please help 77 cj5 304 problems

i have just bought a 77 cj5 with the 304v8. The engine starts and runs fine untill you run it down the road. When I take off and watch the tack it hits about 2800 rpm and the starts missing from there and the tack kinda jumps all over. I have replace plug wires, dist cap, rotor. It had a eldebrock carb on it with a alum intake, i thought it might be the carb so i put on a holly from my chevy and it does the same thing. Also replaced fuel filter. When running 55 or so it also runs rough. any suggestions?

Thanks
Deano1938

 

I had a car that did that to me, when the car heated up the coil would loosen up and start jumping the timing arould. In your case warm up the engine then check your timing at idle. Then see if the timing is advancing when the engine is reved up. Also search this site for some of 'teamrush" articles.

 

If the tach is jumping around at the same time that the missfire occurs then that would indicate that the ignition primary circuit is breaking up (module or pickup coil). What ignition system does it have?

 

A '77 should have the Prestolite electronic ignition. The Prestolite distributor is easy to identify by its plastic vacuum advance canister. Search for "Prestolite" here for more discussion.

 

Possibly bad tach causing pblm?