Ignition Timing

[hendred]

When I got my Silk running some years back I set the ignition timing as per the handbook method (I am still using the Lumenition system).  Recommissioned the bike last week and decided to check the timing again as it hadn't been touched for years.  It was spot on using the handbook method.  After calling Clive on another matter, we got round to ignition timing and he suggested checking the advance mechanism was working correctly.  So removed the alternator cover and marked up the tdc and advance positions.  Connected strobe and started engine. The running advance was about 3 - 4 degrees less than the static. Must be backlash in the mechanism. Easily corrected. I was also interested in engine speed for full advance and on my set up it is 2200 rpm. Has anybody else checked their mechanical advance and what speed did you get?

The performance on the road has been transformed.  Before it was very peaky, only really getting going at over 5000 rpm.  Now it pulls very well from quite low rpm.  Much more fun to ride!

The handbook quotes ignition advance by piston position in most places, the trigonometry isn't difficult to convert to degrees of rotation, just repetitive and boring.  Found this little program on the internet for calculating crank rotation from piston travel:

http://www.torqsoft.net/piston-position.html

I believe the con rod length of the Silk is 130.5mm and stroke 72mm.  Put in the numbers and it will convert piston position to degrees of rotation or degrees of rotation to piston position.  Quite interesting results.  One place in the handbook quotes 37.5 degrees of advance with 0.3 inch of piston travel - put 72mm stroke and 7.6mm (=0.3") in the boxes and the con rod would need to be 1 metre long to get 37.5 degrees!  I'll stick to setting the ignition by marking up the alternator rotor and stator from piston position and using a strobe - works well on my engine.  

Always learn something new when messing around with old bikes!

[ukpco]

Very interesting but I'm a bit confused about what change gave you the increased mid range torque as you say you still set timing to the same piston position (at full advance) and you reach full advance by 2200rpm.

I'd also be interested to know what advance in degrees you get on the alternator to match the piston position.

I'm particularly interested at present as I've now given up on the Boyer system, although I hate being beaten, and am refitting Lumenition and want to set it up in the next few days. I'll let you know the revs at which I get full advance once I get it up and running.

[hendred]

Apologies for the confusion.  My static timing is 5mm or 0.2" btdc.  Set statically as per the handbook it results in a running advance about 3 - 4 degrees retarded when checked with a strobe. I think it must be because of wear/backlash in the mechanical advance unit.  I made the strobe marks on the alternator stator by setting the piston on the correct advance using a dti down the plug hole.  There are two convenient lines cast into my alternator rotor.  I also marked tdc on the stator as well.  

On starting the engine the strobe showed the static setting of the timing resulted in a retarded ignition timing when running.  I advanced the Lumention back plate to give the correct strobe setting when running.

I didn't measure the degrees of advance using a disc.  Using the Torqsoft program I get 27.14 degrees for 5mm (0.2") advance and 33.73 degrees for 7.64mm (0.3") advance. 27 degrees and 34 degrees for practical purposes.  From my recent experience I will set the timing using piston position and strobe, using the handbook method for getting an approximate initial setting.

Unless you have the handbook supplied with the  bike I don't know how you can tell which advance setting to use.  I think all the 38mm carb bikes are 5mm.  Hopefully somebody will come along with a definitive answer.

[hendred]

Curiosity got the better of me.  Took the alternator cover off again, made up a small timing disc and my advance is 27 degrees btdc.  Took some measurements with digital calipers and using trigonometry got 29 degrees (at small angles a small error in measurement creates a big error) so the 27 degrees advance at 5mm piston movement agrees with the Torqsoft program calculation.  Gives some confidence in the previously calculated advance for 7.62mm piston movement.

The studs holding on the alternator cover and stator make using a large timing disc impractical without replacing with shorter ones - I've had the alternator cover off once in 12 years before this week - six times in a week is more than enough!

Hope this answers your question.

[ukpco]

Thanks very much, you've answered my questions beautifully.

A definitive answer to the correct piston movement for all bikes is probably beyond us, but no doubt we could get close by asking all owners with their orignal handbooks to provide mk, engine number, carb size and handbook ignition setting.

To start the ball rolling, mine's a mk2, engine number 80, 34mm carb and 0.3" btdc, fully advanced.

[ukpco]

I've just finished a trial re-fitting of the lumenition system and all is well.
Interestingly I also found that the static setting gave 3 or 4 degrees retarded at full advance when checked with a timing light.
I haven't accurately measured when full advance is reached, but it's in the range of 2000 to 2500 rpm, so ties in well with Hendred's 2200 rpm.
Within the limitations of accuracy of measurement, the timing at 0.3" btdc equated to 33 degrees measured on the alternator rotor, agreeing with the calculated value.

[gilesorton]

To add to the data, my handbook gives the following info: engine no 138, 34mm carb, Lumenition 37.5 degrees BTDC on full advance.
One might also be encouraged by the statement:
"The Lumention system has no wearing parts and, once correctly set, should be trouble-free and require no further adjustment"

[hendred]

Engine 126, 38mm carb.  The handbook now gets confusing, it is a heavily corrected Second Issue dated May 77.
 Page C1: ignition timing 37.5 deg. BTDC with (0.2" or 5mm) added.
Page G2: 9.1 Timing.  0.3" crossed out and replaced by 5mm full advance
Page Z2: Ignition timing. 0.3" crossed out and replaced by 0.2" or 5mm  on full advance.  
The corrections have been made using a stencil.
Talking to Clive, he confirmed that the ignition advance was reduced on the bikes with the Peter Green porting which I think includes the 38mm carb. The correction to 0.2" or 5mm would seem to be correct.  It certainly goes well at a 5mm advance.  The Lumenition system when set shouldn't require adjustment however the mechanical advance could wear over time resulting in a change of timing.

The problem is 37.5 deg advance would require a 1 metre long Conrod to give a piston at 0.3" before BTDC, not really credible.  I suspect the 37.5 deg is the error as it is only given on one page, elsewhere only piston position is given.