My 1991 Chevy short van started the day running good as it always has. On the way home leaving a stop sign it would not accelerate. If I tried to increase throttle it would backfire and run worse so I kept the throttle steady as we ascended a long hill. Speed was limited to 20mph. Cresting the hill I could pick up speed for the next hill but power was way down and running rough. Half throttle was max or power was less as it ran worse if too much throttle was applied. The van starts easily hot or cold. I pulled the housing and started the engine after dark but no arcing was observed. When brakes are applied at idle there is no change in RPM. I was thinking if there was a vacuum leak, it may show symptoms when the brakes were applied.
I've tried searching for vans with these symptoms and while some are close, hard starting is an issue and my van starts easily as it always has with the same short cranking time.
I'm hoping someone who understands this vintage of G10 or an owner with this problem can tell me what solved it. I like doing my own repairs to save money, and also learn more about the machine I'm driving as I intend to keep this van(my 1st one) as it is very practical and I like it.
Thanks

Maybe your timing slipped or it isn’t advancing as your rpms go up?... that would indicate distributor mis alignment or advance issues(vacuum or mechanical.

It does feel like that. I'll look what kind of advance it has. It was running right with no hiccup whatsoever. I was coasting about a half mile downhill to a stop sign. I made a turn uphill from a stop and immediately had no power. I was taking everything very easy because I had groceries in the back. I never abuse my vehicles, but I was extra careful this time. I'll add that this van only has 82k on it, I got it from a police dept with 67k so it has been serviced regularly.

Thinking out loud here.....

Always check for codes even if the CEL isn't on. The '91 ECM is not as good at registering and storing codes as the later models so it may or may not show a problem. There is info on the web on how to retrieve codes from older systems.
From what you describe I would suspect a possible fuel problem. If you trust that your last fuel purchase was good (and not contaminated with water for instance) then I would suggest changing the fuel filter. If that doesn't improve the situation then you've, at the least, removed a suspect. Low fuel pressure can also cause the problem if the regulator is failing in your throttle body or if you're having a fuel pump issue.

If you have the stock air cleaner make sure the warm air/cold start damper in the air intake is opening/operational. This can cause a fuel to air imbalance, that the ECM can't correct, when under a load but would idle just fine.

The EGR can stick open and cause a problem but that will, usually, generate a check engine light. You can find info on the web relating to how to test your EGR and EGR solenoid.

The Cat can clog and give you the old "potato in the tailpipe" feeling which feels similar to what you describe.(ask me how I know) This would usually generate a code as well but is, of course, easy to check. Usually you'll smell it as the Cat will overheat big time.

Pop the dizzy cap and see if the inside cap/rotor looks normal and isn't broken/worn out/or wet. You'll need a 5.5mm torx for the screws.

Hope these thoughts help......

Thanks CatFish for the ideas. I changed the fuel filter, but didn't have time to test drive after. We have the stock air cleaner and was curious to see that damper blocking the intake. It works, but even when open it seems to be a restriction. Maybe not with this small of engine, but wondered if mileage could improve without it. This part of Oregon has few days below freezing so I also wondered if the air warming function was really needed. It was about 40 degrees out after changing the fuel filter. Started the engine with the air cleaner housing off and the TBI totally exposed so I could see the spray pattern and it seemed to run possibly smoother at idle, without the damper and air cleaner in the way of air flow. The spray pattern looked good and both sides looked equal. The air filter is still good and not a problem, in fact when I removed the engine cover someone didn't install the air cleaner lid correctly where it left a gap letting unfiltered air into the engine. It has run about 15k miles like this since I regrettably never pulled the housing until now. I will give it a test drive tomorrow.

I took it for a test drive and it ran fine with the new fuel filter. I may not be out of the woods so to speak until I take it on a longer trip, but I feel good that it probably is fixed. I've not had a clogged fuel filter do this without some warning, but this was the 1st time driving the van. We used it as a base and storage for our small u-cut christmas tree operation since Thanksgiving so it was only moved 100 yards twice a day and wasn't ran at more than walking speed during that time. I think I might carry an extra fuel filter with me, in case I have a lot more crap in the tank. Thanks CatFish for starting me on the simple fix. When something goes wrong I sometimes first think of the worst.