Issues after tune up

[Scowling]

Hello everyone. I bought a 1986 GMC C3500 (with a 350 and 4 speed) to use to haul hay and such around, mainly to have something to tinker on during the winter. The previous owner totally neglected this truck, but according to him he maintained it well. Anyways I got it home did a total tune up on it. Plugs (broke 4 of 8 taking them out because they were so old they basically rusted to the head), wires, cap, rotor, ignition coil, then by luck the fuel pump crapped out in the shop so I did that as well, along with all filters and fluids. My problem is, just after doing the fuel pump the truck ran great but something felt missing.  Seemed to be lacking a bit of power. Sure I could get up to highway speeds no issue but when I put my foot into the floor it doesn't seem like the 4bbl's kick in. Got looking around and the rod for the 4bbls seems little sloppy.Then after about 100kms of changing the fuel pump I've ran into main issue. When cruising at 90-100km/hr with my foot into a little the truck will back fire then shut off.I have to turn the key off and start the truck back up, I can't even dump the clutch and jump start it.  If I back off the throttle a little it coughs and sputters a bit but then works itself out and drives fine.This also happens from time to time while in town, truck will back fire and up and die.
So I'm at a loss for what's going on. Thinking maybe it's getting too much fuel and flooding itself ?the 4bbls not opening and all the fuel is going to the 2bbls?Maybe the two are related or maybe all the gremlins are coming out at once.
Thanks in advance

[Captain Swampy]

Check for vacuum leaks, change the fuel filter (again if you already replaced it) and run a good cleaner like SeaFoam through it. You may need to replace the rubber fuel lines. It could also be a bad cam, worn out timing chain, or worn out distributor.

[Irish_Alley]

electric or manual fuel pump?

[jg1977c20]

sounds like the dizzy needs a rebuild - that happend on a friend of mines nova .

[blazer74]

Ditto on the fuel filter,, do you have external or the one in the carb inlet or both.  Quadrajet ?

Runs ok if you ease in to it? How does it run first started.?

Some aftermarket mechanical fuel pumps can put out too much pressure also. Later quadrajet can take 7-8 psi.


How long did the truck sit?  Bad gas, junk in the tank drawn into the carb can cause issues.

A lot of variables like was mentioned. Process of elimination and depending on your abilities.


The rod from the choke pull off to the secondaries will be a little sloppy. All it does is delay the secondary air flaps from opening too fast when you floor it. Sounds like a quadrajet. Check that the choke pull off that that rod is connected to holds vacuum. Affects cold starts and secondary's. Check the distributor advance can holds vac also, that can cause issues at idle and cruise due to not enough timing depending on how it's set up.