Spub,
Ignition timing is mostly dependant on combustion efficiency, which in turn depends to a great deal on engine load, fuel quality and compression ratio.
The 750 combustion chamber and inlet tracts are far removed from modern designs, valve configuration and plug position add to the disadvantages. The less efficient a head is, the more advance is needed to get the inhaled gases burning at the right time without creating too much heat.
The combustion process is just that... the atomised fuel burns, it does not explode. The general idea is to have the mixture burn as quickly and completely as possible, this creates a minimum amount of heat that the engine components need to cope with. A compact combustion chamber without too many nooks and crannys and a centrally placed spark plug will allow the compressed fuel/air mixture to burn quickly and evenly. Add a couple of deep valve pockets and a plug placed way off-centre, the mixture will take a lot longer to burn completely. This is especially evident in large-displacement cylinders, smaller pots will hardly show much difference. Big bore 2-valve Guzzis e.g., profit greatly from a twin plug set-up, this really speeds up the combustion process.
At full advance, the piston is still on its way to TDC. When the plug fires, it only initiates the fuel burn process. Advance is chosen so that the full pressure resulting from combustion, quite a bit later once all the mixture has caught fire, acts on the piston once it has travelled OVER TDC! Having the combustion pressure acting on the piston before it reaches TDC would result in a massive power loss.
The average Laverda 750 will require something around the factory specified 40?BTDC at WOT under load. Up the compression a bit and you might get away with 35-36?, getting more of a charge into the combustion chamber via different cams or decent porting might allow even further full advance reduction.
The triple combustion chambers are quite a bit shallower, piston crown flatter with shallower valve pockets, all adding up to a more compact chamber shape. In the case of a 1000 triple, we also have less piston crown area, ie, a smaller chamber. This allows the full advance to be reduced to 32?BTDC on a stock engine to around 27-29? on a tuned engine.
The path the ignition curve folows up to full advance, also depends in part on fuel quality, engine state of tune and of course the load the engine is subjected to at any particular revs.
The procedure for choosing a particular curve should roughly follow the procedure for choosing carb jets. First look for maximum performance at WOT, then start tuning how to get there....
piet
Basically the advance curve is to overcome the fact that fuel does not burn fast enough to maximize the resulting pressure on the piston if you spark at TDC. So - the question is why? Well, a flame needs time to hop from one atom to the next (or basically "expand the flame front"). How much time is needed to do so depends on several factors, which makes up for a need of different pre-advance timings during different running situation... the advance curve depends on:
