ignition timing

Benton said:

Hello!

See my info on shifting/separated rotor on hub above!

My Commando was produced December 1972.

Initially it confused me alot when trying to set the timing.

Then I realised the rotor had come loose from the hub…!!

Benton

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Yet again, this is (apparently) a new Lucas rotor. These have a 'solid' core.
Edit: Also, from the information given by qside, it's the crankshaft keyway that's in the wrong location.

olympus said:

Joe Lucas....Prince of Darkness

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He got a bum rap.

yep. if you want to see out right junk just look no farther than 60s and 70s Italian stuff.

concours said:

He got a bum rap.

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I'm quite impressed with Lucas. My stuff is 45 years old and still delivering, albeit with an occasional hiccup. Elliot Lucas had it going as far as I'm concerned and the blame should be laid at the feet of bean counters & those expecting 100 yrs. of service from lowballed items. My nips and hd's never provided me the same reliability, and most issues can be resolved roadside quickly with a decent travel kit. Just my limited Limey experience speaking....Oh, and a small flashlight for night work.

I agree that Lucas is terribly criticized unstifiably...though the jokes are great and I tell them a lot!

Also, I find it amusing how we sometimes criticize parts that "fail" after many years beyond their expected life. I use to run into this a lot on a Hatteras boat site I was on from 2004-2017 - people upset that their boat's original water heater (or whatever) started leaking...after 30+ years!

I did a post called "Linear Piston Travel and Advance Degrees." Below is the chart. It is compensated for the spark plug hole angle. I have a dial indicator that screws into the spark plug hole. The tip is ground into a rounded ball which slides across the piston easily. I find TDC by turning the rear wheel in high gear until the dial indicator is at it's peak. This method allows you to find thirty one degrees or twenty and any in between without taking off the rotor or the primary cover. Just set the piston .362 inches (9.205mm) before top dead center and run around the other side to see if your mark is correct on the primary cover degree markings. What to do it is is off is another story and I would try it several times to make sure the dial indicator is repeating accurately.

31°= .320" (8.127mm) BTDC (Before Top Dead Center), 0.362” (9.205mm)
30°= .300" (7.630mm) BTDC, .340” (8.642mm)
29°= .281" (7.147mm) BTDC, .319” (8.095mm)
28°= .263" (6.678mm) BTDC, .298” (7.564mm)
27°= .245" (6.224mm) BTDC, .278” (7.049mm)
26°= .228" (5.784mm) BTDC, .258” (6.551mm)
25°= .211" (5.359mm) BTDC, .239” (6.070mm)
24°= .195" (4.949mm) BTDC, .221” (5.605mm)
23°= .179" (4.554mm) BTDC, .203” (5.158mm)
22°= .164" (4.174mm) BTDC, .186” (4.728mm)
21°= .150" (3.810mm) BTDC, .170” (4.315mm)
20°= .136" (3.462mm) BTDC, .154” (3.921mm)

The figures in GREEN are corrected for the 62° angle of the spark plug hole. When the gauge measures 1 inch of travel the piston will have moved .8829" down so the gauge needs to read more than the actual piston travel to give you the actual piston travel. According to Jim Comstock the crank offset he has measured was only .5 to 1.0mm on 750 and 850 Commando engines which is not much. And, of course, if the piston is domed or dished the dial indicator will be hitting a different part of that dome or dish at the different distances from TDC as it moves across the piston because of the 62° angled spark plug hole.

The calculation which converts degrees crank rotation to distance from TDC uses the rod length and stroke. It is not the same for every motor. I use that conversion when I am porting two strokes. With two strokes, the relationship of the heights transfer and the exhaust ports is important and I usually work in degrees. When I last did it, I had access to a computer which would print a table. I now have Python language which I think can do similar, but I've been to lazy to develop it. There is a similar calculation which could calculate an optimum advance curve which would compensate for the various internal geometries of rod length and stroke giving different rock-over times. You could then simply advance or retard the static timing to move the whole curve.

I never use a dial micrometer through the plug hole to measure distance from TDC, if the plug hole is angled. In fact I use a soap bubble in an old spark plug with the electrode removed, to find TDC. It is extremely sensitive. And I use a degree disc. In the end what is important is knowing where the ignition system fires when you look at the trigger. Unless you want to re-jet, you need to get the timing in exactly the same place every time. - So use the strobe light. If you are rejetting it doesn't matter much within reason where you lock the timing - you simply lock it and jet to it. Moving the timing slightly has the same effect as adjusting the fuel/air mixture. If you are using a fuel with more antiknock, you can use more advance without getting detonation or burning a piston. But most guys with road bikes never get into that situation. With two strokes on methanol, you can really get them going, but the wear rates are horrendous (unlimited antiknock, they usually seize before they detonate) .

Sorry, I forgot to mention that my chart is for the Norton Commando motor and will work for any Norton engine with that same stroke. Also, the purpose was to find out if the degree markings on the primary cover in relationship to the marks on the rotor are correct for that engine. If they are off then you can also know how much they are off and write it down. Then use the strobe light.

motorson said:

Sorry, I forgot to mention that my chart is for the Norton Commando motor and will work for any Norton engine with that same stroke. Also, the purpose was to find out if the degree markings on the primary cover in relationship to the marks on the rotor are correct for that engine. If they are off then you can also know how much they are off and write it down. Then use the strobe light.

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Jim Scmidt's longer rods will probably give you an error if you use that chart. The maths involve the rod length.

Acrotel, that is correct. The formula involves rod length. The cylinder offset also has to be taken into account on a lot of engines. The Norton happens to be close enough to centered over the crank that it does not make a significant difference.

What is 'significant' varies with how lean you jet the carburation. Within reason, it usually does not matter much where the ignition timing is, as long as it is fixed and you jet to it. If you already have the jetting already as lean as it can safely be, and you get a small error in the ignition timing, then you can start doing damage. If the timing error is in the advance direction, it has a similar effect to further leaning-off the jetting. It is not normal to run a Commando engine like a two-stroke, but if you are seeking best performance it might be the way to go - particularly if you are using petrol as the fuel. Even with methanol as fuel, doing it that way gives better results.

When I tune my bike, I set the ignition timing first, then jet to it. I then strongly avoid changing the ignition timing. If for some reason, I have to move the trigger of the ignition system, I then start checking the jetting situation.