Ignition advance...

Fast Eddie

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Got talking to a mate of mine recently who was strobing his Boyer equipped 850 Commando.

He had to take ot to 5000 rpm, but in real life he almost never does that whilst riding it. Like many other folk I uspect.

So... most Commando riders have no idea what ign timing they’re actually using most of the time (as it’s somewhere on the advance curve). Even cruising on the freeway / motorway at 4000-4500 rpm still wouldn't be fully advanced.

Some years ago I had a manual advance magneto Triumph. I would retard it to start, then put it straight to full advance where it would stay until I needed to start it next time. And it ran perfectly.

It got me wondering, has anyone ever tried a fixed ign with a manual retard switch on an electronic ign on a Commando?

And / or, just how quickly could a Commando auto advance be set to advance before any downside was revealed?
 
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Until the first hill?? Didn't motor cars have vacuum input to alter advance for an engine under load? Though a drop through the gears to keep the revs up might negate the need?
Maybe okay in a racing scenario?
 
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Dunno, but my old HD's went from retarded for start & moved to full advance once running.... Seemed to have no ill effects on them.

If allowed to load up on hills without downshifting they would begin to ping though.
 
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The advance is only responding to crank speed not load as in cylinder pressure. Time for a more sophisticated EI I guess.
 
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If a stock spring advance unit goes to full advance at 2000 then you might as well have fixed ignition once running because no one in their right mind uses much throttle or torques around below 2000RPM. The only real advantage in the retard is for starting.

If you want fixed ignition with a manual retard for starting you can go with a specially modified fat spark easy starting JH mag as shown below.



PS
When I was racing I called someone at Boyer and they told me how to wire in a resistor to reduce the amount of advance. I did this to achieve something closer to fixed advance to give me more acceleration out of the turns. Max advance was the same - it just didn't retard so much at lower RPM.
 
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My Joe Hunt starts first kick every time without a manual retard, it don't kick back at all, but man does it put out a big fat spark.

Ashley
 
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My 500cc Indian Scout had left hand throttle and right hand advance and retard. I used to retard it to start then wind on the advance as I rode off down the road. As you advance the ignition there is a sweet spot where the engine pulls hard and if you go beyond it, the motor runs with a bit of distress. The other thing on the Indian, is you can adjust the fuel mixture by winding the needle on the Schebler carb, while you are riding. You normally set your advance, then adjust the jet. Once you adjust the jet, you can forget it until you change the fuel.
There are about four factors which must be balanced to get maximum pull from the motor - fuel octane rating, comp. ratio, fuel mixture and ignition advance. With my 650 Triumphs, I always ran fixed ignition then adjusted the fuel mix to get best power. I used to kick the bikes over on full advance. I would never let the motor do full swing when I was starting it. Always push it up onto compression, then just flick it over the top.
With a road Commando, it probably does not matter much if your advance is out by a degree or two as long as you jet to suit it.
 
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If you are racing a Commando on petrol , there is probably something to be gained by using programmed advance curves and different tapered needles.. Methanol is much easier - it hides up the tuning errors.
 
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"The advance is only responding to crank speed not load as in cylinder pressure. Time for a more sophisticated EI I guess."

Isn't that called: 'Engine management?'
Yeah - none of the typical Norton electronic ignitions do anything different than what the AAU does on the OEM Commando ignition. There is no "sensing" of engine load or related engine management, they just advance the spark based on RPM..as does the AAU on the stock system. Of course, they eliminate the maintenance that the OEM system requires which is a good thing. On the down side, some of them will result in a Commando with less responsive midrange performance because full advance comes in later than it should.
 

Fast Eddie

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Considering the wildly different advance curves of some of these units, and the very late advance of some (especially Boyer it seems), and the fact that a great many Norton riders will spend the majority of their miles not even reaching full advance (instead being at some random point on one of these random curves)... it all kinda makes a mockery of the care and attention many put into strobing their igns !!
 
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olympus

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Until the first hill?? Didn't motor cars have vacuum input to alter advance for an engine under load? Though a drop through the gears to keep the revs up might negate the need?
Maybe okay in a racing scenario?
The vacuum advance device gives the engine initial advance only. as the revs increase the internal weights take over
 
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The vacuum advance device gives the engine initial advance only. as the revs increase the internal weights take over
On the car distributors I’ve seen, the vacuum advance moves the points plate, to give extra advance under light load.

The centrifugal system advances the points cam, with increasing rpm.

I’m not sure what you mean by one taking over from the other.
 
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One of these days I will install one of these


There are 3 advance curves but they are 3D so are mapped to revs and load which is obtained by a separate MAP sensor with a pipe running to the inlet manifold to measure air pressure.

First curve is for starting, cuts out when the engine first gets to a programmable number of revs and switches to the normal curve.

2nd curve is main curve, fully programmable to revs and inlet pressure

3rd was developed for running on LPG on dual fuel cars but could also be for hi octane fuel as you decide what the curve is, selected by a switch.

So you could have low advance profile on the starting curve for kicking and to prevent kickbacks on electric starters, then have a virtually fully advanced curve for normal running with the advance tailed back at lower revs and full throttle openings. A second mapped curve could have more or less advance, more for hi octane or a lower advance for hot days in the mountains.

There is firmware developed for Boxer twins which uses a cam mounted toothed wheel and gives sequential ignition.

As the ignition driver is 5V digital the difficult bit is the -ve earth needed and the COP (coil on plug) ignition coils.
 

olympus

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On the car distributors I’ve seen, the vacuum advance moves the points plate, to give extra advance under light load.

The centrifugal system advances the points cam, with increasing rpm.

I’m not sure what you mean by one taking over from the other.
Once the vacuum advance is fully advanced whether that is operating the base plate or cam itself that's it... the centrifugal weights take over the advance process and the vacuum device is just held at its max position
 

SteveA

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I have mentioned elsewhere on here recently that in about '77 I bought a Lucas Rita set up with timimg cover from Tony Smith. It had come from Thruxton.

I figured the advance curve would be useful, clearly Thruxton thought otherwise, the advance circuit was jumpered out, it only ever ran full advance!

For a motor started by bumping, this worked absolutely fine.

I also mentioned that I bought it because the AAU had exploded, it was only after that happened that others said: Oh I always weld them up so that doesn't happen! Advance, well who really needs it! It is only for kickstarting

And as for running with half the normal advance, well I did that too, no problem. I bought a crank triggered Boyer from Steve Maney, when the rotating pick up piece was wrecked I just fitted a two coil standard Boyer pick up in the points cover, of course the cam only runs at 1/2 engine speed. But since you set at 5000 plus....that is the only figure that mattered!

Changed to a fully advancing Pazon and can't say I really noticed a difference!
 
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Fast Eddie

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Interesting stuff Steve, and certainly makes a mockery out of carefully strobing according to the instructions!

Seems to back up my own conclusion from playing around with ign timing on the dyno, with old Brit bikes at least, moving the timing several degrees either side of ‘correct’ seldom seemed to make any difference.
 
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One thing I always try hard to avoid is moving the rotor on the Boyer. If it moves and your carbs are jetted lean enough to get best power, you can start burning things. I'm too lazy to strobe my motor, but it is worth doing if you have shifted the rotor - just to get the ignition curve back in the same place each time. Even at 5000 RPM on a road bike, you are usually on the taper of the needles and for best acceleration getting that right is critical. If you get the advance curve moved forward by one degree, that leans the carburation off, probably enough to do damage. The other thing which can cause the combustion process to run too hot is the exhaust system. If you fitted megaphones to a Commando which was normally running with back pressure, you would probably burn a piston.
 
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Interesting stuff Steve, and certainly makes a mockery out of carefully strobing according to the instructions!

Seems to back up my own conclusion from playing around with ign timing on the dyno, with old Brit bikes at least, moving the timing several degrees either side of ‘correct’ seldom seemed to make any difference.
Moving the ignition advance has the same effect as changing the jetting or altering the compression ratio. If you move the ignition advance on the dyno and the power does not change, you are probably jetted too rich. As you approach the optimum by leaning-off your jetting, there is usually a distinct power increase before you go too far. The situation is a balance between ignition advance, comp. ratio and jetting. You can get to the optimum by changing any one of those three factors. And don't forget the exhaust system and the octane rating of the fuel - if you change either, you need to start again.
 
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