Hello all
I have been fighting a problem for two drag racing seasons and cannot figure it out. In a nut shell this is the problem. The car begins to launch very hard the wheel almost come off the ground and as quick as this happen the car falls flat on its face and it takes about a second to a second and a half before it will begin to go aging. Some interesting points, it only does it when the car hooks and I never take my foot of the gas pedal. I have done ever thing that I know to do. Please if anybody has had this problem or has any ideas let me know what you think. The key note to remember is it only does this at the drag strip when the car launches good.

 

Provide some details about your setup....

 

Sounds like a lean bog but without any information, nothing can be suggested.

 

It is a 1988 camaro with a 350 40 over 750 holley, msd ignition system tci street fighter tranny, 411 gears, holley blue fuel pump. I have tried different adjustments on the carb and get the same thing when it hooks hard. I can punch it on the highway and it never falls but it is also just spinning the wheels. In the shop I can the hit throttle while the hood and there is no skip. Let me know what other info would be helpful. I have put jet extenders in the carb also

 

that would be while UNDER THE hood

 

I'm kind of thinking the same thing. What size jets are in the carb now? I'm also curious how big a cam are you running?

Dan

 

squirter size? Mine done that and I plugged the power valve and changed the primary squirter to a larger one it worked quite well

 

x2

 

What camshaft do you have? Those street fighter converters only stall to 2000 (if you're lucky). Sounds to me like its just taking a while for the motor to get up in the powerband.

 

Is/was the carb new or did you buy it used? Many times the 600-750 Holley carbs are sold off for a reason. They're worn out and not worth rebuilding and someone else buys them thinking they got a good deal. Something simple like a new carb may fix a lot of problems.

There are many different carb adjustments which may help or hurt the situation. As already mentioned, squirter size may be wrong. When you go to WOT you are putting the engine into an instant lean condition while under load. The accelerator pump (normally 30cc) gives the engine an extra shot of fuel to compensate for this lean condition but there's more going on than just shooting fuel in.

The accelerator pump is controlled by a cam. Different cams installed in different positions control how much fuel is shot in and how much at different throttle positions. The fuel then needs to go through the squirters. Different sized squirters allow different amounts of fuel to pass through. They're all tuning aids and what works well on one car may not work on another.

After running a mechanical injection system for a couple of years now, I'll never go back to a carb for a race engine.

 

get jet extensions. If you are leaving super hard, the fuel will slosh back in the bowls and u will starve the motor. or you need to raise your bowl levels

 

Also besides the jet extensions make sure you have the anti slosh whistle installed on the vents. You may also try a piece of hose from one vent to the other with some cut outs in the middle.And check your float levels.

 

Thanks for all the info, I will try to answer all of the questions sorry if I miss one.
Jet size are 70 in the primary and 74 in the secondary- I have had an assortment of sizes in there but this was what was in there for the last run
I will have to get the cam specs over the weekend. It was a custom cam from crane cams
Last weekend I ran 37 squirter and it bogged bad coming off the line(but it ran) so I changed it to a 35 and the first run it spun the tires and the second run it fail on its face like usually. The only difference between the first and second run was the rpm I left at. The one that the tires spun at was 2000 and the one it fail was from a dead idle. I did not get a chance to make another pass.
Power valve is a 10.5 changed from a 6.5 because of what the vacuum reading at idle was. 87draggta what is the reasoning of plugging the power valve. As for the stall converter it is a 3400 stall
As for the carb it was bought used but I knew the guy that had the carb. The accelerator pump is a 50cc and as for the accelerator pump cam it is the green one in the first hole. I also have jet extensions and fuel bowl baffles. The fuel bowl level is set so that when you take the sight plug out fuel will just start to trickle out of the hole. This last weekend was the first time that I got it to run without falling but It bogged pretty bad coming of the line and my 60 foot to was real poor. Could this have anything to do with my fuel tank or is everyone in agreement that it is carb settings. Last weekend was the first time I started changing things in the carb and the first time I had a complete run although it was slow. Now did it run because of the carb settings or because it did not hook as hard so the exciting problem did not accrue thanks for the help.

 

are you running on slicks? no reason for a car with such a low stall to be spinning the tires. Also if you're running the power valve in the front and have a larger cam in the motor I'd be putting in a 6.5 power valve or smaller, not sure how you got the 10.5 valve in there?

Jetting should be more like a 70/78 if you have the power valve still installed.

 

Yes I’m running slicks. The reason for the 10.5 power valve is because in the holley tech book I have it tells you to set you power valve at 1 ½ to 2 inches below the vacuum at idle. My vacuum was 13. The cam is not large it is just custom built for my car and stall

 

what size slicks and what pressure? with such a mild combo there shouldn't be tire spin.

 

Power valves are good for street driving but not that good for drag racing. They can richen or lean the fuel with little control. Typically when you remove the powervalve, you install a powervalve plug and jet up 8 jet sizes in the primary and secondary.

When tuning, ignore the secondary jets. You play with the primary jets until you get your best MPH then start adjusting the secondary jets to improve the MPH even more. I doubt the jetting is causing a bog off the line.

As mentioned above, run a rubber hose from one bowl vent to the other and cut a notch at the top of the hose loop. This allows the bowls to still vent and prevents fuel from sloshing out the vents on a hard launch.

Put the 6.5 PV back in or drop to a 4.5 if you want to use a power valve.

You could also be dumping in too much fuel. The 50cc accelerator pump is huge for that setup. The 30cc pump would be better. When you go to WOT off the line, you actually may be flooding the engine with so much fuel. 50cc pumps are good when running alcohol since they need so much more fuel.

 

What ET/MPH are you currently running?

What's the advertised (total) duration of the camshaft?

 

I'm sure you meant plug the primary power valve and jet up 8 sizes in the front, the rear typically has a plug already and wouldn't need any jetting differences. You would want all 4 jets to be the same size after finding the sweet spot.

Also just want to add that if this car is street driven then the advice to plug the powervalve is poor, you need the powervalve for street driving as I'm guessing with such a mild combo it is street driven often.

 

i don't drive my car on the street and the power valve was just something extra that could go wrong so I plugged it and went up a few jet sizes.

 

i wasn't talking to you 87draggta the above post was meant for the thread starter.

 

I know, I was responding to a question he asked me several posts up about why I blocked off my pv, sorry for the confusion.