How to use a timing strobe

QI’ve bought a strobe to set the ignition timing of my Hillman Minx. The pulley mark darts about a lot at lower rpm. At high rpm it’s blurred to the point that I can hardly see anything at all. What should I be seeing and doing? Norman Steele, Rowsley

ASetting the timing with the engine off (ie, statically) is normally more-or-less adequate. It can be fine-tuned by a degree or two during a simple road test. The centrifugal mechanism in the distributor advances the timing as the rpm increases. The maximum ignition advance is reached between 1200 and 3000rpm, depending on the car. A strobe light helps you to see exactly how the advance behaves at different rpm.

Connect the strobe to the HT lead of cylinder one and direct its beam toward the crankshaft timing marks. It’ll flash every time the plug sparks. You’ll see an almost continuous picture of the marks and you can read when the spark’s triggered.

Your manual should state how much centrifugal advance is added at certain rpm, up to a maximum advance figure. If a vacuum advance mechanism is fitted, it’ll give a maximum advance for this, too. Add the static timing figure to the maximum centrifugal and vacuum advance. This gives the maximum advance you should see at high rpm. It’s usually between 25 and 35°. This is a critical figure, beyond which the engine may start to pink or knock, which can harm pistons, valves and crankshaft bearings.

Disconnect the vacuum hose to measure the centrifugal advance alone. Reconnect the hose to show how much advance is provided by the vacuum capsule, which will be at its maximum at mid to high rpm in neutral. Check the figures against the manual.

Ideally, the mark shouldn’t move at all when the engine is at a constant speed. In practice, a little flutter is perhaps inevitable – but at no point should the mark be leaping around or completely blurred. If it is, something’s amiss with the coil, the distributor or its drive mechanism.

Run the engine at a very slow idle and watch the strobe. If it seems to miss an occasional flash, this indicates a failing coil. If all’s fine, take hold of the distributor body and shake it. If it’s loose, try tightening the retaining clamp. If the clamp’s OK, look for some other failure. In some goblet-shaped units, the

‘cup’ is a separate piece pressed onto the ‘stem’. If they’re loose, degrease the gap between the two and inject threadlock.

A worn distributor shaft or shaft bush will cause the points cam to flap around, leading to inconsistent timing. The points cam itself is attached to the centrifugal advance mechanism. Check it’s not loose. It should be held by a screw, found under the oiling felt under the rotor arm.

Nudge the pivot post of the points to see if the whole baseplate moves up, down or from side-to-side. This will also throw the timing. Don’t overlook the points – try another set, regardless of age or mileage.

If the distributor and coil seem OK, try another condenser. If that doesn’t help, examine the drive to the distributor. It’s just a couple of gears on your Minx (and most cars), but some may use a number of gears and long wobbly shafts. These may call for an involved repair if worn.