I'll try to keep it simple, and if we need more info we'll go from there. 454 GM '81 Quadrajet, custom dual exhaust, new filters, original manifolds, rear and right side tanks.
Problem; After running for over an hour, if idling by the pedal, either at a stop or going down a hill, when accelerating again she runs strong for about 8 -10 seconds, then cuts out and sputters for a few seconds, before catching and smoothing out again. She never quits running, but it's enough to scare you in heavy traffic. Fuel line-in line filter too close to exaust? Fuel line too close to intake Mani? Is there a tape to isolate the fuel lines from the heat? I know how you guys love a challenge so let's hear some input. Thanks in advance, DZ

Doubtful it's vapor-lock, since the line from the fuel pump is pressurized.

First thing I'd suspect would be vacuum advance on the distributor - maybe a vacuum leak or a bad diaphragm, or even a leak elsewhere in the vacuum system causing low vacuum in the advance.

I'd next suspect the fuel pump, and then the carburator float being either set to low or having a leak.

Finally, a weak coil.

Dan,
Interesting problem, I've been thinking about many secenarios, and keep wondering if you have changed anything lately before this problem began. It does seem like a fuel starvation issue because it runs strong for the 10seconds and then you get an extreme lean condition. I would look to items that have been changed from original, ie additional fuel filter may be causing fuel to boil. When you are idling or going down a hill, the engine isn't demanding much fuel so the float will be full and the needle and seat closed. This creates a dead head in the line from the pump to the carb. If the additional filter is resting on a hot intake manifold and the fuel is temporarily not flowing, it may boil and aerate, then when you step on the throttle you use up what's left in the bowl, the needle and seat opens up, but draws in aerated fuel that won't atomize properly. Just a thought. Pull over next time and check that in line filter, then try suspending it in the air (not resting on something) so it isn't being warmed by conduction.
ps sometimes things I have added thinking they would be a benefit have been part of the problem. I have added extra in line filters to some of my vehicles, but is it neccessary when someone engineered one that threads into the carb? More isn't necessarily better.

I had a somewhat similar problem, though it was purely a case of fuel starvation under heavy throttle. A change of all the fuel filters solved that one.

Nonetheless, I've experienced episodes of fuel starvation accelerating after start up when the weather & engine were both hot. So far, a judicious pumping of the throttle has cleared things up, though it makes for a few nervous moments - Is she gonna go, or not? No fun on a freeway on-ramp

It's probably a matter of heat soak vaporizing fuel, most likely in the line between the fuel pump & carb. Fuel from the tank is not likely to get that hot.

If this is the case, the cure would seem to be to make certain you run at a fast idle long enough to bring the engine temp down a bit before proceeding.

Thermostatically controlled electric fans that run for awhile after shut down help, & might help in your case, pulling away from idle. It wouldn't hurt to check and see how close your fuel pump, filter, & fuel lines are to the really hot stuff, & insulate, or move, as appropriate, if you can.

I had a similar problem pulling out after a shut down in an S.O.B. It does get awfully hot under that engine cover.

quote:

Originally posted by modeleh:
I have added extra in line filters to some of my vehicles, but is it neccessary when someone engineered one that threads into the carb? More isn't necessarily better.

The Qjet in-carb filter will bypass if clogged, dumping all sorts of who knows what into the carb. An inline filter catches and holds contaminants.

Danny, is there an electric fuel pump in back?

I wouldn't completely rule out vapor lock, but the fact the engine's been running normally would tend to eliminate that. Even at idle, there should be enough fuel passing through the lines to cool it, and the fuel line is under pressure.

Fuel starvation would appear to be the prime culprit, and so I'd suspect the fuel pump. GM mechanical fuel pumps rarely go much beyond 50K miles, and these are symptoms of a leaky diaphragm in the pump; low pump pressure could also allow vapor lock.

I'd replace the fuel pump and the at-carb filter as a matter of routine.

On my Chevy SOB, I got a new fuel pump when it hit 55K miles, intending to install it before a 3,500 mile trip @ 57K miles. But then I figured, GM surely has improved the design, and didn't. So I got to replace it in an Advance Auto Parts parking lot at 106F (it cooled off to 99F by the time I finished). This was the same parking lot where I replaced the alternator the previous year...

Thanks all for your input. Modeleh is the winner when it comes to the consensus of those on board. It acts exactly as described. The first thing I'll do is replace the fuel pump, isolating the line at the same time as best I can. I haven't changed anything but the in line filters,[replaced only], but she's done this since she adopted us so your inquiry still stands. The fact that it happens with both tanks the same leads me to eliminate any in-tank pumps. The lines from both tanks and the electric crossover are very near the right side exhaust, which appears to be the "non- stock" side of the custom made dualies. We tied them back after last year but it's hard to get them very far from the pipe. We'll try a pump and some heat proof tape this weekend and see how it goes.

Dan,
Aeroquip makes a fiberglass hose wrap that has an orange silicon outer jacket. You buy it by the foot and cut it as needed. I have used it on spark plug boots that rest on headers and have used it as a sleeve over a rubber brake line that became close to a "custom" exhaust system. Unfortunately, I'm not sure the cost or supplier as my supply comes from tossed out scraps by the turbine engine overhaul shop I work alongside. It is definitely handy stuff to have some scraps in your barth junkbox. I'm not sure how effective the heat tape is but I've heard the header wrap products help.

quote:

Originally posted by modeleh:
Dan,
Aeroquip makes a fiberglass hose wrap that has an orange silicon outer jacket. You buy it by the foot and cut it as needed. Unfortunately, I'm not sure the cost or supplier.

I get it from http://www.aircraftspruce.com/catalog/appages/aqfiresleeve.php

I have also had good luck with Thermo Tec products, which are fiberglas with aluminum foil. http://www.thermotec.com/ I get it from www.summitracing.com