Forum bmwbike.com

Hello and thanks for showing interest in my problem.
I have a 72 R75/5 that has been running great all spring. I had been cruising for a hour or so the other day when my bike started running really rough. It would bog down when I gave it the gas and it started popping and back firing too. I though I had some bad gas...Filled it up- no change. Got up the next morning and inspected the fuel bowls- they looked great, no water, debris, or gunk were present. Started the bike and it ran great for a few miles, then it started running like crap again. Made it back to the house and pulled on each of the carburetors throttle cable (independently)...left side was the one that got me home. It seemed to be firing properly. When I pulled on the right side it would bog down and try to shut off. I Disassembled this carb and the only thing that looked bad was a needle jet's o ring was in pieces. This jet had tiny holes in it, I'm guessing for atomization. Everything else seems to be in decent shape.
Any advice or suggestions would be greatly appreciated.
Thanks in advance,
JoeP

JoeP -

I wonder if your tank is not venting properly. If it builds a vacuum after a few miles, gas won't flow the carbs. Next time this happens, quickly and safely crack open the gas cap and see if the problem goes away.

Another thing would be the height of fuel in the float bowls. If the floats are set right or the floats aren't floating anymore, the fuel level will be too rich or too lean. When this happens again, you could quickly and safely turn off the petcocks, kill the ignition, and drift to a safe area. Drop the float bowls. The height of the fuel in the bowl should be about 24mm, measure with a stick in the center of the bowl. You can also so this at home before you ride. Petcocks on for a few seconds, then off. Pull the bowls and measure the height of the fuel.

Have you checked your rate of fuel flow? It should be at least 350cc per minute. Find a graduated cylinder and pull the hose off of the stub where it goes into the carb. Then turn on one petcock for a specific period of time and compute the rate of flow. Try it with each petcock and try it on each side of the bike. You want to see if fuel can adequately flow from say the left petcock to the right carb through the crossover.

Kurt in S.A.

Kurt,
Sorry its taken me so long to respond. I did a full rebuild on the carbs and replaced the floats. Bike is now running better than even. I suspect that bad O ring had something to do with it, but mostly i think it was the floats which had turned dark brown in color and were not level anymore. In any case thanks for your suggestions!
JoeP in AVL