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jdemaris
02-21-2005 07:57:24
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You need to take something into account when working on a two-stroke-cycle engine -
especially chain saw engines like Homelites. Fuel delivery is dependent on the crankcase
being sealed. If it's not, the rest of the fuel system won't work properly. Yes there are
more obvious things to check i.e. & e.g. the fuel filter in the tank, the fuel filter in the
carburetor, the fuel pump diaphragm, the vacuum port or the vacuum hose that hooks to
the fuel pump in the carb, reed valve if it has one, etc. But . . . again - the engine depends
on a sealed crankcase. I was a Homelite and Stihl mechanic for years (back to the 60s).
If a saw had erratic fuel related problems, such as bad idling, hot starting issues, lean
running, etc. - you first pulled the muffler and checked the exhaust port and cylinder liner
in that area. That is usually the highest wear area - especially if the saw was running
lean. If that looks okay, you then block the exhaust port, and also block the intake port -
and then pressure test the crankcase. Very often the crankshaft seals leak - especially
with Homelites because they are cheaply built and usually do not have ball bearings on
both sides of the crank. In fact, some Homelites only have a needle bearing that rides
directly on the crankshaft. We threw a lot of Homelite saws in the garbage due to worn
out crankshafts. Not the problem with Stihl saws - at least not the older ones except for
the little el-cheapo 015s. As far as just the crankshaft seals leaking, it is common for the
seal on the sprocket side to get trashed and it's pretty easy to replace once the sprocket
and clutch is off. So, if you have a persistent problem, you need to pressure check the crankcase. If it
leaks, the air-fuel mixture will not be consistent no matter how much you fool around
with the carburetor, hoses, gaskets, etc. Along the same line and being a little more general - two-stroke-cycle engines tend to
use one of three common fuel-charge delivery designs. One is the roto-port which is rare
- so I won't describe it. Another is the piston-port sealing design like Stihl uses, and
another is the check-valve reed design like many Homelites use. When the piston is on
its down stroke, it pressurizes the crankcase below the piston so the fuel charge can be
delivered later to the combustion chamber above the piston. So, there must be a way of
holding the charge, separating it from the combustion chamber, and then delivering it. Thus the three systems aforementioned. So, with a piston-port engine like most Stihls - if
the piston or cylinder wall gets scored by the ports, fuel cannot be delivered properly. In
a reed-checkvalve type engine like many Homelites - if the reed breaks, you get the same
sort of fuel delivery problem. If your engine has a reed valve, it will be directly below the
carburetor.
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