Accurate ignition timing, Degree wheel or ruler?

 Note – this is about my iron barrel Royal Enfield Bullet – or any engine with contact breaker ignition. Bikes without points (like my BMW which has a Hall sensor for the ignition timing) usually do not need attention as there is nothing to wear over time. 

When such vehicles do need the ignition timing to be set, a strobe is really the only way. 

SUMMARY

If maths is not your strong point, then this is the jist of the rest of this page:-

Always set the timing fully advanced. That is where the engine spends most of its operating life.

Measuring the distance before TDC is the simplest method and very accurate.

On my Bullet, if I can set distance that the points open to before TDC to between 9mm and 8mm the timing will be 32^o +/- 1^o. 

A lot of rubbish is talked on line about setting the timing using degree wheels, timing lights etc.  Timing lights are fine if you have a proper reference mark somewhere on the rotor and ideally a hole in the chain case to see it. The Enfield has neither.  Degree wheels are fine if you can find TDC to within a tenth of a mm and find a place to attach the degree wheel.  That means removal of some bits.

Measuring the distance before TDC to set the timing is often classed as primitive, but it is the most simple, and usually the most accurate way. 

The Bullet manual advice is to check the ignition fully retarded. The distance is 0.8mm 
All that is needed to know is the stroke, con rod length, and some GCE “O” level trigonometry to convert between distance and degrees before TDC. Here  we go:-

Distance to Degrees before TDC conversion
Con rod length	(a)	175	mm
Stroke		90	mm
Crank radius	(b)	45	mm
Distance before TDC	(d)	0.8	mm
Crank to small end	(c) a+b-d	219.2
Cosine Rule	COS A = (b2 + c2 – a2)/2bc
COS A	COS A	0.986
Degrees before TDC	A	9.7

c is longest at TDC when A=0 and c=a+b, 
or half the stroke + the  conrod length. (220mm in this case)
At other time c = this figure – distance before TDC.

0.8mm corresponds to 9.7^o before TDC. With a ruler, I can probably only measure the distance between TDC and the required distance before TDC to +/- 0.5mm.  This puts my timing range to 5.9^o to 12.3^o .  That is within 3 degrees. So my timing could be out by +/- 3^o and what errors are in the Advance and Retard Mechanism?

OK, a dial gauge could probably measure to 1/1000 of an inch so we could get the fully retarded position accurate.  But the bike is usually running in the fully advanced position. Here are figures:-

Distance to Degrees before TDC conversion
Con rod length	(a)	175	mm
Stroke		90	mm
Crank radius	(b)	45	mm
Distance before TDC	(d)	8.50	mm
Crank to small end	((c) a+b-d	211.5
Cosine Rule	COS A = (b2 + c2 – a2)/2bc
COS A	COS A	0.848
Degrees before TDC	A	32.1o

 Using a ruler I can still only measure to 0.5mm, but the error now is only +/- 1^o

NOW if I want to get the same accuracy using a degree wheel, (I’ll assume that the degree wheel has ZERO errors)  I need to measure TDC to within +/-1^o. That is within .009mm or.1/3 of thou! (of an inch)  I’m not that lucky. 
If I can only measure to 1 thou, I can only measure TDC to +/- 1.7^o. AND I need to fit a degree wheel to the rotor meaning removing the chain case outer and probably the centre nut on the rotor. So the best I can aspire too is around +/- 2^o. In reality its probably +/- 4 or 5^o

THE MORAL – measure the distance before TDC in the fully advanced position.

So, my method is

• turn the engine forward until the piston is 9mm before TDC,
• Ensure the ammeter DOES NOT show a discharge when the advance retard is pushed into full advance position using my fingers.
• Then move the engine forward until the piston is 8mm before TDC,
• Ensure the ammeter DOES a discharge when the advance retard is pushed into full advance position.

This beats a degree wheel into a cocked hat.  The only better method is a factory made mark on a rotating component and a timing light – if you trust the factory!

I have invested in a digital vernier caliper which can be used a depth gauge. Out of interest I measured the fully advance position as 8.71 mm (32.5^o) and the fully retarded as 1.35mm or 12.6^o. So there is 20^o movement in my A/R mechanism.  The design value is 22.4^o.

If I go by the book, and measure the 0.8mm with a dial gauge my fully advanced position would be 2.5^o too far retarded. If I only had a ruler and could measure to within 0.5mm then it could be between spot on (if the errors all went my way), and 6^o too far retarded.

UPDATE, since fitting the electronic point saver I have made a LED indicator with a two foot lead and a couple of spade connectors.  This fits in between P1 and S1 in the schematic. When the primary chain case is off (for an oil change) it is easy to turn the engine with a spanner on the alternator rotor and see the vernier caliper,  (or ruler) and see when the points are opening without having to keep looking at the ammeter.