BMW Airhead R100RS problem diagnosis

Evening All,

Bought an R100RSthe around 7-8 months ago and dint get chance to run it until recently, although I did turn it over regularly. After some minor adjustments to the Mikini carbs (by ear!) it seemed to be running spot on despite being guilt in 1979. I did a short 30 mile ride into the Derbyshire Peaks and all was going well until I turned for home. Going downhill I got back-firing, racing engine as though the throttle was being twisted, then cutting out. Struggled home and started scratching my head as to what the issue was. Came across downhill deceleration backfiring, and the symptoms sounded exactly what I’d had.

Stupidly I played about with the carb settings, and since then I just can’t get it to run right. I’ve checked the plugs and it’s suggesting too rich a mix but the minute I cut back the fuel it stutters and cuts out. The strangest thing I’ve noticed is that when its nearly running right, if I tilt the bike to one side (the right) the engine struggles and tries to cut out...I bring it back to level standing and the engine picks up. Getting blue-ish smoke from the left hand pipe but none from the right hand pipe.

Been looking for a good old-fashioned mechanic to take a look if anyone can recommend anyone from near Stockport.

Any thoughts on what the issue is based on the symptoms?

The carbs- every 1970’s BMW R100 I have had has Bing carbs fitted.

If it’s running too rich that is where I would start. Anybody’s guess what jets Should be. The only other options were on the 90s - very similar to the Guzzi ones but not exactly the same.

If you park it on the side stand they always smoke from one of the exhausts. Apparently oil pools in the rocker covers and seeps down the vale stems - a recognised “character” of the old airheads.

MotoWorks BMW might be able to help. Also has the exhaust been changed - they are a bit sensitive to that as well.

Thanks M620....I have a set of Bing carbs that came in a box of spares so might give them a whirl.

I think I’ll strip the Mikuni’s down and give them a good clean first and go back to factory settings.....start there and see where I end up!

I'd be all over the rubber boots on either side of the carbs first, have you tried spraying WD40 at them while it's running? If the revs rise you've got a leak.