HOME and how to join Forum Open Area General Scott topics Advance-Retard: Lever Position? Re: Slow-speed behaviour. Refined or rough?

#10904
Mike Fennell
Participant

This must rate as an important topic for most owners, and worthy of a Yowl article – but which I don’t feel fully qualified to write.

Some thoughts occur : Brian refers to “spasmodic snatching, jerking, and popping on over-run” – largely eliminated by allowing the slide to close fully. Idling speed is then controlled by the twistgrip ( friction setting as per Dave ) but it could still be irregular. Likewise, country lane cruising, needing just a whiff of gas can also be quite unpleasant. These are the conditions where I find a retarded spark very helpful. How much? At least 20 degrees back from full advance.

Irving ( “Motorcycle Engineering” ) discusses an early generation of American outboard engines which were quite similar to a Scott. Two-stroke twins with deflector pistons. They had to have very smooth and quiet low speed performance for trolling – slowly dragging a fishing lure along. This was achieved by a throttle linked magneto control retarding the ignition 50 degrees back from full advance! To run at all in this condition they needed about a quarter throttle, giving the engine enough fuel to eliminate four-stroking. The delayed combustion ensured very low rpm and power output. A Scott could theoretically be set up to do likewise but it would quickly overheat, so such extreme retard is not possible. The outboards of course had an infinite supply of cold water.

Brian mentions a tailpipe restrictor, also recommended by others. I have just fitted one but not yet tested. A heavy flywheel is another smoothing factor and Roger Moss adds weight here for that purpose.

Final thought – oil in the combustion chamber is a familiar bugbear and robs the engine of its inherent smoothness. It needs to be minimised.