383’s with AVS not launching well

Last night a buddy and I took 2 B-bodies to the dragstrip. Both were 383‘s with Edelbrock AVS2 carburetors. One was a stick car and one was an automatic. Both cars are reasonable “street builds“. Nothing too radical, basic cam intake carburetors and the stick car has aluminum heads. 3.91 gears in the auto and 3.23 in the stick; both sure grip. Converter in the auto is a TCI that is supposed to be around 3000 stall. Both of these cars are flawless on the street. They both run super smooth from idle through red line with great power and “light switch“ response off idle. On the strip it is a different story.

Both cars would launch on the green light but then fall flat on their face, even to the point of stalling occasionally. My automatic car almost stalled and I had to feather the throttle and struggle to keep it lit. I ended up launching with it hard up against the converter and then going to about three-quarter throttle and progressively going to for throttle in order to get a semi-decent pass.

Both carbs are at Edelbrock jetting with no change to spring tension.

We are going back in order to try and dial this in and get these things to launch well. One thing I want to look at next time out is the spring tension on the secondary flapper. Any other ideas would be welcome.

-MHZ

That’s where I would start.

Tighten up the spring a half turn until the bog disappears.
Check the pump shot too. But for it to stall usually is the air door.

always floor it for a blip... just before staging,... this loads the secondary wells with fuel so you don't open the secondaries with no fuel there for a few seconds..... float levels may be low also.. never hurts to double ck

check fuel pressure and fuel pumps. if your using the old factory type 2 valve/5psi pumps they were always a problem at the track.

Headers? Did you uncork them? If so instant lean condition. slicks or street tires? If both leave great on the street from a dead start find out what you’re changing at the track, that simple.

Great points everyone. Here are comments to all the posts above:

Transman, my thinking is much the same as yours at this point. I need to verify pump shot and I want to make the flappers tighter.

ph23vo, I do always give it a wing between the pre-stage in the stage lights to clear it out. I agree with you that internal set up should be checked.

lewtot184, One car has an original style pump on it and the other has a Carter hemi style pump which has recently been rebuilt. for a while I was concerned about pressure and vapor lock, but it was so cool last night and we had so much time between rounds to cool off and I do not think this is one of the issues right now. Absolutely fuel pressure is worth checking though.

cudaman1969, both cars have headers. Both cars were street driven into the track, raced, and street driven home. We did not uncork. Both cars are running drag radials. Honestly, the only difference that I see is traction. In fact after one of my runs in my automatic car with a huge bog when I got to the return road I stopped and brought it off and it was perfect; literally 20 seconds after the bog on the line.

cudaman1969, both cars have headers. Both cars were street driven into the track, raced, and street driven home. We did not uncork. Both cars are running drag radials. Honestly, the only difference that I see is traction. In fact after one of my runs in my automatic car with a huge bog when I got to the return road I stopped and brought it off and it was perfect; literally 20 seconds after the bog on the line.

Well that tells the story, blowing the tires off shows the problems, timing and carb. Ump timing up to full in (37) by 2000, tighten the air door a turn or two.

This.

Age old problem back in the day with the factory AVS. Car's fine on the street. Sticky tire/hot track - bog. Tighten the air valve.

what was the ET and MPH of both cars?

back in the late '60's nhra had a pure stock class (not to be confused with todays pure stock). a lot of us guys would run that class. there were a lot of 383 road runners and super bees and they all suffered from what we thought was vapor lock. truth was that the crummy 2 valve factory pumps rated at 3-5psi were usually around 3-3.5psi. they'd drive ok and street performance didn't really tell the story of how bad they were. when at the track the pumps would get hot in staging and then there wasn't nothing but crummy starting line and total track performance. we were doing everything including the clothes pin trick to solve the problem but the only solution was a better pump.

there was something else i've noticed in the past several years with a rash of 8:1 compression 383's; performance is dismal. i worked on a '70 road runner, basically stock, 8:1 with a 3.91 gear and automatic. even after a good tune-up that turd couldn't spin the tires! 383's aren't good torque engines and need a good ignition tune-up, not be over cammed, and do like a little gear. i've always used 2 1/2 turns from the air valve just seated on the carb top. float level should be looked at and make sure you don't have a fuel filter on the suction side of the pump.

Was never mentioned

650 AVS 2 OR

800 AVS 2

I've seen plenty of cars that work fine on the street but when you put slicks on them at the track the great traction bogs the motor down.

I removed the air valve from my Carter AVS 4640 (Charger 440 R/T Auto 3.23 rear) in 1976 and have never looked back.
I don't remember what was wrong,. But it has been fine ever since. Never bogs. Although it was never at the track and
is a peg leg.

I have been running an 800 thunder AVS for 8 years? This is what I have have found. 410 CI 3445 lbs 4:10 gear auto. I found that mine likes the pump linkage in the hole the closest to the pivot, or the biggest shot possible. Next mine really didn’t like the stock jetting. In fact it likes CRAZY jetting. I run .416 jet in the front and .419 jet in the rear. I have actually run no rear jet at all. I found that until recently before some intake work it like to be rich up front and lean in the secondaries. I ran 3 stages rich up front and 3 leaner in the back. So you would think, ok, bring them both back to stock and there you go, no way! I think it likes it because the fuel up front helps to cover the transition to the secondaries. The AFB has a small port that the butterfly swings by to help pull fuel to cover the transition. There is no provision for that in the AVS. Even if there was the door opens so fast it probably wouldn’t help. Some of the NSS with stick cars do remove the door, but they can launch at crazy rpm! I also drilled the shooter nozzle, but they now sell a kit with different nozzles. I needed to install the large needle and seats so it wouldn’t drain the bowls. I run a big mechanical and a holley red pump. First try a bigger pump shot. Then start moving fuel to the primaries. I have tested to determine when and how quickly it opens the air door. On mine it takes .3 of a second and starts to open 2500 rpm. Be careful on tightening the door spring. It is easy to break, I would not go more that 1 turn tighter than stock. Some of this will not translate to your car. I’m pretty far from this carbs comfort zone.