Loss of power at high RPM

[253862656971]

I was out making a few quarter mile runs tonight and while accelerating my motor died.  I don't mean that it blew up but the RPM's dropped like a rock at about 4000.  If I got off the gas the motor ran fine but if I stayed in it it would almost pulsate with the motor dieing then picking up RPM then dieing again doing this for as long as it was floored.  I was running at normal temperature and had driven my car about 30 miles before making my runs.  My first guess is a fuel problem but as long as I was at 3000 it ran fine and it probably wasn't vapor lock.  The one run I did make that didn't pulsate was the first run and it was a full 2 seconds off of my norm and 10 mph slower.  The car is a 69 383 with a stock intake, Carter AFB carb, and Accel points distributor.  This is the first time it has ever done anything like this.  It wasn't out of gas and it ran fine before and after the runs driving at normal highway speed (65 mph).  So does anyone have an idea?

[firefighter3931]

Sounds fuel related. How much gas was in the tank when you made the pass ? It's possible the pickup became uncovered under hard acceleration and starved the motor. If you had less than 1/2 tank this is likely the problem. Easy problem to diagnose....just fill er up and take it out for a blast to see if the problem persists.

Have a look at the fuel filter as well...it could be clogged up.


Ron

[Chryco Psycho]

the pulsing sounds like the fuel pump pushing fuel up a little at a time , you most likely have a fuel line or pump problem

[253862656971]

I had an almost full tank when making the runs.  I filled before I left and it should only have been down a few gallons.  Something I got to thinking about last night is my sending unit.  I know it leaks around the fuel nipple where the nipple is soldered to the unit.  Is it possible under hard acceleration that a few bubbles are sucked in and that's causing the pulsation?  I'll also run out and get a filter when I get time and see if that helps.

[Chryco Psycho]

if there is a pin hole in the fuel line it will have an affect on the fuel flow from the tank

[firefighter3931]

Pull the pickup and repair the tube. Install a new sock on it while you're in there.

Ron

[Bobs69]

I pulled my pickup, naturally I wrecked the sock while inspecting it (long story short).  Do I really nead to put a sock on it again?  Or can I run another filter inline outside of the tank??

[craigandlynda]

i had a similar problem while racing my 69 1/2 superbee 440-6....it seemed to run low or run out of gas 3/4 way down the track...turned out i was using a straight thru filter, which was the wrong thing to do...i replaced it with a 3 legged type, and problem disappeared. some kind of internal fileter vapor lock  or something...anyway, it solved the problem for me.

[Bobs69]

A 3 legged type?  Hope they know what I'm saying at the autoparts store.  Not much support around this city for dodge people!

[ITSA426]

The way you describe it my first thought is exhaust problem.  Muffler bafffle or something free to move in there.  Particularly because it gets better as you let off throttle.  What do you have for exhaust?  Do you have vacuum advance on that distributor?