' The high swirl at higher rpms means you need less advance as the rpm rises. Staying at 32 degrees at 6000 rpm on a performance motor is tough on pistons and cases. Jim'
In reading the Dynatek 2000 literature, they achieve retard at high revs by using a VOES vacuum switch to change from the high advance curve to one with less advance. I thought that the reason less advance was needed was because there is less depression over the jets in the mixing chamber of the carburettor, i.e. under the slide, if the throttle is wide open at high revs. Do you have much experience with tuning commandos that have had the head fully hemisphered, or varying the ports which has indicated the swirl effect has a major effect on the need to retard the timing ? In Nortons you rarely have a high piston dome stuffing up the flame front.
I think there are two things which affect the required advance at mid throttle openings, if the comp. ratio, and fuel composition are constant. That is the jetting (needle shape) and the change in the time the piston takes to rockover TDC as the revs rise because the piston is experiencing a change in acceleration, and the time taken for the entire combustion event is almost fixed, being a chemical reaction. It would make more sense to me to estimate the time the combustion takes from the standard timing of 28 degrees at 3000 revs and relating the deceleration of the piston to it as the revs rise by calculation, the algorithm would give the shape of the curve needed purely to compensate for the geometry of the situation. Then you could simply jet to that, and change the static advance if the fuel altered.
I don't know whether the Dynatek 2000 system of retarding when there is little vacuum actually works. On reading their stuff, it seems they sometimes earth the wire that goes to the VOES switch, so the advance curve does not change if they lose vacuum.
I have never worried much about trimming the main jets as long as they are rich enough and the needle doesn't obstruct at full throttle. To my mind what is important is that you have the optimum of jetting and timing as you wind the throttle on coming out of corners, and that would be particularly so if you are using petrol to race. With methanol, you can get away with a lot, however even there if you raise the needles one notch on my bike there is a considerable drop in performance as you come out of corners.
When I practise, I do a lot of laps and there is one hairpin bend in particular which I use as a measure, and I know if the bike is going quicker or slower at the end of the straight after it. The trick is to know when you can raise the overall gearing.
