Advance-Retard: Lever Position?

[pplatt]

Hi folks. Here another basic question from a Scott newbie.

I have spent some time poking around inside the points cover on my mag and it has raised some doubt in my mind about the appropriate position of the timing lever. If I move the handlebar lever from its fully forward position I can see the cam ring moving counter clockwise. The points also rotate counterclockwise.

Does this mean that the timing lever is fully advanced when fully forward? I want to make sure I get this the right way around for obvious reasons.

Thanks.

[dave bushell]

If the cam-ring rotates in the same direction as the rotation of the magneto then you are retarding the ignition. If the cam-ring moves against the direction of mag rotation, then you are advancing the ignition.
Dave

[pplatt]

Thanks Dave. That all makes sense.

[BRIAN MARSHALL]

The vast majority of BTH and Lucas mags on Scotts are ‘slack wire advance’, but there are a few exceptions, notably on bikes that had a left-hand twist grip to control the advance and retard, instead of a lever; like some Sprint Specials, some early TT Replicas, and some of the ‘works’ TT bikes. If you look at some of the late 1920’s Isle-of-Man TT photos, the L/H twistgrips can be clearly seen on some of the bikes. Quite useful on short-stroke bikes, where they were sensitive to the A & R on the petrol of the day, and both twistgrips were opened together, but of limited value on a long-stroke bike where they can usually be left on full advance all the time.

[Mike Fennell]

“… of limited value on a long-stroke bike” This may well be the case with an engine-speed magneto ignition which has very limited retard capability.
Coil ignition can be retarded a lot further by rotating the distributor and this definitely helps to smooth out the slow-running.

At Abbotsholme last year Jim Baxter, having watched me trickle past the main entrance, made the comment that it’s quite difficult to ride a Scott slowly.

Although a number of members in the past have successfully modified coil ignition to retard the spark as far as TDC or beyond, the idea has not caught on.
It does work though and can be controlled by lever or LH twistgrip.

If you wanted to get the same benefit with a magneto you would have to follow Phil Irving’s suggestion ( in his book “Motorcycle Engineering” – published around 1960 )and arrange to rotate the instrument bodily! He also suggests linking advance/retard with the throttle. I haven’t tried that yet.

[dave bushell]

Brian, I think that you have rather over generalised with your statement that the vast majority of Lucas and BTH mags are slack wire advance. Of course it could be pure coincidence that the mags fitted to my Scotts and in my stock are otherwise. I have five Lucas mag/dynos that are pull to advance and none that are slack wire. I have three BTH mags that are also pull to advance and only two that are slack wire advance.
On the effect of advance and retard on long stroke engines, on my 1938 DPY engine, advancing the ignition from fully retarded to fully advanced, the revs rise by 500 rpm from tick-over. (I do have a rev counter fitted).

Dave

[BRIAN MARSHALL]

Well, I was generalizing, but with my long-stroke bikes, from 1981 onwards, where they have had magneto ignition, I have never found much benefit from playing with the advance and retard. This is perhaps because I never set up the throttle stop to give me a tickover, and this gives infinitely better behaviour when ‘coasting’ on a closed throttle, with none of the spasmodic snatching and jerking, popping back, etc., on the over-run, that you get when the carb is set to give you a tickover. A restrictor in the exhaust tailpipe also improves the slow-speed torque and general running, but that is quite another story….
Brian

[dave bushell]

I don’t use the throttle stop either but rely on the throttle friction screw to allow me to set tick-over to the speed that I want. I’ve never liked self closing throttles – very difficult when it comes to hand signals – and I’ve had many an argument with MOT testers who have wanted to fail the bikes because of that. It’s not until I point out that the MOT manual, Section 6.2 clause f, covers the subject, in that the throttle is operating in the manner intended. No more MOTs, so no more problem!
Dave

[Mike Fennell]

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.

[BRIAN MARSHALL]

Somewhere, though finding it may be difficult, I have a 1960’s-era scientific paper that was submitted by Silkolene Lubricants to an unknown university (?). The main topic is plug fouling in deflector-piston two-stroke engines, but it includes other issues that may play a part in the general running characteristics, such as fuels and ignition timing. I think that Scott engines got a brief mention, but Mercury outboard engines are the main player. Both fuels and lubricants have changed a lot in the half-century since it was written, so perhaps we ought to now ignore it, but if I can find it I will read through it again and see if it could form the basis of a Yowl article.
I just love how a little remark on this website can kick off a flurry of very interesting replies !!
Brian

[Tony Kitching]

Hi everyone, I have a 1947 FS with a 1937 DPY engine. Some 20 years ago I had it converted to petroil lubrication and have had no fouled plugs since. It runs with a Lucas magdyno (slack wire advance) and if I remember I retard it a little to start and then run on full advance. I think that with an engine speed mag you can only get about 12 deg. of retard. The other advantage is that I don’t get the dreaded fourstroking at low revs, the twistgrip is set to close the throttle completely. I know it’s not for the purists, but I find it’s just something less to worry about.

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