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looking for suggestions on what to look at...

well used murray lawn mower
b&s 12.5hp engine flathead model 28v707
smokes on startup and while running, oil fouled plug, misfires when running but gets better when hot

pulled the head and quite a bit of oil in cylinder...installed new head gasket and spark plug.
installed breather tube between carb and breather (wasn't there before) still smoking (maybe a little less)
cranking compression 105psi

if rings were the problem would the compression be lower? or is the oil causing them to seal during testing

looking at the diagram there's no valve seals? so just the bronze bushing seals them from the crankcase?
maybe i should give it the seafoam treatment and see if it improves

any help is appreciated

350ss said:

smokes on startup

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Typically valve seals.

350ss said:

and while running

Click to expand...

Typically rings.

350ss said:

pulled the head and quite a bit of oil in cylinder.

Click to expand...

Cylinder has no oil control. Either oil rings are seized to the piston or they are not sealing. Time for a rebuild or replace.

Okay I got to do it. Or try this.

is that a real thing? lol

why is the compression as high as it is if the rings aren't sealing? is the oil sealing them kind of like when you check if low compression is rings by adding a teaspoon of oil to cylinder? hope that makes sense just trying to learn here

On a good set of rings that seal proper, the oil does most of the "final" sealing. Dry cylinders typically result in low readings.

oil can get into the cylinder to burn thru either bad oil control rings, or valve guides. Oil can also be diluted by a defective fueling system where gasoline dilutes the oil, and allows it to be so thin it passes by the rings. Do make sure that the oil is clean and does not have a lot of gasoline in it that thins it.

I would do a dry and wet compression test, then followed by a leak down test to troubleshoot the problem.

Most pistons used in small engines use at least three rings. Two compression and one oil. Athough the second ring is regarded as a compression ring it also does a little oil control.
If the engine has been fed clean air the compression rings will show little wear and the engine will have good compresion.
If the oil has not been changed the oil ring will wear and fail to controll the oil passing into the cylinder, causing oil burning issues.
The opposit is also possible.
Any oil that lays in the bottom of the cylinder when the engine is shut off has a tendency to leak past the rings. Kohler had that issues on V twins.
With the breather hose missing it is interesting the carb end did not suck a bunch of dirt into the engine and destroy the top rings.
The crankcase is supposed to be under a vacuum when running, if bad rings, blown head gasket or other leak, the breather hose will have a lot of blowby passing thru it.