Back firing 6 cylinder

I am working on my son's '93 Grand Cherokee, 6 cylinder with 4WD. It recently started back firing and losing power all of a sudden. Up until this happened it was running fine.

The spark is good, plugs are good, and there does not appear to be a fuel problem. The timing is at 12 deg BTDC. I couldn't find in the manual what is supposed to be.

Any suggestions from anyone?

Thanks,

WD Perry

First thing, check you dist. cap for any cracks,arcing. It sounds like the engine jumped time. For the proper timing look at your emissions label under the hood, it should list spark gap, and timing at a certian RPM. How many miles are on your Grand? Also you siad spark is good, is it a good solid "blue" spark, if not anything else may be too weak to effectivly ignite the fuel.