This issue always seems to be difficult to fix. So many things can cause and all we have is a description. I'll take a stab and add to the already given causes. Keep in mind we chase this issue all the time. It may take awhile or you may get lucky right away. Keep exact info coming.
No need to check timing when changing points. But after a few days we may say "check timing".
Cleaned air cleaner? What kind? Hope it's not a 4" diameter paper filter hot rod one. Running rich? I think lean. If it needs choke on it means it's running lean. Needs more gas, less air. If it is not rich, then the black plugs are a spark issue. (and that's a another half dozen causes). Don't look at the plugs anymore. Soot is not a worry, it's temporarily caused by whatever is wrong, carbon buildup, gas wet or oil is of more concern.
Drives bad and you discover .004 points gap? Don't think it would have run with .004. I am disregarding that info.
Changed points, runs good for awhile?
When a truck runs one day and not the other and it needs choke to run under load, there are things that can change each day, but NUMBER ONE is clogged gas delivery. Gas tank dirt floating around blocking tank pick up, fuel lines clogged, fuel filters clogged...........and of course, fuel pump going out, intermittent vacuum hose leak, intermittent accelerator pump, intermittent condenser........and here we go.
Keep in mind that whatever I left out is what is really wrong. I am 100% sure.
