Technical advice: Sorting a misfiring engine

MCN expert Chris Dabbs is here to answer any questions you have on anything bike related.

If you have any questions, ask an expert here

Q. I’ve got a 1985 BMW K100RT which has been misfiring for ages, it used to only be the first 1500 revs which I could live with, but now it’s up to 2000rpm and it’s also popping and banging on the overrun.

I’ve fitted a new battery, plugs and leads with no effect and tried an injector cleaner but it’s no better.

Peter Knappett, e-mail

A. If you check the back end of the throttle bank there’s a little black box with a wiring harness going into the top.

This is the throttle switch and should cut off the fuel injectors when the throttle is closed so that the engine has extra engine braking, it saves a bit of fuel and also stops the bike backfiring on overrun…

These switches could get filled with water if the bike was jet washed and the contacts inside can corrode. As the throttle bank starts to open, a click should be heard from this box as the ‘closed’ contact inside it is a micro switch.

There is a second contact in the box which is the full throttle contact, but you will not hear a click for this one.

I would also suggest looking for air leaks. This will cause the engine to run lean. There is the crankcase breather pipe which is a rubber hose from the crank to the air box. This perishes and allows air to get into the engine without going through the airflow meter.

Another point to check is that the flap in the airflow meter is not sticking at idle position and that it moves freely. Sometimes an oil deposit can build up and cause the flap to stick. This can be cleaned off easily with a rag and some brake cleaner.