'Madditive'

[groily]

When does the advance start and at what revs is it all in? One really needs the engine on a dyno and map the advance required at every 500rpm. And then create a mechanism to deliver that.

Tests we did years back, with a decent ATD that offered 12° mag movement, 24° crank, with new correct springs, not preloaded, showed the advance started at 750rpm of the ATD and full advance was reached at 1,160rpm of the ATD - so it did its business between 1500 and 2320 rpm engine. This changed a fair bit if the springs were preloaded, with full advance being delayed by about 200 rpm at the mag, 400rpm engine and the start of the curve being similarly delayed. Wear in springs etc had the opposite effect, as one would expect.
Many ATDs are probably a bit tired and will also have more movement than when they came out of the factory. All sorts of things alter the relationship between degrees of advance / rate of advance and engine speed. But, absolutely always, the thing will be at full advance at quite low rpm compared to some of the programmed electronic curves that don't get to full advance until maybe 3500 or 4000 rpm.

Most car dizzies benefit from a vacuum system sensitive to inlet manifold pressure as well as a centrifugal one, so comparisons aren't apples to apples. I would think that IF an alloy head A10 had an equivalent vacuum retard facility, much of the pinking problem many of us have grappled with on those engines might disappear as heavy throttle and load would make the timing retard itself, replicating automatically Andrew's manual camring in conjunction with an ATD.

It is interesting ruminating on compression ratios, head design etc. I have run some of my own bike engines on pump 98 octane with up to 10:1 cr without pinking, and 2 of my current beasts are 8.5ers, yet none of them pinked like some of the alloy head A's I've played with for other people. I have concluded, as Andrew says, that the design 'isn't great' - but ignorance precludes me from saying 'why'. Beesas don't need the arguably excessive advance of Trihards and some others, yet they are more sensitive from what I have seen than other alloy head high-ish compression engines that run with a good 5° more advance. A problem for the latter-day Ricardos and Phil Irvings of this world, way beyond my pay grade.

[CheeserBeezer]

Thank you to those who have contributed interesting. additional, information; particularly Groily ad RDfella. The pinking thing is something I have been working on, off-and-on, for a number of years. As a forum, the combined wisdom of those who have contributed, we should congratulate ourselves on resolving something which is a PITA to many who own alloy head A's. It would seem to me that there are a number of tools in the box. The first, and most effective, seems to be a half size increase in needle jet, i.e. from a 106 to a 1065 or a 106T and a couple of sizes on the main jet. I am surprised that I haven't seen this before on any forum as it's quite a simple and obvious fix. However, most people, when referring to fix for pinking go straight to 'retard the timing'. It would also appear that there are so many pattern parts around and 'A' models have been fixed and fiddled with so many times that it is difficult to be sure what's inside them (valves, pistons). Furthermore, the quality of workmanship on a cylinder head, for instance, will impact on the engine's ability to burn the fuel properly. So, each bike will have to be treated differently but tinkering with carburation should get you there or thereabouts. Fitting an ATD to a manual mag is definitely helpful. Further assistance can be found by retarding the timing, though I haven't found this necessary. A bit of fuel additive may not be required but it does seem to be the icing on the cake. I know the specs of the Super Rocket and RGS I've been working on recently and I've not tried to exceed 8.25:1 pistons and both are running really well now. I think anything over 8.25:1 is testing the tools even further and, given the way I ride my bikes, there's no benefit to me to pursue this line of experimentation any further. Thanks for all your help, guys.