View Full Version : Carb for autocrossing?

09-28-2004, 12:44 PM
I'm trying to prep my 69 camaro for auto-x.
I have a 383 with a holley street avenger 750. Currently this carb will stall if I brake really hard or corner hard.

Does anyone have any recommendations for a carb for racing?????

09-28-2004, 06:39 PM
Try using jet extensions, they'll keep ya in the fuel when it sloshes for and aft. If that does'nt work you can try an off roading 4x4 type carb.....if all else fails, efi!

09-28-2004, 06:50 PM
Dont get extensions as they will lean you out on braking. I am not sure what the offroad carbs have? If you are running out of fuel on braking (stop the car hard and let it die, then look at the plugs) then a 4 corner idle circuit would help. If its dying from too much fuel try a hose going from your front to your rear vent tubes with a hole on the top of the hose. That keeps fuel out of the engine (usually) Baffled float bowls are available as well for corners. I also wonder if your fuel pump is up to the job. If it dies on hard braking after a while of WOT then you may not have enough fuel left in the bowls and then it slams forward away from the idle circuit. Lateral fuel slosh is not usually noticed until cornering forces get pretty high (not sure where your car is at though).
Good luck.

10-15-2004, 05:01 AM
I modded the crap out of my Holleys (750/800/850 DPs, 870 Street Avenger) and nothing helped on hard braking. I used front and rear main jet extensions, bowl vent whistles and even custom bowl baffles. The Holley is just a poor design for braking. The fuel bowls have a lot of fuel and the rear flow paths are a straight shot rear-to-front. My Qjet is a completely different story. The fuel bowl is in the center of the carb and all of the flow paths go from front to rear, so braking has zero effect on it. I've slammed on the brakes at 60 mph and my engine never skipped a beat. My Holleys would have immediately flooded, stalled at around 50 mph and then my ps/pb would go bye-bye. No good. My 750 Edelbrock/Carter would be ok under braking but would stall under serious cornering. The Qjet just takes it all in stride and keeps doing its job.

10-15-2004, 04:21 PM
The Qjet just takes it all in stride and keeps doing its job

I am also a big fan of the Q-jet, It is a really good street/race carb and can be modded to feed some pretty healthy engines, I highly recommend reading a book called "Rochester" by Doug Roe, it explains everything that you could possibly do to a Q-jet.

If you don't want to get that involved in building a carb, I recommend JET Performance. I have dealt with them before and they are great people to work with. :icon996:

You can buy a carb from them ready to bolt on or you can send your carb to them. For a modded Q-jet go directly to jet because the ones from summit/jegs etc... aren't going to be built for your engine. I made the mistake of buying one from summit and it wasn't quite right for what I needed, so I called jet and they had me ship it to them and they made the required mods for no charge, the only thing I had to pay for was shipping.

the people at jet will take your engine specs/cam specs etc... and build to your needs.

10-15-2004, 06:15 PM
So how do you deal with the whole small primary/big secondary thing? Even the mechanical secondary Qjets are difficult to make a smooth transition. In an AutoX course you may not notice that though as you arent often seeking smooth throttle application in my experience (stomp gas pedal, stomp brake pedal :D ). In a long sweeping curve though I cannot stand Qjets as I always seem to find myself at the secondary opening point of the carb as the car is in full scrub and any abrupt changes will slow me down. I find myself fighting the Qjet where the holley is so smooth that you can open the secondarys without noticing it in that situation. Then you stomp on the brakes and the car stalls :lol:

David Pozzi
10-15-2004, 08:45 PM
I had pretty good luck autocrossing with my Holley 800 DP.
I first had flooding under braking, added jet extensions, and vent whistles. Most of it went away. I then lowered the float levels just below the threads of the side plugs. It was real good then. Turning up the idle speed helps burn off the extra fuel when braking. I've allways found it to be flooding problems, not leaning out with a Holley carb.

The next thing I'd like to try are the foam road race floats by BG. the sides are cut on an angle, for a better float level when cornering hard.

10-18-2004, 07:28 AM
So how do you deal with the whole small primary/big secondary thing?

There is a tension spring on the secondary air doors that can be adjusted so the secondaries dont come in quite so strong. the adjustment is at the back rh corner of the carb at the top, there should be a set screw that comes up from the bottom and a flat head screw on the side, you loosen the set screw(usually a 3/16 allen wrench) and then tighten/loosen the flat head screw from the side to tune the airdoors, and a small turn on the screw makes a big diff. in the way the air doors open. it just takes some time to get it adjusted to act the way you want it to.

10-18-2004, 06:16 PM
Yes my last Qjet was adjusted about as tight as possible while allowing the secondaries to open. Better, but still not nearly as smooth as a equal bore size carb. I definatly am planning on efi for the next toy as it will seriously out-handle anything I have played with in the past. I dont think a carb would do the job.

11-10-2004, 11:13 AM
If you don't want to go through a bunch of trouble, just switch to an Edelbrock style. Since the jets are at the bottom of the bowls, gas is always there.

Just get the off road needle and seats. Basically they ad a spring to the needles, to keep the floats down. Without them, the floats can flop up and down, thus not allowing enough fuel to enter the bowls.

12-15-2004, 09:57 PM

Take a stab at it its not as hard as you think.