I have a 90 Iroc w/350 TPI motor. After rebuilding the tranny I noticed there was a miss. The number 8 plug was carbon fouled after replacing it about 1500 miles earlier. So I replaced it and it seemed to run great for the first 5 minutes then it went back to missing again. I pulled it and it was carbon fouled again after 10 min of driving. What causes that to happen? It is also idling high, 1200-1500 RPMs....that because of the fouled plug? What do u guys think? Thanks

 

If you have no other mechanical noise to indicate a valve problem, it is likely ignition related. Check cap, rotor, wires...
Is the plug wet?

 

Depends on what you mean by "carbon fouled". If it's crusty crud that's building up that fast, it's from oil consumption. If it's black and sooty, and rubs right off, then that cylinder is running rich.

 

The cap and rotor are new with fairly new MSD wires that look good. Right before the tranny went out I replaced all the injectors so I didn't get to drive it very much on the new ones.

 

It is sooty...no oil deposits

 

Could be a bad plug. If they haven't been changed in a long time, a new set might make your engine happy.

 

Do you have stock ignition, or does it have stand a full MSD set up to work with your injection system?

Since its only that cylinder, either you have an injector that is putting too much fuel in, or you have an ignition problem. Like 69 said, just because its new, doesn't mean it can't be bad. I would put another plug in making sure its gapped, making sure that plug wire is good and making sure cylinder 8's point on the cap is good, then you may be able to rule out ignition. Hearing how you changed injectors does make me curious if one is bad, or if it has the correct flow. I know most of these points seem simple, but just about every problem I have had with any of my cars has been simple. I have a 67 mustang that after I changed plugs, wires, points, etc it ran worse, it ended up that one wire wasn't making good contact in the cap (points sytle cap) anyway, it cost me two sets of wires when just better attention to how deep the plug should have been in would have saved me a lot of time and aggravation.

 

Cap, rotor, coil, wires are all MSD. I don't think it's an ignition problem with the different ignition parts I've swapped with the same end result...dry soot on the plug. Would a bad injector cause the motor to idle high 1200-1500 RPMs?

 

Does anyone know how much resistance a good injector is supposed to be?

 

So have you tried a different cap, wires and plugs? Is your distributor stock, or is it MSD as well? I would completely rule out ignition before moving to induction.