I have a 1987 LT with the 305 carburated engine. I store it all winter and do not run it for approx 5 months. The car starts up quite easily in spring and idles well. I am able to drive the car slowly and it will run as long as I accelerate VERY slowly. As soon as I need to put the car under any load to accelerate it sputters and jerks to the point where it stalls. It will also stall as soon as I take my foot off the accelerator. Once the car stalls, it will not start. After this initial attempt at driving it, I let it set for a few days and then it runs PERFECTLY for the rest of the summer.

Any suggestions on a fix?

 

I'd try putting in a 5 gallon container of fresh, premium, good-quality gasoline. I say premium to boost the octane that the gas that's been sitting has probably lost.

Also, just FYI, when a car sits for that long it's a good idea to change the oil.

 

I've tried 100% fresh gas and the result is the same unfortunately.

 

Try putting stabulizer in the gas before you store it. Old gas will gum up your carb and make it run like crap.

 

I do that every time as well...

 

Sounds fuel/carb related. Change the fuel filter lately ?

 

It's been a few years and could use changing. Not sure this will be the 'cure' though, since after the first drive of the season, the cars runs perfectly.

 

There's a few possibilities there which are all pretty easy to address.
Being 44 years old and having a full military career, I've had to store many vehicles of all types for extended periods, so I've been through this a LOT.

One thing particular to carburetors (especially with alcohol in the fuel) is the vulnerability of the tiny air bleeds to moisture and corrosion. Aluminum carbs are worse about this, but it happens to them all. The added alcohol in the fuel pretty much negates any of the lubricating properties of the fuel, and even ignoring this, a free flowing air filter system lets in quite a bit of water vapor over time. I lube my carburetors before putting them into storage in-car or not.
There are different ways to lube, but I prefer to use a spray bottle of 3:1 WD40/motor oil mix. 75% WD40.
I cover the entire carb with spray and make sure it goes down into the air bleeds and fuel bowl vents. I also stuff a towel or something similar into the air-feed tube to keep moist air out.

Another factor with either carb or EFI is that the sparkplug ceramic insulator around the tip that is in the chamber is often thinly coated with some carbon and fuel residue. Even with a clean plug, the ceramic will absorb moisture which will cause the spark to fail as the energy shorts down the insulator to ground. Top this off with a dead-cold start and the resultant extra rich AFR (cold start enrichment) and the plugs can get fuel fouled very easily.
You can pull the plugs, sand-blast the tips, then cook the tips with a propane torch to cook out the moisture & fuel, and the plugs will perform as new again.
To avoid this problem you can pull the plugs before engine storage and spray some WD40/oil mix into the chambers to keep moisture away, then reinstall the plugs or plug the holes. If you keep the plugs out, then oil soaked paper towels make a pretty good plug for the holes. I usually just put the plugs back in for storage, then later pull them and clean them up again just before the first start-up. Don't spray WD40 or oil on your sparkplug tips.
It is important to keep moisture away from the plugs and out of the combustion chambers as much as possible, so I also plug the exhaust pipe with an WD40/oil soaked rag or paper towels.

I also won't ever leave a battery sitting in the car. Store the battery fully charged in a dry and cool place. Sometimes they just die from sitting around but this is the best you can do.
For long periods of storage, sometimes buying a new battery and a fresh set of plugs is unavoidable.

For small engines like lawn-mowers and such, I mix oil into the fuel and let it run through the carb & engine before storage. For cars I just fill up the tank with 87 octane.

When gasoline sits for a very long time it loses much of its high-end volatiles. This makes it act like much higher octane fuel - slower burn rate and all. Any alcohol in the fuel tends to allow more moisture to actually mix with the fuel, which further slows the burn rate. This is not good at all for cold starting or street driving. It causes very late burning of the fuel. Starting off with a full tank of 87 octane will help out there.

In some cases for both small and larger engines I add a little acetone into the fuel before starting after a long storage. Acetone is a powerful solvent and has the effect of speeding up the burn rate in the chamber. That's why it works so well to improve cold-starts, and often improves warm-up and fuel mileage to an extent on non-CC carbureted engines. It (acetone) has an effect similar to lowering octane and increasing the engine's sensitivity to knock. This is a big plus for cold-starting, low-load driving, and start-ups after prolonged storage. I don't recommend going WOT with 87 octane, acetone and/or oil in the fuel. Just drive normally for a little while to run it out.
Acetone also negates any lubrication in the fuel, so there again oil in the fuel can be helpful for some engines.
Make sure all the oil and/or acetone is run out of the tank before loading the engine hard.

Again, the acetone & oil added to the fuel is for the carbureted engines. It could really cause problems with a computer controlled vehicle running in closed-loop mode. I run only a couple of my engines in closed-loop, but that's beside the point that additives can mess up the O2 feed-back system.

Sorry if this is too much of an answer. I hope you can take something useful out of it.

 

Just an FYI that I have an answer to my problem. An older mechanic said that it is likely the accelerator pump cauing my problems. The seals/packing get dry following extended periods of inactivity. He suggested running the car and then let it sit for a few days so these seals/packing get "wet" and function properly. I did this and the car ran great on the initial drive of the season for the first time in years.