This has me baffled. I started my YM240 (24 hp diesel) this spring and it runs absolutely fine for about 1/2 hour - plenty of power under load, no smoke, no blowby, everything seems normal. Then, after about 1/2 hour it starts losing power and in a period of about 3-4 minutes it loses so much power that it can't climb a small grade unless I drop it into 1st gear. Yet, once on the level it will return to the original RPM. (Maybe it will eventually not regain the RPM but I haven't waited that long yet before putting it away.) I've done this at least 5 times now. I've even roto-tilled a 50x50 garden completing 2 full passes on the whole thing before it started losing power. In other words, I wasn't running under a light load for that initial 1/2 hour - at least not in my garden due to all the clay. Once it cools down, it runs fine again - for about 1/2 hour. I've replaced the fuel filter and loosened the fuel tank cap to make sure it wasn't pulling a vacuum. I also removed and checked the fuel return lines to make sure they weren't restricted. Nothing made any difference. When it starts losing power there seems to be light white smoke - I had to get the exhaust between me and a dark wall to see it. If it was broken rings, sticking/cracked valves, etc. then I would expect it to have a problem as soon as it started. That's why I've been focusing on the fuel system. Tonight I tried to pull the injectors but I found out that's going to require a special puller and probably lots of penetrating oil considering the age of this thing. I think it's about 30-35 years old with about 1800 hours and never rebuilt. It's one of the older green ones. The serial number is 1000104 in case that means anything to anyone. Any thoughts? OK. I'll admit one more thing but I'm not sure it's the cause unless maybe it has gummed up something. But why would it only cause problems after 1/2 hour? When I went to fill the tank I added maybe 1/2 gallon of gasoline (probably more like 1/4 gallon) then filled the tank the rest of the way with fuel oil. I figured a 1/16 mixture (approx) wouldn't be a serious problem. The first time I had the power loss I took it back to the barn, siphoned off all the contaminated fuel, and refilled it with fuel oil.