I've got a 81' E350 motorhome about 75000 miles 460 . # 3,4,7 cylinders seem to be dead . Pull the sparkplug wire off and no change to the motor . I've had the carb rebuilt ,new cap ,rotor, plugs, plug wires . Compression in each cyl. is about 125 psi. I've even tried a new /used duraspark module .Anybody got any ideas ?

 

Pull the valve covers and look for bent pushrods and broken rocker studs.

If you have 125 psi on all cylinders, then you are not looking at burnt/bent valves or blown head gaskets. You've ruled out spark plugs, plug wires, and cap. Look for a mechanical problem. Since you get good compression readings, most likely bent/broken parts on exhaust side rather than intake.

 

Check for air leaks or a bad EGR plate under the carburetor. 3 and 7 are on the same runner, 4 could also be affected by an air leak or exhaust leak on the passenger's side.

 

Hook up a vacuum gauge before you go ripping though it.

How does the motor run at idle and higher RPMs?