I'm sure I already know the answer, but just wondering. I haven't been able to get the 88 to run smooth sounding like the other pair of 6 cylinder Olivers that I have. Of course, initially I attributed the lack of a smooth sounding engine to having a poorly fitted straight pipe. I replaced wires, cap, and plugs. Points and rotor looked ok. I've rebuilt the carb and think I have it adjusted where I want. 

I had the tractor running at high throttle and as an experiment I removed #5 plug wire at the cap. Had a great spark that would jump the distance showing that plug is firing, but absolutely no change in engine sound. Same result with #6 and to some extent #3. None of the plugs were oil fouled when I changed them although they were &quot;black&quot;. Engine does not smoke at all and seems to start easy enough. I have not checked compression.

I'm guessing it probably needs a valve job.  Anyone have any similiar experience? I guess I won't really know until I pull the head.