Allis 712 K301 problem

I parked it and let it cool down.  After an hour or so, I tried again, and it worked perfectly for another 20-30 minutes when the same thing happened.  I shut it down again, waited for it to cool, and did it one more time.  This time, even right after start-up, it wouldn't rev up to full RPM and ran unevenly at full throttle.  Idle and low speed are normal, though.

I parked it in the shop and once it was cooled again, I pulled the head to see if there might be an obvious valve issue or a tear in the head gasket.  Gasket looks brand new, very little carbon on the head or block.  The intake valve (larger one) is black, but not thick build-up at all; exhaust valve is white - is that normal?  The valve seats look shiny and smooth - nothing obvious that might be an issue that I can see.

The only other thing I can think of is a bad coil or something messing with the ignition so that the timing is screwed up; or maybe the spark plug isn't getting enough juice for a proper spark?  I'm not an expert on ignitions, so I tend to think if there's something wrong with a coil or something that the ignition will just not work.  But this thing runs...just very poorly.

Any ideas?  Thanks!

Kyle Sands <>< Brandon, MN

If it was an icing problem, I should be able to squirt fuel into the throat and it should clear up, right?  Well, I removed the air filter and did just that.  Using a squirt bottle, I fired a little fuel into the carb throat and that just about stalled the motor -  so that tells me it was getting too much fuel.  I also tried playing with the choke.  It was pretty touchy, but I could get it to smooth out some with just a hair of choke, but any more and it would flood out.

With the air filter off while it was doing this full-throttle, loaded up/boggy condition, there was a lot of spit-back through the carb.  I stuck my hand close to the throat and it was quickly soaked with fuel.  There was also a great deal of smoke or mist that was wafting behind the carb.  I discovered there is a "vent" on the (what I assume) is the tappet cover behind carb.  It was spewing quite a bit of something...it smelled like burning oil, so I wonder if it was an oil mist coming out of there.  Later, after the engine cooled down and I ran it again where it was running normally, at full throttle there was NO spit-back from the carb throat, and NO "mist" or smoke coming from anywhere.  This engine does not smoke normally; it's pretty clean burning (either low hours or has had a recent rebuild).

Does that description bring to mind any ideas?

Kyle

Edit: would a timing issue cause the spit-back and abnormal smoking?  The timing is all mechanical correct?  So if it runs fine for 10 minutes it seems unlikely that it would suddenly starts acting all weird due to the timing going haywire...and then start working correctly again...and then go haywire.  Or could it??