cub engine problems i diagnosed myself into a corner

ericlb

Well-known Member
about 2 weeks ago my cub which i use to mow around the place and a few acreages in town ,and pull a pickup bed trailer around the animal pens started smoking, to me its black smoke indicating unburnt fuel, maybe a little oil, not much , carb was new last year its the ih type, now today i did all the mantenence i can to a cub, thinking the air cleaner wool might be clogged, causing it to run rich i took it off and cleaned it completely it wasnt clogged, changed the engine oil, as well as the oil in the air cleaner cup, and oil filter,also disassembled and cleaned the carb, nothing in it either,and it seems to be working properly, no choke half on ect, cleaned the fuel sediment bowl, then decided since i had the hood off anyway, id run a compression check, not good, if my gauge [ kd brand] can be believed, 90, 25, 50, and 90, front back , adding a small amount of oil to the number 2 cylinder only brought it up about 5 lbs, rings should have brought it up more right? now at 25 lbs the cylinder should not even fire, neither should the number 3, assuming it takes 90 psi to make a normal engine run, but the cub doesnt miss, and at full throttle if it had bad rings it should smoke like a truck on acold morning, but adding rpm actually takes the smoke away its barely noticable, and i still claim its black smoke, the carb setting is for highest idle speed, which is about 2 turns out from the stop, so what next? would valves not seating due to carbon cause the low readings? but there is no popping thru the stack or intake, and the crankcase breather where the oil dipstick is has some blowby but no more than it always had, i need ideas, here this is a daily use tractor, and im in nm, which means there is no getting a rebuilt cub engine around here, i dont have the measuring tools to accurately do a engine overhaul here, and there is no machine shop for 200 miles. and by the time i pay for that i could just buy another tractor , if i could find another tractor id consider it or upgrade to an a, super a, or even a b,provided it came with a good belly mower, but none west of ia that i can find,
 
should also add i did a pressure check on the radiator, pumped it to about 6 psi, no leakdown, then started the engine and let it idle about 10 minutes, no increase of pressure indicating no combustion leaking into a water jacket
 
I've found breather tubes on H's and M's blocked by dirt daubers. Then you have pressure building in the crankcase. That also happened to Hugh Mckinney on this forum but caused blue smoke. I don't have a cub though and don't know how the crank vent is configured. Something worth checking.

A leaky needle valve on the carb float or high float setting might cause excess feeding a low rpm. At high rpm the consumption catches up with the leaky valve.
 
I bought a Cub good shape and new tires BUT engine didnt run good. After compression check i removed the tappet cover to find out two valves couldnt close because the tappet wouldnt let them close. Removed the valves cleaned them up were just like new but the seats were gummed up . So with the correct stone from my valve grinder kissed the seats put back together and now it runs like new again. So you mite just want to check the tappets as you do have a problem on that low cycl and that would make the black smoke.
 
If it has been running for several years and this just started, I would pull the head (pretty easy) and see if the valves on #2 are burnt. If so proceed. If not, look for oily clean edges at the piston top. If found, rings are your friend. Jim
 
thanks guys i'll pull the head as soon as it quits raining, hope its valves this tractor has run real good up to now
 

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