrow another carb foot brake it see what it does if the carb is close to the same and it doesn't fall on it's face copy that and your in the ballpark
falling over when you leave you're not even close to were you need to be adjustment wise
OR carb wise If your leaving real hard on a street carb you could be slossing the bowls

it's your carb plain and simple
try leaving at idle that way you can flash the converter and you can leave a little higher rpm too

 

Change to a Hollow squirter screw (Optional choice at this point)and #32 squirter noozle for the accelarator pump and put the accelator pump cam in #2 position.

 

Actually I would not recommend the carb you have for what you are doing, you would be better off with a Double Pump style carb. That 670 va*** is more of cruising carb than performance. If you want to still run this carb what is your va*** reading at idle with the nice and warm.

 

Oops I screwed up put a stiffer spring in the secondary not a softer one.

 

I appreciate all the advise, and I'll do what I can to try all the possible solutions that have been suggested. The problem seems to be narrowed down to the carb, which is a big help. My car is mostly street driven, and I kept that in mind when I was setting it up, but since I live very close to the Maryland International Raceway in southern Maryland, I also wanted to be able to do some Brackett racing.

 

That carb will work fine. It just needs tuning.

It sounds like you need a larger accellerator pump shot to cover the hole in the fuel curve when you stomp the gas. You could also have the wrong power valve and need one that opens sooner (higher number on it). What is your idle va***n? The power valve should be about 2 numbers lower. IIRC your carb comes with a 6.5 PV. If you idle at 14" of va***n, go with a 12 PV.

If you PV is already setup, then you are looking at a fatter accellerator pump shot. IIRC that carb has a 28 squirter in it. I would move up to something like a 32 and see if that helps.

 

Thanks for the reply. I don't know what the idle va*** is, but I'll find out and take your advise from there. It's also been suggested that I try a stiffer secondary diaphragm spring?

 

I agree with changing the cam that governs the acc pump. to come in quicker