We are struggling to figure this out. We have had this issue since the start of the season. Car runs great first few rounds. Saturday for example the car went 9.190 - 9.197 - 9.200 - 9.197 then started falling off 9.215 - 9.230 - 9.263

Sunday the car went 9.158 - 9.158 - 9.203 - 9.277

DA was pretty stable between runs only changing a few hundred all day and this is a methanol setup so it should not affect it this much.

On the majority of the passes the car just stops pulling, I can feel it lay over and struggle to get to its 6800 shift point. Fuel pressure is a steady 7.5# so fuel delivery to the carb does not seem to be an issue.

Float levels are 50% into the glass front and back.

We have checked valve lash and its perfect compared to where we set it a couple months ago. We replaced plugs, wires, checked and cleaned cap and rotor, installed new pick up in distributor, tested MSD and coil with MSD tester. Timing is locked out at 35* Installed new intake gaskets also.

Pump is Magnafuel 500. Front mount fuel cell and all lines, fittings etc. are new at the beginning of the season (-10 from cell to pump and pump to regulator, (2) -8 from regulator to carb).

We are going to try a different carb this week as one of the big changes this season was adding the Alky SV1 from prosystems. Not sure why the carb would be the issue since it runs great the first few passes but we have got to try something. We also disassembled the carb and cleaned it thoroughly just in case there was something floating around in there blocking a passage?

Trans shifts good and the car still 60's great it is generally at the top of second and in third when It lays down.

Anyone have any clues as to what might be happening?

I know you said DA is staying consistent but how about humidity? I know alcohol moves more with humidity than overall DA.

If it stops pulling my first thought would be fuel, Have you tried jetting up with the carb and see if this helps?

Yes we have been up 10 # from where it is now. It lost MPH so we went back to where it is now. Humidity etc did not change much all weekend. We never varied this much in ET when we were running gas and I know alky is much more forgiving.

Could be converter slipping when oil temperature goes up

Okay if humidity wasnt changing it may not be carb related. Where is the pressure regulator mounted? I had an issue where the regulator was too close to the headers (mounted on the inner fender) and I was actually boiling the fuel before it made its way to the carb. Would get 3-4 good passes in then would start dropping off on the top end.

Are you running an alternator or charging the battery between rounds?
Is battery in good condition? May be that the voltage is getting a bit low and the ignition is laying down on you..

When you say later rounds…..

What's different or changed?
Do you run the headlights in the later rounds and not the earlier ones?
How many amp alternator?

Voltage to the fuel pump stable? at speed?

Are you monitoring tranny temps?

I know it SEEMS like fuel, but from what you are posting, not sure.

Its been a head scratcher for sure.

We do not have a trans temp gauge but the fluid color and smell are like we poured it from the bottle.

We do not lose fuel pressure when it happens.

We do run a GM one wire alt. (80 amp) and 2 batteries. I charge after every couple rounds just to make sure.

Did you guys ever actually swap out the 6al box?
Remember, like we were talking, that tester is only looking at THAT moment in time your testing... And that most likely isn't a moment in time the box may be producing a fault.
From troubleshooting computer systems earlier in my life for 15 years, I found setting up conditions to force something to fail can be tough.
I would definately swap it out, think you said you had a spare?

Hopefully get the carb swapped in time for TNT at Milan Wednesday. If it acts up then we will swap in our spare 6AL. If that doesn't fix it then.............?????

Probably should check valve spring pressure. Problem doesn't sound like the usual signs of weakness, but a check couldn't hurt.

Hmmmm.... track conditions going away??
Might try short shifting; say 6500-6600 RPM. Are the fuel bowls running out? Do you have the spacers to increase the fuel bowl capacity? Just throwing out some food for thought.

Seems like if it was sucking the bowls dry it wouldn't wait till later in the day? that magnafuel 500 pump shouldn't have a problem keeping the bowls full.

Had a few guys say it was "definitely " the sprag in the converter going away? I would have thought my 60' would suffer also if that was the case?

How do the 1/8 mile times compare?

Take a close look at your incrementals. They should tell you where you are loosing ET. Back track from there, in otherwords confirm where it is loosing the ET.

You say it feels like it will not pull up top but do the incremental times show that? Or is the problem form the get go. If it is that you are loosing ET the entire track I would be looking close at the converter or trans itself. Maybe pump pressure is being affected as the trans heats up? Know where it is loosing the ET will help diagnose where the issue is. Takes a BIG problem to loose that much ET on the top end of the track. I suspect it will be found elsewhere.