misconception of this problem is thinking the bearing seal
is no good. The seal on ball bearings is not designed to retain
pressure, their only purpose to to keep dirt and moisture
out of the bearing. The pressure developed inside a model
engine's crankcase is much higher than this seal is able to
a model engine prevents the pressure in the crankcase from
leaking out is the precision fit between the crankshaft and
the crankcase, mainly just behind the front bearing. If this
fit is loose the engine will blow fuel out the front bearing.
Many engines do not have seals or shields on the front bearing.
The balls and cage are completely open, however no fuel leaks
the precision fit a groove is usually cut in the crankcase
just behind the front bearing. Another groove or hole connects
this groove to the inlet passage just below the carburetor.
The intake stroke suction of the engine is directed to the
groove so any fuel is sucked back into the engine. Many K&B
engines use a spiral on the crankshaft to pump the fuel back
into the intake stream.
the above mentioned is correct your engine can still leak
fuel out the front bearing, This is caused from excessive
blow by past the piston. This can be because the piston ring
is not seated or the engine isn't broken in. If this is occurring
in a new engine the problem may disappear as the engine gets
more running time on it. See Ringed
engine break in
have a ringed engine (See ABC, Ringed)
with running time on it, the problem may be a stuck ring.
If an engine has been sitting for some time the ring can become
gummed up and stuck in the groove. This makes its sealing
function ineffective. See How
to unstick a piston ring
have an ABC engine it could be worn out and require a new
If fuel is leaking from the front bearing while the engine
is not running it probably because fuel is siphoning from
the tank and filling the crankcase, not the issues mentioned
above. If the crankcase fills with fuel it has no where to
go except out the front bearing, back out the carburetor,
out the draw bar, and up into the cylinder. This condition
may cause "Hydraulic
Lock" and this will result in damage to your engine.