Forum bmwbike.com

Hello, I'm stuck. One plug has a fat spark while the other one has a weak one, weak enough so that the bike is running only on one cylinder .After a while the engine dies completely; I wait a few minutes, curse the bike terribly and then it runs for about 200 m until same story happens again: reducing to one cylinder and then stop. Where can I find the trouble? Valve clearance is correct and timing is ok. Thanks in advance!

Try swapping the plugs and wires from side to side to see if the problem follows them.

How old is the coil? It usually fails slowly indicated by hard hot starting and then missing at idle. The coil usually gets weak on both plugs but not always.

Also clean the pionts with some stiff paper and try a new condensor but that will afect both plugs at once.

Sounds like the coil may be going bad. Both plugs normally fire at the same time with the same energy going through both; but if the coil is shorting internally one side will be weak. When the coil heats up the problem becomes worse.

Hi, I had a similar problem with my R51/3. The engine would initially fire on both cylinders initially and backfire after a minute or so. On closer examination I found the spark on the on the right side weak. A friend & self pulled out the magnet, on checking it we found the coil was defective. I had the coil rewound and now both the cylinders are firing and the bike also starts in half a crank. Please check the fuel flow to the carbs. I hope your problem is sorted out soon. Regards

Thank you all.