Differential Timing Part 2 – Causes and Cures

Differential Timing Part 2 – Causes and Cures

(Archived from Peter Ardren’s vintagebmwbikeservice.co.uk site)

There are three main components in the timing process (I’m discounting the points here, for they are unlikely to cause split timing). A problem with any one of them – or in the way they connect together – can cause you timing problems. And as our machines age, these problems become more likely. Twenty years ago it was rare to find a machine with differential timing problems. Nowadays most of the bikes I am asked to look at have differential timing problems to some degree.

Differential timing is caused by one of the two cams in the auto-advance timing mechanism opening the points earlier than the other cam. The earlier opening on that cam causes one of the cylinders to fire in advance of the other.

With three main components involved, there can be several causes of this and THERE IS NO ONE-FIX-CURE-ALL .

The American guru, Duane Aushermann, is someone who deserves our respect for all that he has done – and is still doing – to further our knowledge and keep old be-ems on the road. His website is a fount of huge knowledge. BUT, I think that on the topic of differential timing, he has caused more harm than good.

Duane describes a method used by his technicians to reseat the magneto. They knew what they were doing. Lesser mortals don’t. I have seen occasional damage done to the crankshaft taper and frequent damage done to the magneto spindle as a result of injudicial use of a hammer. No matter how tempting it seems, DON’T USE DUANE’S TECHNIQUE. I don’t know anyone who has used it successfully – but I know a lot of people who regret trying it.

Instead, just work logically to find the cause(s) of the problem on your machine. With a bit of luck the problem will be that the advance mechanism is worn and needs replacing. But before you buy a new one (Salis has NOS at 556 euro or a repro version at 202 euros), check whether the problem hasn’t started further down the line. If so, then even with a brand new, perfect Advance unit, your split timing problem will continue.

The end of the magneto rotor shaft – the bit that the Advance units sits on – MUST be running exactly true with the camshaft on which it is attached. If it is at all out of line – if there is the slightest wobble – then that is the cause of your differential timing problem. Even 0.01mm out of true will equate to about 10 degrees difference in timing between the two cylinders.

Checking this is simple, though you need a dial gauge and a means of attaching it to the crankcase with the tip resting on the magneto shaft just below the indexing notch for the Advance unit. Turn the engine over slowly using an Allen key ( or 11mm spanner on very early models) on the crankshaft. There should be ZERO run-out. If that is the case (phew) then your problem lies in the Auto Advance Unit. See the third part of this article on what to do next.

IF you have run-out at the end of the magneto spindle then there are two ( arguably three) possible causes:

1. The rotor spindle is bent. Most probably because someone in the past has hit it with a hammer, following Duane’s advice. I always check the magneto rotor shaft for run-out on the lathe. If there is a way of repairing a bent shaft I don’t know it. It’s a bin job. The real problem is that good second hand rotors are like hens teeth. As for buying new… currently, in Europe there is only one very poor quality repro rotor available. I am told that Mark Hugget is having a batch made which will be available soon. If that’s the case then these will be of good quality and worth the wait.

2. The taper between the camshaft and the magneto rotor has a problem causing the run-out. This can be on either the male camshaft or the female rotor shaft – or both. I have seen damage done to either and both through trying to adopt Duane’s technique. I have also known a brand new repro magneto rotor have a taper which didn’t properly mate with that on the camshaft.

To cure taper problems see part three of this article.

3. I’ve never come across this and it’s highly unlikely that this will be the cause but an engine builder I respect told me he once came across a camshaft which was itself slightly distorted – this distortion was amplified as it went down the line of other components. If all else fails check for run-out on the end of the camshaft. But don’t expect to find any!

So, now you’ve found the cause of your split timing problem, how do you rectify it? See part three of this article, coming shortly.