[85Sierra]

The truck is a 85 sierra with a 350 4bbl,

now the problem just started to happen about a week ago, i was driving like normal and it started to really stumble like down low and its never done this before so what i though it may be was the carb being dirty so i changed the fuel filter and used some carb cleaner seemed to run smoother but didnt make a difference so i check to see if both jets were squirting and they dont miss a beat. So i thought maybe a dead plug? changed them all and didnt make a difference so im thinking maybe its a vaccum loss not 100% i checked alot of the hoses and there arent any with big cracks but i may be wrong

any ideas? its really pissing me off leaving work going up hill its just sick

[Restore60s]

I would check the basic's first like the timing, normal visual inspection of the cap & rotor. look for corroded cap terminals or rotor firing pin burned away. I'm not sure about your definition of stumbling whether it is a miss, no power or a bad hesitation.
If the timing and secondary ignition are correct then I would look to the carb. An 85 model Q-jet MAY have a power enrichment solenoid that could be at fault. It's on the (looking in) right top with a 2 wire connector. If it's just a bad hesitation on take off you could have the carb pump going out.
Other problems with power loss could be restricted exhaust.
On a vacuum leak you would have a high idle in most cases.

[85Sierra]

well it idles fine and doesn't rev up at all so i guess it may not be a vaccum loss.

the exhaust is just a y-pipe to a glasspack so i know its not that

as for power its non existent when it stumbles it just sorta feels like it dies then comes back to life really big hesitation

[Restore60s]

Sounds like the pump in the carb is about gone. I'd just buy a kit and re-build it and put the new pump in (comes with the kit). GM used to sell the pumps buy themselves that were modified by being shorter to give a quicker pump shot for hesitation problems. But you still had to buy a top gasket and nowadays you can get a whole kit pretty cheap.
Make sure to check the timing also.

[jimfulco]

The idle passages and/or transition slots on the carb may be getting plugged up. Try a jug of fuel injector cleaner (I like Berryman's) in the gas tank before you do anything drastic. Also, you may have just got some bad gas.