Carb problem at track

After I get 1-2 qualifying passes my carb starts to act up.First pass or so runs perfect. My car normally idles at 1,200 . After a pass on the return road my idle will go down below 1000 . If I blip the throttle it will increase to 1700. If go rounds this same thing can become a problem on the starting line. After the burnout it will sometimes idle slow enough that when I go on the two step it will bog.
I have all new carb and intake gaskets so I do not think it is a vacuum leak. It is a Quickfuel 1250. I'm thinking maybe a change in low or intermediate air bleeds may help or maybe cleaning the air bleeds.?
When I'm running it at home it idles and runs perfect. Only after you beat on it a little at the truck does it act up.
Any suggestions?

Coil? Getting hot? Do you have an AFR gauge on it? Is it going lean?

What fuel are you using?

Sounds like the distributor advance is not retarding back to what it needs to idle at your regular RPM

Might want to check the base plate to make sure it's tight to the carb body.....it could be loose and sucking in air as you go along. I've had that happen before.

I agree. Pull the cap and check the springs and weights. How old is your fuel?

My car does almost the same thing. When it gets hot sometimes it starts to idle lower and almost die out. Sometimes I have to hold the throttle on the line so it wont die out I am thinking maybe the power valve is opening and flooding it.
I am going to try a lower vacuum power valve and see if that helps. This a Quick fuel RQ 950. Keith

Power valve doesn't affect idle mixture unless ruptured. Lower that vacuum number, your going to create a bog. The power enrichment will come in later, if at all.

Check the float levels, fuel pressure. Do you have an insulator between carb and intake? Sounds like it's going rich after it warms up. I also don't think it's the PV. Timing?

Yes I have checked all that. I also tuned it for the best mph in the quarter so it is pretty close to optimum.
I was just thinking maybe the power valve was opening do to low vacuum when it gets hot.
I have the timing set at 38 degrees it is a fresh 440 build.

When the power valve opens it only provides extra fuel to the main well which feeds the idle well. The idle well is drawn through the idle feed restriction. Having more fuel available at the main well doesn't change the fuel at the idle discharge ports. The extra fuel available at the main well is for the main boosters when they are in operation.

I recently replaced all the ignition system chasing a down track misfire. cap-rotor-wires -coil-distributor- plugs ended up fixing the problem even though they looked ok. Yes i kick myself in the butt for that. Should have replace the plugs to start with but live and learn.

I replaced the carb because the new engine needed more cfm. also replaced the fuel pump when it went bad. I had replaced the regulator too as part of the fuel pump diagnosis.

So it seems the only thing I haven't replaced is the msd box it is a 6al
maybe i will try a different msd box. I cant think of anything else. Any other ideas?

On the cheap i would look at the Crane boxes and if ya want to spend a little more the Daytona sensors box looks to get good reviews. I run the Ice Ignition and like it, but i do get bored and when i do i will be switching to one on these.

Have you tried 34 and 36 also?
If not maybe you should
Keep in mind that not all timing lights read the same thing, not every body sees the same thing as everyone else, especially timing marks when the motor is revved up
A racer friend of mine saw 38 when I saw 34 on his motor with his timing light

What dist. are you running? Your problems sound like the same problems I was having. Is your timing out of gear the same as it is in gear? Assuming it's an auto. When you roll into throttle without bringing in the boosters or pump shot does the cam sound roudier? What's your initial timing? How much fuel pressure do you have?

To the original poster, I think you are relying on the transfer circuit for idle mixture. I would make sure that the idle speed screws are in the same ballpark. In other words after putting the carb on you only moved the primary screw to adjust idle. I would try opening the mixture screws after checking that.

I've been waiting to see if he gives an update.

This is a Quickfuel 1250 without power valves. Timing is at 35 with crank trigger ignition. All new carb and intake gaskets. MSD distributor with new cap, rotor and plugs. Fuel pressure 6.5 steady float levels at bottom of window,fresh Sunoco 112.
Did not check if idle screws are at the same place. will do that next but am leaning towards air bleed or pump shot problem???
Mike

IAB= idle air bleeds is where I start . put some smaller ones in it and see if that helps .

after you check to see if the butterfly's are in about the same from front to back . How many turns out are the mixture screws?

Here is a thread that contains a few possible answers.

https://board.moparts.org/ubbthread...07/re-idle-tuning-issue.html#Post2924307

Cab I will try backing down the timing a little.
It is a MSD distributor. I haven't checked the timing in gear I will do that. not sure what the initial timing is i put the strongest spring in the dist to limit the advance. Fuel pressure is set at 6.5 psi.
I use a MSD timing light

Try using the weakest springs you can get in there
I use the Mr. Gasket 925B spring kits in most of the Mopar distributors I use in Hi Po street and race only cars, I shorten up the advance slots so I have between 14 to 18 degrees at a low idle RPM and all the timing in before 2000 RPM. I really concentrate on having no more than 34 to 36 degrees total timing, higher altitude use 36, lower, below 2000 FT above sea level, I use 34 total
I also verify my timing tab and balancers when assembling the motors to make sure I'm getting what I see with the timing light and timing marks