Misfire problem

I have a 87 b2200 5 spd and it has carb. It drives fine until it gets up to operating temp but then it will start to misfise when its under load mainly in 3rd- 5th gear. Once it starts misfiring it gets worse and worse until i pull over. If i let it cool a little it will drive fine until it gets hot again. I checked the plugs and they looked good and the wires are new any ideas.

sounds like the timing belt needs to be replaced. while your in there might as well change the water pump too.

o that sounds fun thank you

Do you have a timing light?
If you do, have you checked your vacuum advance?

Have you replaced your fuel filter? If you have a bad filter, full of water, that sort of thing, the engine is going to starve for fuel. . .it isn't as much of a problem while the choke is closed. Once it opens, more air needs more fuel.

Maybe a fuel pump is starting to fail.

You have described "something" under load.
But the word "misfire" is ambiguous.

The only thng I can figure out is that the problem is replicable.
It only occurs when the engine reaches operating temperature.

It probably isn't a plug.
Or a wire.
Or a timing belt.

Maybe it is a bad distributor cap.

or a coil breaking down under load when it gets hot.bj