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Need for speed

I’m working on a 1985 4.3 V6, The firing order is right, the timing is set, plugs are firing properly… This thing starts excellent, however when I drive it and it gets to about 20 MPH, it falls on its face. At 20 MPH it starts to spit and sputter and backfire!..Is it loading up? or is the distributer bad? I don’t understand, I’m sure its something simple.

Carb or fuel injected? Getting enough fuel?

Have you checked the fuel pump for pressure and flow? If the idle pressure is okey, rig up a way to view the pressure while you are driving and having the problem. I bet the pressure will fall on its face when “it starts to spit, sputter, and backfire”. You might just have a clogged fuel filter, a collapsing fuel hose, or an air leak into the fuel line. But, my bet is on the fuel pump because modern gasoline seems to deteriorate the valves in the pump.

Hope this helps.

Assuming it’s carbed, have you checked the vacuum advance (for ignition timing) and for vacuum leaks overall?

Have you checked the exhaust system for restriction(s)?

Thanks for all the replies…It is carburated, I didn’t think about adding that piece of information, I appreciate the suggestions. I will start a process of elimination, to nail the problem down… Thanks!!