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At the time these motors and their predecessors were designed, it was the norm to drive ignition off the cam. As you mention, the cam travels at 1/2 the speed of the crank. But as you allude to, cam chain slack can make cam driven ignition to be less precise.

Most modern cars use crank driven ignitions. If the trigger is in the flywheel, the diameter supports a fairly exact timing. A cam position sensor is used to determine which stroke the motor is on, but the primary trigger is the crank position sensor. On top of that, the ignition controller (computer) can advance the timing and listen to the knock sensor to determine if the timing needs to be retarded. This is why some motors have premium fuel recommended, but will run on regular fuel with lower power output.

Hey Seth, yes that was my point. For these mildly tuned motors, super accurate timing is less critical than a high revving race motor.

Gibson wrote: ↑Thu Dec 13, 2018 10:53 am Hey Seth, yes that was my point. For these mildly tuned motors, super accurate timing is less critical than a high revving race motor.

But let's not forget that the octane rating my /7 was designed for no longer exists in the USA. In fact the highest octane I can get (93), is still something like 5 points below optimal.

So accurate ignition timing is not a "good thing" becasue I'm racing, but is instead highly desirable due to generally lower fuel quality in my riding area.

Wobbly wrote: ↑Sat Dec 29, 2018 7:11 pmBut let's not forget that the octane rating my /7 was designed for no longer exists in the USA. In fact the highest octane I can get is now (93) is still something like 5 points below optimal.

I think there's a difference in definition between what was written in the manuals versus what we see on pumps today in the US. There are three primary values - Research Octane Number (RON), Motor Octane Number (MON), and a combination which was the average of the two. Mostly in Europe where the manuals were written, the RON was written on all pumps. In the US, we have the average octane number listed on the pump. I think an (R+M)/2 of 93 and an RON of 98 are essentially the same in terms of octane.

I never had problems running my /7 on pumps in the US showing 93. I have since put on an aftermarket top end and have lowered the compression ratio just a bit for good measure. I still don't having any issues.

Kurt in S.A.

Pumps in my state have always shown the (R+M)/2 formula, and the highest octanes have dropped from nearly 100 in the mid-70's to the 93 we have today.

Perhaps that uncertain fuel quality is one of the reasons that BMW dropped the compression on the later model airheads.

I think im settling on the alpha v3 unit. Does anyone know well/bad it works when using with the kickstarter?

The other variable is nominal compression ratio vs dynamic compression ratio as determined by differences in valve timing. The mild valve timing of the R60/5/6/7 with earlier closing of the inlet valves made them prone to pinging because the dynamic compression ratio was higher than other models. You make very small changes to dynamic compression ratio just by altering valve clearances.

No problem kick starting with Alpha