Yesterday, at Island Dragway, first nostalgia race this year, I was racing my 77. It was 85 degrees and I went 12.6 and 12.7 on my time shots, with no problems. During my 1st run in eliminations, I noticed a slight drop in acceleration and a slight change in engine sound at the top of second gear, and at the finish line, about 5,700 rpm (TH350 trans). I have made hundreds of runs with this car so I am sure some problem was happening. Also the car went about .05 second slower than I expected, but I did win that round. For round 2, I raised my dial in. The same thing happened at the top of first and second and finish line this time. I slowed down another .05 this time, and lost the race by 0.0002 second, just to add to my aggravation.
I've been using a stock type, AC Delco fuel pump, with vapor return. I have a Lars Quadrajet carb. The car has been fine on the street. To me this seems fuel related. I'm going to try it on the road later this week, when it is cooler. If it runs OK that still leaves me guessing it is a fuel pump problem. I don't mind changing the pump, but those pumps aren't available now.
Anybody have good luck with a mechanical pump, with vapor return, rated at 6 or 7psi without a regulator? Or is the problem elsewhere?

 

Are you losing fuel pressure?

 

I would definately rig up a fuel psi gauge and see if you are running low at high rpm after a long pull.
Taping it to the outside of the windshield is safe for a quick test.

 

Thanks guys. I was wondering about fuel pressure. Since I never had the problem before and I don't usually race in 85 degree air, I was thinking about partial vapor lock being a possibility. According to AI it can happen at high rpm, WOT conditions in addition to when you normally expect it.
Meanwhile I'm going to try it on the road later this week, in much cooler air.