Dean,
The answer may depend on which sort of starter you have. There are two main
approaches to engaging the starter motor -
1. The method used on Spitfires and many older vehicles, where there is a
helix on the shaft of the starter motor, and the gear can move along this
shaft. When the motor starts spinning, the inertia of the starter motor
gear against spinning means that it moves along the shaft instead, which
moves it to a position where it engages the gear on the flywheel. While the
starter motor is pushing the flywheel, the gear is held in place. Once the
engine starts, it will try to spin the starter gear faster than the shaft,
driving it back along the helix and out of engagement. There is also a
spring that pushes the gear along the shaft and out of engagement.
2. Solonoid engagement (used on most modern cars), where a solonoid is
used to move the starter motor gear along the shaft and into engagement with
the flywheel. Once the gear is in position (and not until) an electrical
contact is made which completes the circuit to the starter motor to make it
spin. When the starter motor is turned off, a spring pushes the gear back.
OK, I know a third, but have only met it on my fathers Fordson Super Major
tractor-based digger - a variation on solonoid engagement, except the driver
pulls a lever to engage the gear, rather than using a soloniod!
Anyway, you can ususally tell the difference just by looking at the starter
motor - if it is entirely round it is type 1, if there is a big bulge
sticking out on the side, or there is a separate, smaller cylinder on the
side that joins up at the business end, then it is solonoid-engaged. If you
ever try and start the engine and the motor spins but does not engage, that
will be type 1.
Assuming you have type 1, if the engine is running really badly, it may not
spin fast enough to push the gear back along its helix, so the starter motor
will remain engaged. Although, if the starter motor is no longer turned on,
it will want to stop spinning completely, so any engine speed would try and
push the gear back. Hmmm.
If you have type 2, then I can't see any way the gear would not go back when
the starter motor was turned off, unless the gear actually got stuck on the
shaft.
In either case, it cannot hurt to take the starter off and make sure the
gear can move easily on the shaft. On cars I think a graphite lube is
required, you don't want grease or oil around there as it will muck up your
clutch, but there's no clutch on an aeroplane so the recommended lubricant
may well be different.
Whether the starter remaining engaged is the cause of the rough running, I
have no idea! But your symptoms don't sound unusual for an old engine in
cold weather...
Richard
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