[Oak]

I got a problem on my 1863 with the Command engine. I took this tractor on trade about 1 year ago and the PO told me the engine had overheated and the valve seat came loose in the head. He took it to a machine shop and they repaired it and he put it back on. We traded and I never used the tractor except for a few times pulling a sprayer. It has been parked in the barn and now I'm wanting to sell it.
Over the weekend I noticed the engine doesn't seem like it is running at 3600 rpm. I used my contact tach and found it was only running about 3000 rpm. I adjusted the throttle cable and the high speed adjusting screw and got it up to 3500 but it runs rich and doesn't sound good. When I move it to idle it dies. I thought that maybe the carb is dirty so I removed it and cleaned it but that didn't help. I did notice that I'm getting some oil out of the breather tube when I had the cover off the air cleaner. I checked the crankcase vacuum and have about -5"WG at the dip stick tube.
I ordered a cylinder leakdown tester today to check for problems with the valves and rings but got the thinking maybe it's in the ignition.
What do you guys think could be my problem?