Airhead Tech Pages


Parking Lot Carb Balancing Act

John Petty


 

The following may be considered idiocy by some but it is what I found to work after reading comments from contributers of the IBMWR and Airhead lists but mostly my own experiance. Some of this is also detailed in another article I wrote on the blue "K" coils from Bosch. Of course, YMMV. This is what works for me ... and probably your Airhead, too.

After using the shorted-sparkplug method and needing to buy a second set of coils for my electronic ignition motor, I thought up a way to do the job by cutting off fuel supply instead of risking burning out my coils again - not to mention unburned fuel coming out the tailpipes. I drained one carb's bowl and pinched the fuel transfer line passing through the rear of the starter cover (using a tool made for the job that will not cut into the rubber - you could also slip some fuel line over the jaws of a needle-nosed vise-grip) and switching on/off the taps to get the bike running on one cylinder. A small cutoff valve permanently installed in the transfer line (or one for each carb) would be a better solution.

Fully warm up bike if not already. Shut off motor, shut off fuel to one carb and drain bowl, restart and set idle to as low as it will run at a stable speed, stop motor, switch fuel line clamp, and drain other bowl. If you have a clean container to hold the fuel, or just remove the bowl, you can dump it back into the tank. Restart motor and set idle to exactly the same RPM as the other side. Release pinched transfer line. Your motor is then balanced at idle without any gauges except your tach - and without shorting the high voltage ignition circuit.

Cable adjustment requires a throttle lock to get them right. With only one carb getting fuel, bring RPM up to about 1500, make sure the throttle lock holds the speed setting, and shut down the motor. Pinch the fuel line to one carb again and drain the bowl. Start the motor and make note of the RPM - maybe use some masking tape and a pen to mark it or just tweak the throttle lock a bit to get it on the nearest tachometer marker line. Switch carbs as described before without disturbing the throttle lock, restart motor and set the cable adjustment so that the RPM is the same as the first side. Release the fuel line to refill the carb bowl and release the throttle lock screw.

Better accuracy is easier to obtain if you set the tach needle directly on the middle of one of the marker lines on the tachometer face. You should be able to do this quickly enough not to need a fan but do take care not to idle too long without good airflow between the fins on your cylinders. Different temperatures can make a difference in the idle settings when switching cylinders so a fan can do more than just keep your motor from overheating.

I only suggest this method for emergency parking lot adjustments far from home. If you can get home to your tools, then do it right with some gauges of some kind. I actually did this to the Mikunis on my R100RT and someone else's R90 with Bings but it should be similar with DelOrtos or any other brand/type.

In my own humble opinion, NEVER, EVER balance your carbs by using the shorted-spark method!!! This may have been marginally OK on Airheads with points and lower current (much lower) but the higher current you will get from the blue "K" coils (and especially on an electronic ignition motor) will kill your coils. Yes, BTDT, bought new coils AGAIN! I guess it was the feedback from the shorting or some other reason but the rougher running went away when I got the new coils. I used the popular little spark plug extensions and always had a path to ground either through the spark plug or a metal grounding strap attached to the extension. It happened over the course of three adjustments but got a little worse each time. Yes, I know some people will tell you that I'm full of manure but I'm the one who had to go buy new coils because of it. If possible, use some mercury tubes, a Twinmax, vacuum gauges, a good musical ear, or the method I described above.

John Petty ~ 1983 R100RT and 1996 R1100RT-P


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R-Tech pages maintained by: John Petty for the original author(s) and the Internet BMW Riders
Last Update: 11 September 2010