could fouled plugs idle but not rev?i

[505challenger]

im having issues with figuring out why my car isnt running right.  one local guy said 340's are notorious for fouling plugs.  so i was wondering if it is possible for the bad plugs to be able to idle, but when you rev, or put a load on the engine then they wont spark

[Cooter]

There's a number of things that can cause your symptoms..First off, how do you know if the plugs are fouled? just going by dude's word? Need more info as to what the engine is actually doing...How do you know for sure that the plugs aren't firing? Did you perform a resistance test on the wires? Did you perform a spark test at the cap and plugs? What type ignition? What type carb? What size camshaft? What year engine? (Needed for comp. ratio if stock). All of this plays a role in diagnosing your deal here...Did you recently change something to make this problem start?

[Chryco Psycho]

if the plugs are blackened they may never fire properly again even if cleaned up .
A plug is more likely to misfire under load

[Aussie Challenger]

What Chryco said.
Pull a couple of plugs and inspect, maybe worn out, wrong heat range (causes fouling). Does the engine still have a points distributor, could be burnt or closed up or the ground wire may be broken or the plate is loose, distributor is 40 years old.

[505challenger]

its electronic ign.  i have changed  the ignition module, ballast resistor, coil and  pickup coil.

[Aussie Challenger]

I would still check inside the distributor to make sure that the shaft is not seized, many service people didn't bother to lube the shaft on electronic distributors and the breaker plate could still be worn and the reluctor could be hitting the shaft.