Won't hold WOT

[azathoth]

83 Chevy 4x4, 350 with headers. Carb has been on the truck a little over a year and been fine. Now at WOT or even high throttle settings it will not maintain power. It's like the opposite of a surge, almost like going back and forth from WOT to idle quickly. If you back down it will smooth out and run fine, but if you start to ask for more power it starts again. Idles good and runs great otherwise. No pop or backfire, just sounds like the secondaries are coming on and off rapidly. No vacuum leaks I can find.

[429bbf]

sounds like your running out of fuel (fuel filter , pump.kinked line, etc.) if you hold it flat on the floor does it go like heck and then go flat and then go again ? if thats the case the fuel bowl is going empty.i had a couple of problems . a kinked line on my front tank (the tank had slid in the brackets) also i had to small of a hole in my seat . it was only .125 but you said it used to run fine so the seat is not your problem. hope this helps . let us know what you find.one last thing .had a car one time that had a bull duram sack in the tank and was plugging up the inlet .

[blazer74]

Fuel filter and fuel supply first. Check the rubber lines from the tank to the frame rail steel lines and to the fuel pump for collapse to start with. Like 429bbf says running out of fuel

I have a similar problem with my 74 blazer that falls on its face  and surges also, but not all the time. Random at times especially after driving somewhere, letting it sit and then going again, like getting on it out of a parking lot.
I have replaced all rubber fuel lines from tank to fuel pump. Blown out all metal lines on frame, blown into tank thru sender. I have bypassed the metal line up against the block thinking it was vapor locking.

I have done everything possible to the carb using Cliff's book and components. Replaced fuel pump also. I have an inline fuel pressure gauge that reads 6 psi at idle, but I have not plumbed it to the cab to see if fuel pressure maintains while I'm having the problem. Other than that all I have left is to drop the tank and take a look inside.

I haven't gone any further with it lately (frustrating) I think its good and boom it does it again. But still on the list of things to do.

Good luck on yours, hope I gave you some insight.

[blazer74]

Oh and by the way I even tried a different distributor with not luck.

[azathoth]

Little update. I changed the filter and it has gotten worse. Last time I has it out on the interstate it pulled for about 10 seconds then fell on its face and didn't really want to recover. I let it coast a bit and it was ok at low throttle settings. 429bbf described it perfect. Dropped the right tank (feeding from) and it was pristine inside. Going for a pump next. Sadly I'm down for the filler hoses for the tank. Hopefully they will be in by my next day off. Thanks for the advise everyone.

[blazer74]

Any luck?