20.5 twin cylinder what is the minimum compression?

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b1mmuo27

Active Member
I have a 1998 Craftsman lawn tractor with a Briggs & Stratton 20.5 HP twin cylinder engine that seems to be losing power as my 42" cutting deck is really lugging. The deck is fine. the tractor we got with the farm last year. It smokes when engaging the mower. I put oil in in every time I cut the grass all 2 acres. Up until now its been fine. Today more smoke than ever. I did a compression test & both cylinders are at 90lbs & holding. I'm thinking the motor needs a little freshening up. What should the minimum compression be? The tractor had probably sat a couple years before we bought the place.
 
I squirted a little oil in the cylinders & had the compression jump to 150psi. The engine is YBSXS.7842VE 274567
 
The number you posted is the EPA emissions certification number. the number I need is stamped in to the valve cover or the blower housing. While actually say Model, type, code.
 
If its holding that pressure for a 98' machine it cant be to serious...how long is it holding??....head gasket, valves, drive pulley id look at for that year..:2cents:
 
Michael72 said:
If its holding that pressure for a 98' machine it cant be to serious...how long is it holding??....head gasket, valves, drive pulley id look at for that year..:2cents:
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It held the pressure all night. But still smokes when I put the mower blades on. I know it can't be the rings.
 
Can you do a leak-down test on it ? That will determine if it's top end or the rings..

Although for the compression reading to jump 60 lbs to 150 after adding oil & re-testing, does indicate the rings seal is not ideal.( that is the purpose of that old school test).
 
Any viratin more than 20% on a wet & dry compression test = worn out rings.
Replace them ow & it will just be new rings & a hone.
leve it or much longer & it will be a new piston & rebore.
 
I use a digital leak down tester and have had different readings depending on position of piston in cylinder on older machines...as mentioned above its def a leak up..that extra ask when blades are engaged is a good test for rings....good luck :smile:
 
Here what I know for sure. Model 61707 Type 0145 E3 Code 9912205A I definitely leaking from the crank shaft seal underneath. Smoked for a while this morning blades weren't spiinning too fast just kind of died in the grass. Motor kept running though. Last year all was well it just was using a lot of oil. This year I have had it out for about a month now I its getting weaker. I also know its using almost twice the fuel. All I have is a standard compression gauge. I'm tempted to take it apart. Its not in the cards for a new tractor this year. Right now I'm all ears.:smile:
 
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