Forum bmwbike.com

Sometimes when I am at a red light traffic light on my R26 it starts to rev very high for no reason and without me advancing the throttle. I have cleaned the carburetor and it still does it. Any suggestions?

Steve

For this to happen, surely the throttle slide must become raised. Is it happening after you blip the throttle while waiting at the traffic light? If so, perhaps the throttle slide becomes stuck and does not return. You could check the slide to see if it moves up and down easily without binding. If the slide is binding, you may be able to see scuff marks and smooth the area with emery cloth. Or, perhaps the throttle spring is has also become too weak to hold the slide down, and you could replace the spring. You can watch the action of the slide if you remove the connection to the air cleaner and turn the throttle grip.

If the throttle slide is not responsible for the problem, then the advance could be sticking. If the advance unit is sticking in the advanced position, then the revs will stay high when you come to a traffic stop, and then may decrease. If this is the case, the advance unit may be worn and need replacement. You can watch the timing marks if you attach a stroboscopic timing light. After high rpms, the S mark should reppear as soon as you release the throttle.

I understand from your email that the rpms increase spontaneously with no action on your part.

My R27 does not maintain its idle speed very well. Sometimes it idles a little faster than at other times, and I fiddle with the idle stop screw. This may be normal.

But if the idle were to spontaneously begin to increase suddenly with no action on my part, then I would suspect the carb's idle mixture (air) screw, the idle gas jet and the knurled slide stop screw.

When you cleaned the carb, I wonder if you removed all the jets and screws and blew through them with compressed air. Some say that the best way to clean jets is to pass a fine wire from a wire brush through them.

Check that the tank and petcock filter is clean, so more dirt cannot get into the carb. I have also added additional visible fuel filters to my bikes.

Check that the short hard spring under your idle stop screw still strong enough to lock that screw, so it cannot change adjustment by itself due to vibration.