Ignition advance...

The ignition advance you use is determined by the fuel octane rating and the comp. ratio. Adjusting the fuel mixture does the rest. Once the advance has been set and you have tuned the carbs to get best performance, if you change the ignition advance you need to re-adjust the jetting. That is the reason I always try to avoid pulling the timing chest off my motor. If I do that, it is back to the drawing board - ride and test.
I suggest it is important to strobe your motor so you always get the ignition advance at the same position, then you won't need to adjust your jetting every time you pull your motor apart. I am too lazy to strobe my motor, so if it comes apart I need to check my carburation.

With methanol fuel at 12 to 1 comp., it is usual to use the standard ignition advance you would use for petrol. With methanol at 14 to 1 comp., it is normal to go back 2 degrees. I use methanol at 9 to 1 comp, and I have gone forward 4 degrees. It does not really matter where the ignition advance is, as long as when you jet the carbs you don't get detonation or burning. The main difference between methanol and petrol is methanol has unlimited antiknock rating ( thus the difference in ignition advance) - it also has a supercharging effect due to it's high latent heat of vaporisation. Using petrol for racing is twice as difficult.

acotrel said:

Once the advance has been set and you have tuned the carbs to get best performance, if you change the ignition advance you need to re-adjust the jetting.

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Not on a street bike. A street bike is tuned to optimize the entire throttle range from idle to WOT. "Jetting" is set just once. The only thing that would change with a few degrees of timing is idle speed.

If you inadvertently move the ignition timing forward a couple of degrees when your carbs are jetted very lean, that has the effect of making the combustion temperatures even higher. You can start burning valves and pistons.
What you have said is correct if you are jetted too rich to get the best performance.

"Very lean" isn't the way to tune a street bike.

acotrel said:

If you inadvertently move the ignition timing forward a couple of degrees when your carbs are jetted very lean, that has the effect of making the combustion temperatures even higher. You can start burning valves and pistons.
What you have said is correct if you are jetted too rich to get the best performance.

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When you say forward, which way do you mean advance like I think you mean, or retard?
I wish you would think about your reply not every body is on the same wavelength as you!

Bernhard said:

When you say forward, which way do you mean advance like I think you mean, or retard?

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Advancing.

johnm said:

.....I do think it is worthwhile calibrating the primary cover timing marks and then strobing the bike to the ignition manufacturers recommendation.....

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I drew a degree "half moon" that could be stuck to the alternator rotor with a bit of double sided tape and positioned the engine to 31BTDC. When I replaced the primary case, the casting mark on the rotor indicated 30 degrees on the primary case scale. Having previously strobed it to 31BTDC, that implies that the current setting is actually 32BTDC, so I will retard it a smidgeon.

Thanks for the discussion, a lot of clever people here..

maylar said:

Advancing.

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Then what is the point of his reply to this thread, it appears he has not run/ raced a Commando on pump petrol for the last oohhh, 20 years, as he is always banging on about his beloved methanol where you retard the ignition a couple of degrees, whereas if you set the ingition to run on pump gas, advancing a couple of degrees will result in a kickback on the kickstarter, because it will be too far advanced?

I don't think it matters much where it is because you jet to it. When I jet, I go there and come back - I don't start rich and stay rich. If your motor is running too lean, you stop and increase the jetting. Best power is just on the rich side of too lean. With methanol there is a moderately wide margin for error. But you guys use petrol, which is much more critical. If you are even slightly too rich, your bike will be a real slug. But would you notice that when you are burbling along in traffic ?
The ignition advance is usually set to suit the octane rating of the fuel. Avgas has an octane rating of 100 and the rest up to 135 is achieved with tetraethyl lead. If you use that, you can use more ignition advance and the combustion process takes much longer and you probably get more power. MotoGP bikes use Elf racing fuel which I believe is simply a more highly refined hydrocarbon fraction. You would probably find they use more ignition advance and jet to it.

Bernhard said:

Then what is the point of his reply to this thread, it appears he has not run/ raced a Commando on pump petrol for the last oohhh, 20 years, as he is always banging on about his beloved methanol where you retard the ignition a couple of degrees, whereas if you set the ingition to run on pump gas, advancing a couple of degrees will result in a kickback on the kickstarter, because it will be too far advanced?

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I have never raced a Commando on pump petrol and I would not attempt to do it. I am not a Pommie and I don't play cricket. To get the most out of petrol, you would need the patience of Jove. When you ride a road bike, it does not really matter if it feels a bit sluggish. But a bit sluggish on a racing bike means a lot. With methanol, the tuning band where the bike really gets up and goes, is fairly wide, so it is much easier to get within it.

The point about mentioning methanol was about octane rating and comp. ratio determining the ignition advance to be used. Methanol has unlimited antiknock. With pump petrol your RON is probably limited to about 110. What was it in 1973 when the jetting and ignition advance for the Commando were specified ?