Q I have a 1984 A reg 520i and the idle speed and ignition timing is proving to be a nightmare. I can’t get the idle speed to remain constant and setting the timing is just impossible because with the strobe light, the marks keep moving about. I fitted a new idle speed valve as this was all corroded up and cleaned out the coolant supply stub as you suggested, so this is working. Someone has suggested the vacuum capsule on the air box is faulty but I’ve disconnected this to no effect.

Tom Davies, email.

A The variable idle speed and moving ignition marks under strobe lighting indicate a knackered distributor. This early setup had vacuum advance and retard and that vacuum capsule, and you can bin it. You don’t say if yours is a manual or an auto – the autos don’t have vacuum retard.

What we’d do is find a later (post-1985) 520i or 525e with the plastic airbox and fit that, thus getting rid of the capsule (a coasting shut-off valve). Also replace the distributor with a good used one. The one you want is from a post-1984 car, which works better. A 320i unit will work fine, and you can use an automatic non-retard unit on a manual car. Make sure the vacuum advance pipe is connected to the take off stub on the throttle body on the manifold side of the throttle disc and set the timing to 17 degrees at 3,000 rpm. The BMW figure is 23 degrees at 5,000 – don’t even attempt that on an automatic as chances are, the gearbox won’t work afterwards! You can experiment with advancing the timing until it pinks under medium to heavy load and then back it off.