92 Chevy Sivarado Timing problem?

My son has a 92 Chevy P/U that recently started acting up.
It idles ok but when you give it gas it missfires, shudders and want advance. If you try and drive it it will back fire and you have to pump the accelerator to get it to responed.
I have added some "Heet" in the fuel and changed the fuel filter because it acted like it had water in the fuel. This did not help.
It is not putting out any smoke.
Any thoughts on what the problem could be. I have no experience in timing issues. Thanks for the help

sounds like the fuel pump is headed south

First thing I would do is check the plug wires for arcs and replace if needed also plugs. Crawl under the truck and make sure the knock sensor on the block, above the starter, is still intact, if it isn't it will run like SHIT! Get a fuel pressure gauge and find out if you pump is up to par. Let Us know what you find.