Chevrolet Repair: Truck runs bad, power brake booster, manifold vacuum

Question
" Hi Van,
I have a 1983 Chevy truck. 305 engine, 4 barrel carb. The truck runs bad intermittently. Has new cap, rotor, plugs, wires, pcv, coil, ignition module, catalytic converter, fuel pump, fuel filter. Carb was rebuilt. Truck runs fine sometimes but all of a sudden it seems like it's not getting enough fuel or is clogged. It seems like I have to push down the gas pedal twice as far as normal. It will run bad for a few miles then clear up. I've noticed that sometimes (but not all the time) it might start running bad after I've come to a stop. When I go to take off is when it will bog down. Go a few miles or stop at store and come out and it might run fine. It's already cost me a fortune and I need to stop the bleeding. Any ideas would be greatly appreciated. If you need more info, let me know. Thank you, Larry"


Answer
Hello Larry,
A couple things come to mind.
First...is there any chance you have a slug of water in the gas tank? It will be on the bottom, and may be missing the suction screen till you slosh all the fuel ahead at a stop. Just a possibility.

Second....each time you hit the brakes, you use your vacuum power brake booster. it has a valve that could stick open, and just be a major vacuum leak. You could test that by disconnecting the vacuum hose to it from the engine, and plug the hose to see if it clears up.
Hitting the brake pedal a few times might also reseat the valve, but you need to try to isolate it to be sure.

Third...There is a heat riser valve in probably the right exhaust manifold. Vacuum closes the valve, and forces the exhaust from that side through a crossover port in the intake manifold, and out the other side.
If that valve sticks shut, or closes at the wrong time, it robs the power pretty bad, but usually causes the truck to overheat.

Fourth...Look in the end of the tailpipe, and see if there is a screen up in there about six inches. I have seen them get covered on the inner side with rust flakes, not allowing the exhaust to escape, till you come to an idle, where they fall off, only to repeat later. Acts like someone shoved a potato into the tailpipe.

Fifth...I believe that truck has an EGR valve. The high vacuum of closed throttle slowing down for the stop possibly opens the EGR valve, or actually just the normal operation of it, and if it seats back down on a piece of carbon, and doesn't seal completely, it forms a major vacuum leak.

A vacuum gauge connected where you can see it during the problem would help on a couple of those possibilities.

Really, that last one, the EGR valve, sounds most likely.

Good luck,
Van