Possible causes of occasional backfiring of old BMW Motorbike?

I have a '56 BMW motorbike that has started to backfire occasionally on deceleration. This ranges from a moderate pfft to a loud blap. I'm unable to find the cause maybe someone has some suggestions as to the cause. The bike itself is running the best it's ever run except for this.Possible causes of occasional backfiring of old BMW Motorbike?In this case, it's probably unrelated to timing. If the engine runs properly under load than the timing can be discounted. What you are describing is not un-common in old vehicles.



When you back off the throttle, during the transition when the throttle is being closed, and before the engine reaches a steady state at the new throttle setting, there is a period where the fuel that was being introduced remains at the same level (high throttle). However, the throttle slide or butterfly closes almost immediately. There is therefore an excess of fuel which, due to lack of air (closed throttle), is not burned in the combustion chamber. It exits the combusiton chamber and ignites in the exhaust system.



On some engines, this will contine as long as the throttle is closed and the engine, being turned by the rear wheel(s) is spinning at a fairly high RPM. As long as this is occuring, fuel is being sucked in by the pistons at a greater percentage related to air (since the throttle is closed) and the backfiring may continue until the engine is at the speed that the throttle setting dictates.



Faulty carb setting or worn idle jets can contribute to this.



Hope this was reasonably clear and helpful!



Good LuckPossible causes of occasional backfiring of old BMW Motorbike?Backfiring is usually an electrical problem....or it could be a gas prob. but more than likely electrical.

First try to do the easy stuff.

Like check your plug wires and make sure that they are on tight and aren't broken anywhere.

check your timing....this could be the problem

have your coil(s) checked

is it getting enough gas?

Sounds like you've been working on it for awhile now and are so close to havin' it purr like a kitty.

Just go back through everything and don't leave anything out or skip over anything.

Seriously, start at square 1 and go from there.

If you do it step by step and not take any shortcuts you should be able to figure it out.

And if you start to get 'upset' about how things are not going...then you really need to take a break and get away from it for awhile. Sometimes when you take time away what ever is wrong will pop into your head. And it really doesn't hurt to get away from it every now and then anyway.

Good luck to you, it seems that you'll be riding before too long.Possible causes of occasional backfiring of old BMW Motorbike?Backfiring is caused by unburnt fuel exploding in the exhaust so check your ignition timing and that the automatic advance and retard is working properly .Also make sure the mixture is set correctly in the carburretor