chevy truck running poorly

[white chevy]

 I have a 1978 chevy truck with a 350 engine, I rebuild the engine 15,000 miles ago which would be about 4 years ago. Now the problem, the engine pops out the exhaust only when driving down the road with my foot on the gas, but if it is idleing or you rev it up nothing runs fine, only under a load. it has a mild cam, edelbrock carb, and catalytic converter and vapor canister that is all the emissions it has. it just started doing it yesterday.

[VileZambonie]

See if it needs a tune up. With sitting around corrosion gets the best of ignition parts.

[white chevy]

i just did a tune up about 3 weeks ago, i'm not sure if the module, coil, pick-up would do that. and what is a good base ignition timing.