The Allis Model B I use for mowing runs as well as ever when it first starts. After a half hour to an hour of running, it starts losing power. If I shut it off and let it cool down, that "cures" the problem. I suspect a valve or two might be getting stuck open when it heats up, and unsticking when it cools down. I really don't want to take the time to tear into it in the middle of mowing season. Anybody know of a miracle product that will clean up the valves if that's the problem? I suppose there are several that claim to work, but I wonder if anybody here is familiar with one that really does.
marvel mystery oil triple shot to gas cured my problems
Try Marvel Mystery Oil in the gas and cross your fingers.
I had good results with a Stearman that had little use until an estate was settled by adding Marvel Mystery oil to the gas. Have you checked the compression when this occurs?
Sure it's not something more simple like vapor lock?
maybe its spark coil or condenser?
Check the coil, and condenser.
I would also recommend replacing the condenser as a first step. It does not sound like valve or fuel related issue, and adding anything to the gas is not going to affect the problem.
It definitely won't hurt anything to try the Marvel Mystery Oil or a can of Sea Foam. Personally I would try the Sea Foam for now and then tear into when you can. I have had decent luck with using Sea Foam on tractors that have been sitting for quite a while and have had the same problem that you are describing. But I was also thinking that maybe the problem could be that your tractor is the wrong color.
Lucas gas treatment works well.
Does a magneto have a condenser? I don't see any in the parts book.
Peter, I don't know what you mean about it being the wrong color. It isn't green.
The Fairbanks Morse magneto in the Allis Chalmers B uses this condenser Steve:
The coils in the Fairbanks Morse magnetos are known for dying from old age. If it starts hard hot that would be the first indication...
Flathead? It's just possible that you have too little clearance on the valves. So little that their staying open a bit. Not actually open just not closing completely. We're talking thousanths of an inch. Saw it numerous times on smaller engines back in my mower shop days. Would start & run OK but as soon as it warmed up the exhaust valve would loose whatever minimal clearance it had and would stay very very slightly cracked open. Enough to kill off any power but it would continue to run. Let it sit and it got better.
Sorta sounds like my old tow motor forklift, got to where it wouldn't start at all because the valve clearance closed up. Got it down now for a valve job so should be ready soon. KGB