Running rough...Help please

[chucks70]

Took my POS for a 56 mi. each way jaunt today. Had about 400lb of gravel in the bed. Engine is 350, HEI with 1 yr. old module, new plugs, Edelbrock 600 Model 1406 (electric choke) and Edelbrock performer EPS. First 5-10 mi. no problem. After that anytime it was under load ie. going uphill or passing it seemed like it was fighting itself to spin faster. Back off the accelerator and it runs smooth. A couple of times it backfired. Timing is set at 12 before initial, has stock vacuum advance and weights. Average speed was about 65-70 mph. Did very little adjusting on the carb. out of the box. Seemed to run great on local trips.....Uh no I didn't put a fuel pressure regulator on it. Any ideas?

[cableguy0]

are you running headers if so make sure there is enough clearance between the fuel line and the headers the fuel will actually get hot enough to boil if there isnt enough clearance and it will run out of fuel under load

[chucks70]

No headers, center dump ramhorns. Gas line follows the framerail to the front of the motor then rubber line over to the fuel pump. Probably stock location except for the line that runs over top the intake to the inlet at the back of the carb.

[Bill72BB]

I always like to check the simple stuff first.
How old is the Fuel Filter

[chucks70]

Fuel filter, fuel pump, Carb., intake and all rubber fuel lines all about 1 month old, no leaks that I could find. Rotor, dist. cap. and module about 1 yr. Wires???

[Stocker]

Is the vacuum advance working? Had my weights stuck one time....

[71GMC_3/4T]

If you're still running a cab tank, pull the sending unit and check the "sock" on the end of the pickup line. Chances are it's full of rust like mine was.
I had so much crap in my tank & lines I had to change out the filter every couple of weeks until it finally cleared out most of it. I had the tank boiled out, but didn't want to mess with cleaning out the fuel lines - filters were a lot easier to change!

[jhwkns]

Was it popping through the carburator, or backfiring through the exhaust? How many miles are on this engine?

[chucks70]

Backfired through the exhaust by the sound of it, no burn on the air filter either. Runs great not under load. Goin 70 down the highway just cruisin no problem, but going up a grade or trying to pass it stumbles, backfired a few times. Weights on the dizzy are fairly easy to move (just spring tension). Back it off to about 55mph and smooths out. Mileage on engine is negotiable lol Could be anywhere but definetly not new. Clock says 86,000 so I'd say by the looks of things at least 186,000 assuming its even the original engine. Took gas line off and runs free and clear but the sock is still a possibility I suppose.

[Longhorn Man]

I don't know timeing numbers, I'm one to go by ear, but is 12 degrees a pit buch for when you are loaded down?
I think I'd try backing it off a bit, maybe 10 or so. (if it is still loaded down) and try it again.

[71GMC_3/4T]

This is an interesting problem. What happens under load that isn't there normally?

Vacuum Advance? (retards the ignition) I've seen a condenser in an ignition system that had a wire that had broken from flexing w/ the vac advance. The insulation was still intact, but when I pulled on the wire, it stretched. Try throwing in a new condenser. They sometimes sell the whole harness assy with the condenser. On the truck I was working on, it would run fine until he tried to speed up, then it would buck and snort until he let off, where it ran fine. It was about a '76 to '78 Chevy pickup, which might be the same era as your HEI.
Pull the distributor, the rotor, and then actuate the vac advance manually by moving the part that the vac advance diaphragm would move (or apply vacuum if you have the right equipment). Watch to see which wires flex. If one of them is the condenser, look closer and pull on the wire like mentioned above.

Let us know what you find, and good luck troubleshooting! Heck, it could still be from lack of fuel!

[MandarinSixSeven]

I just a story about putting a newer style carb on an older manifold. A friend of mine bought a new edelbrock carb and popped it on his old manifold and apparently the older manifolds dont have some sort of cooling holes that are supposed to connect to ones on the carb, so it was always boiling the gasoline and he had the same problems as you...he would have to pull over and wait for it to cool down before the truck would run smoothly again. Sorry I don't really know the technical terms, but until a couple days ago I didn't even know such things existed. Hopes this helps a little...just another possible option

[chucks70]

Well problem solved. It was the plug wires, the only thing I didn't change when I had the manifold off. The problem got progressively worse, even at no load. One plug wire fell? out of the boot on the cap and another was burnt on the pass. side exhaust but was not arcing that I could see. Put a new set on and when I started the motor the accelerator cable broke Auto Zone had a one size fits none in stock that I made work, but in the future I will be needing a correct set-up. Now seems to run fine.

[Bill72BB]

Glad you found it .

[71GMC_3/4T]

Yeah, Thanks for letting us know! Sure is nice when people actually post what fixed their problem!
Now I have yet another thing to add to my mental checklist for when the ol' beast isn't running up to par.