Ignition Timing On 1930 TT Rep

[Orgasmatron]

There seems to be a bit of confusion in Technicalities on setting the ignition timing (the confusion being in my head).
Would 1/4″ BTDC (2.4″ on the flywheel) be correct for my 1930 TT Rep ? I see 5/16″ (2.7″ on the flywheel) mentioned also, which seems to be for Flyers.
If the cam on the magneto is not exactly 180 degrees, do I set one cylinder to 1/4″ (if this is the correct figure), so that the other is less than 1/4″ (making the second cylinder slightly retarded – much like me really!)?
Chris.

[Barrie]

Scott ‘s own manual say’s , set the engine to TDC with the ignition lever in the retarded position ,then set the mag so the points are just fully open, lock the magneto sprocket.
If the cam ring is worn this can be checked with a degree disc on the mag sprocket and any allowance made to compensate, but why should one lobe wear more than the other?

[Tman]

It needn’t be down to one side worn more than the other, but manufacturing tolerances etc. It’s unusual (unless someone has taken the time and trouble to stone the cam ring) that they are the same side to side, and that goes for any similar mag.
The Technicalities gives a better method of timing than the old Scott manual does; after all what does “points just fully open at TDC” really mean?

[BRIAN MARSHALL]

I have always found it easier and better to set the timing as :- Points just beginning to open at 5/16″ before Top Dead Centre on Full Advance, or slightly less for longstroke engines. Just think about it…. A magneto sparks just as the points begin to open, and that is easily found with a magneto timing light. Also that original advice was for ‘new’ unmolested magnetos. Many have been modified over the decades to give more retard, in the hope of achieving a slower steadier tickover. Do you know how many degrees there are between full retard and full advance on your magneto ?
Few magnetos have the cam lobes at exactly 180 degrees apart, and I have found that BT-H and Bosch mags are better than Lucas in this detail. Always check the timing on both cylinders to make sure that one isn’t over-advanced. Running over-advanced causes pre-ignition/detonation/pinking, and that breaks cranks….
Brian

[Barrie]

@Tman wrote:

It needn’t be down to one side worn more than the other, but manufacturing tolerances etc. It’s unusual (unless someone has taken the time and trouble to stone the cam ring) that they are the same side to side, and that goes for any similar mag.
The Technicalities gives a better method of timing than the old Scott manual does; after all what does “points just fully open at TDC” really mean?

Well it works for me

[Orgasmatron]

Thanks everyone for your input.
As my TT Rep is a longstroke 600, unless anyone advises otherwise, I will assume that the ignition timing should be set at 1/4″ BTDC (or 2.4″ on the flywheel) and that (in the likely event of the magneto cam being slightly out) the second cylinder should be 1/4″ or less BTDC.
I note that nobody has mentioned the dwell period at TDC. It seems that there is a fair amount of piston dwell on Scott motorcycles. So, again, I will assume that the commencement of dwell should be marked on the flywheel, likewise with the end of dwell and the mean taken as correct TDC.
I think that my feeler gauge has a 1 thou (certainly 1-1/2 thou) leaf. I assume that using this on the points, simply has the effect of slightly retarding the timing (which is obviously better than advancing the timing). However, will 1/4″ BTDC still be the figure to go for, if this method is used for setting the points?
Chris.

[Tman]

If you use the “centre point of piston travel” method (as in Technicalities) you can find the exact point of TDC, then 2.4″ on the flywheel periphery to give the optimum max advance figure you can’t go wrong.
And (as the author of said technicalities article says) if you mark the flywheel at that point, it’s makes roadside checking/resetting so much easier.

[Barrie]

@Tman wrote:

If you use the “centre point of piston travel” method (as in Technicalities) you can find the exact point of TDC, then 2.4″ on the flywheel periphery to give the optimum max advance figure you can’t go wrong.
And (as the author of said technicalities article says) if you mark the flywheel at that point, it’s makes roadside checking/resetting so much easier.

My flywheel has previously been marked.so as soon as my dynamo is back in place I will check out this method also.

[Dripfeed]

5/16 BTDC on my 500cc short stroke motors is as good as it gets even on the present urine that is marketed as fuel ,, but I am not a trained engineer just an old metal butcher not even an ex sapper. Regards to all D.F .

[Author]

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