I have read The Tractor Vet's theory on this motor for years regarding the gas, oil, and scored cylinders. I believe what he is saying and have tried to follow his advise. I use premium fuel and IH 30 wt oil. I am not a mechanic....just an acreage owner that loves old tractors. I am just hoping his theory is not correct in my situation!
Background: I have owned this tractor for 9 or 10 years. Right after I bought it, I found out an exhaust valve was torched. I had an experienced IH mechanic nearby re-do the valves. At that time he said the cylinder walls looked very good. I had it completely tuned up: plugs, wires, cap, rotor and he set the timing for me. It has started and ran very well over the years. I know it is very cold blooded in the cold. I usually plug it in for a while on real cold days. I let it warm up before I use it.
It does not ever get worked hard. It is used mainly for pushing snow in the winter and to take tree limbs and brush to the burn pile. I use occasionally to help a neighbor move stuff around. I do not pull a plow, cut hay, rake or anything strenuous with it. I have put probably less than 150 hours on it in 9 or 10 years.
It seemed like it was down on power last year before winter, so, I changed the spark plugs to NGK. It immediately pepped up...started and ran great with lots of power. Like I mentioned earlier, it blasted out a bunch of black soot/carbon/??? out of the exhaust. Still ran great. Earlier this year, I noticed it seemed to chug a little at higher rpms. Last weekend I moved a bunch of tree limbs in @ 20 degree weather, it seemed like there was more blow by coming out the tube. I was not running it hard...just slowly dumping debris into a burn pile. I don't know if it was oil...or possibly condensation in the oil burning out?? It was running for about 1 1/2 hours while I chain sawed and piled on tree limbs. The temp gauge barely got above the C on the low end.
I do not have a compression tester or any scope to check cylinder walls. I plan to check out some of the suggestions given: check the manifold bolts, check the muffler to see if it is clogged...etc. I did check the air filters..both inside and out and they both look good. What spark plug is recommended for the c-291? It ran better with the NGK...but, am willing to try something else.