(Archived from Peter Ardren’s vintagebmwbikeservice.co.uk site)
First of all, what is differential timing? Simple – it’s when one cylinder’s spark fires at a different stage in the cycle to the other. One cylinder might fire exactly when BMW recommended it should, but the other fires, say, 10 degrees before or after this.
The result is that the bike will be harder to start, run less smoothly and efficiently – and one cylinder will be doing much more work than the other. This is particularly bad when running the bike in, because that cylinder will run hotter and be more prone to piston seizure.
How do you know whether you have it? Well, one exhaust pipe may be bluer than the other. But the acid test is when you look at the timing marks with your timing gun. There should be only one mark visible. If there are two then the cylinders are firing at different times. BUT….if you have very bad differential timing then it may be that you see only one mark because the other firing point is outside the range of vision through the timing window.
It is easy to check manually though. Find the point at which the points just begin to open. Don’t worry at this stage whether this is dead on the S mark ( though it should be). Just note where it is. Then turn the engine through 360 degrees and see where the points just begin to open. The mark on the flywheel should be in exactly the same place as for the first cylinder. If it’s not then you have a problem with differential timing and your engine isn’t running at its best.
A note at this stage about finding the EXACT point where magneto points begin to open. The old Rizla paper technique – or using the thinnest feeler gauge you have – is still a little vague. It’s o.k. for setting the setting the timing normally because slight inaccuracy doesn’t matter too much and bot cylinders will be equally out. But when you’re comparing the timing difference between the two cylinders there’s a better – though more expensive way.
This is a buzz box sold by Vech. originally designed for light aircraft ignition systems, it works beautifully on a slash 2 and makes timing checks a sinch. It’s not cheap but it’s well worth it. I’m a total convert. You can buy it direct from Vech at: http://www.benchmarkworks.com/. It’s in the online store – tool section, as tool 028 and costs $69.95.
In PART 2 of this article I’ll tell you the cause – and the cure – for the differential timing problem.