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I am having trouble
starting my tender’s 40-hp, two-stroke outboard as well as noticing
poor performance at low speed and idle. I’ve checked the fuel and
ignition systems as well as done a compression test. Any suggestions?
W.P., via e-mail
Since
you’ve checked everything else (including, presumably, the carburetor),
look at the crankcase seal. Refer to the accompanying diagrams that
show how such a problem could produce your symptoms.
As you can see in
the left diagram, the downward-traveling piston first uncovers the exhaust
port (A), allowing exhaust gases from the previous combustion cycle
to leave the cylinder. Shortly thereafter, it uncovers the intake port
(B), allowing a fresh air-fuel charge to enter. In the right diagram,
as the piston moves up, it covers first the intake (B) and then the
exhaust (A) port, compressing the charge and simultaneously creating
a low-pressure area in the crankcase, drawing in more fuel and air through
the carburetor. After combustion, the piston is forced down, uncovering
first the exhaust, then the intake port, starting a new combustion cycle.
A leak in the crankcase
means a diluted fuel-air charge will be drawn in during the intake stroke,
creating a lean mixture in the combustion chamber and possibly producing
the symptoms you described. The same problem can be caused by failure
of the lower piston ring, allowing pressure to escape the crankcase
via the combustion chamber. A compression test typically evaluates only
the upper ring and not the lower ring, and even though the engine may
run fine at higher speeds, a classic indicator of lower ring failure
is the engine’s inability to idle at its recommended speed.
Check for crankcase
leakage by looking for fuel residue seeping from the crankcase parting
lines, the upper and lower crankcase seals, and the reed valves and
intake manifolds. If you suspect there’s crankcase leakage, rub
some oil on the suspect area; if there’s a leak, the oil will
get sucked in at its location.
Leakage via the lower
piston ring is more difficult to detect, so it’s best to let a
reputable mechanic handle it by first evaluating the situation with
a fuel pressure/vacuum gauge. As the engine is cranked over, a repeating
pressure/vacuum cycle should be observed on the gauge, with the pressure
reading being considerably higher than that of the vacuum reading. A
test of all cylinders should read basically the same.
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