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Setting the timing
Sunday 2 August 2009

The old 'Beam should fire at just the right point as the piston heads to top dead centre (TDC) of the cylinder barrel. Assuming the magneto is working and you have the correct spark plug, setting the ignition timing requires a fairly simple set of adjustments involving:-

1. The magneto 'advance-retard' control lever on the handle-bars
2. The piston's position at which the the points open - setting the magneto drive pinion
3. The points gap on the magneto
4. The spark plug gap

With the 2009 Testers' Run approaching I decided it might be worthwhile checking the timing and ensuring it was set correctly. Here goes ...


(1) The magneto 'advance-retard' control lever on the handle-bars

For setting the ignition timing the magneto's 'advance-retard' control lever on the handle-bars should be set at full advance. This is the the rising piston's furthest point from TDC when the spark plug ignites the petrol / air mixture and causes the explosion in the cylinder.

For Sunbeam side-valve and standard over-head valve model the lever is moved inwards to advance. For racing models 80 and 90 the lever moves inward to retard. Consequently, on my 1931 Lion the lever is fully advanced by pulling inward towards the rider (photo above).


2. The piston's position at which the the points open - setting the magneto drive pinion

Commence by removing the circular inspection cover in the drive-chain case by revolving the flat retaining spring to one side, revealing the magneto drive pinion inside (photo below).

Undo the long retaining sleeve nut using a 3/8 inch socket. I put the motorcycle in top gear to provide sufficient resistance. Photo below shows the long pinion retaining nut being removed - the internal thread on the pinion is visible. This thread is used in its removal from the taper.

To remove the pinion from the magneto's tapered spline, a special extractor tool is recommended. I get by using a spare clutch stop from the primary drive case. This has the same 5/8 inch diameter and 20 t.p.i. thread as the pinion's internal thread. I insert a 3/4 inch long packing piece made from round steel bar - probably 1/4 inch diameter (photo below).

With the clutch stop and packing piece finger-tight in the pinion I then use an impact driver to break the seal on the taper. A light tap should suffice. Not the ideal way to do it but it works (photo below).

The Marston Sunbeam Register's Peter Yates, who works at HS Marston, shuddered at the thought of the impact - encouraging me to make an extractor tool. This only requires that I drill out a suitably sized bolt (5/8 inch diameter and 20 t.p.i.) and tap a thread through its centre to take an inner bolt that can be tightened to achieve the same end as the 'whack' with the 'Brummagem screw-driver' (hammer to you folks outside the UK's Black Country). That said, Pitman's book of the Sunbeam refers to giving this extractor tool a 'sharp blow' once it is tightened up.

With the pinion now loose on its tapered spline from the magneto, the correct position for the piston before TDC can be set up. This can be done by removing the blanking plug on top of the cylinder above the piston to calculate the distance of the piston above TDC (photo below) or by using a dial marked in degrees on the crank to calculate the angle before TDC with a fixed pointer. I prefer the former method and will refer to it.

I use a sufficiently long bolt - with a wide head so it doesn't end up inside the cylinder! - to bring the piston up to TDC (photo below). I do this by putting the motorcycle in first gear and turning the back wheel by hand.

For the purpose of setting the ignition timing the piston must be at TDC on its compression stroke and not exhaust stroke. On compression both side valves will be closed and there will be the normal clearance at the valve tappets (unlike the exhaust stroke, when the exhaust valve will be lifted). By revolving the rear wheel I can watch the engine going through its four-stroke cycle by following the valve movements: induction, compression, power and exhaust.

Once at TDC, I turn the rear wheel backwards (as if the motorcycle is reversing) whilst measuring the 'fall' of the piston using a marked scale - the wooden stick in the photo below, marked in eighths of an inch. The correct setting at which the contact breaker points on the magneto just commence to separate varies between different models. Marston advised the following from its manuals between 1927- 1931:-

(a) Models 1 and 2 (350cc side-valve)  -  3/8 inch before TDC (35 degrees is also given)
(b) Models 5, 6 and Lion (500cc side-valves)  -  7/16 inch (or 35 degrees) before TDC
(c) Model 8 (350cc over-head valve)  -  9/16 inch (or 43 degrees) before TDC
(d) Model 9 (500cc over-head valve)  -  9/16 inch (given variously as 41 or 43 degrees) before TDC
(e) Model 10 (350cc over-head valve)  -  5/16 inch (or 30 degrees) before TDC
(f) Models 80 and 90  -  5/8 inch (given variously as 44 or 45 degrees) before TDC
(g) Models 80 and 90 on benzole and petrol  -  3/4 inch before TDC
(h) Models 80 and 90 on alcohol  -  up to 1 inch before TDC with special piston

Marston's penultimate manual of 1936 includes the figures of:-

(a) 1/2 inch before TDC for 250cc OHV machines
(b) 9/16 inch before TDC for 350cc OHV machines
(d) 9/16 inch before TDC for 500cc OHV machines
(e) 3/8 inch before TDC for 500cc side-valve machines

There is a minor variation in the timing for the 500cc 'longstroke' side-valve engine. Until 1931 it is 7/16; in 1936 it is 3/8 (or 6/16) inch.

My Lion's 1931 engine was at some point in the past bored out to 550cc. I have found no figures for Marston's 600cc side-valve or OHV engines for guidance on these larger capacities.

As for anything before 1927, there is suitably sensible advice from Radco that 'a good average figure for vintage motors is an ignition advance of 35 degrees before TDC' (The Vintage Motorcyclists' Workshop', 1986). Depending on stroke, this corresponds to a lead in millimetres of anything from 7.6mm (70mm stroke) to 11.0mm (100mm stroke) ... or roughly 5/16 to 7/16 inch.

Back to my Lion, which I determine should have its points opening at 7/16 inch before TDC.

Rotating the engine backward, using the back wheel, I then bring it forward until the piston is 7/16 inch before TDC on its compression stroke, using my 'roughly accurate' wooden measuring stick.

Time then to examine the contact breaker points (photo below). As they rotate they are opened by a cam (a shallow bump) on the ring around which they turn in the magneto body ... and then close again.

Firstly, determine which way they turn when the engine is running - clockwise or anti-clockwise. I'm aware there is often an arrow on the magneto - but worth applying the mental faculties and a bit of logic to double check. The Lion has a clockwise magneto. I use the classic method of placing a piece of very thin paper (cigarette paper in the days when everyone smoked!) or the thinnest of my measuring gauge blades between the points and gently pull on it. When it slips out from between the points, this is the position where they are beginning to open.

With contact breaker points and piston in correct positions, I push the drive pinion back on its tapered spline. With everything coupled up I check again. If all is fine, I carefully tighten the long sleeve nut to lock the pinion in position.


3. The points gap on the magneto

The platinum contact breaker points when open to the fullest extent should be 0.012 inch (12 thou) apart. There are a couple of tiny fiddly nuts - a lock nut and adjustment nut - which require a set of magneto spanners to make adjustments. They are visible at the bottom of the photo above.

A little bit of checking and double checking is called for before I am happy with the results from my measuring gauge.


4. The spark plug gap

Finally, a quick check of the gap between the points on the spark plug. It affects the ignition timing and too wide a gap makes starting more difficult.

'The correct gap between the points should be .018 in' (or 18 thou) to quote the Pitman book of the Sunbeam (First edition, 1945 reprint, p.77).

The result, I am pleased to say was that the Lion ran exceptionally well on the Testers' Run - that is until a mishap with the exhaust valve ...


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