My motorcycle wasn’t running very well. There was an intermittent miss fire at varying throttle positions, and it wasn’t idling very well. I checked the spark plugs, which looked fine, and decided that the ignition system was probably all right. That isolated the problem to the carburettors.
Vacuum ports and Adjustment Screws
My motorcycle has 4 inline carburettors, with a shared linkage, 3 adjustment screws between each pair of carbs, and the throttle connected in the middle.
The problem was that each carburettor wasn’t allowing the same amount of air through. This can be remedied with a tool called a manometer that measures the pressure difference at two points. Most multi carb setups have a small vacuum port between each carb and the engine, which is usually plugged with a screw. I took that screw out and found out it was an M5 x .8 metric thread. I then bought a hex bolt with that size, and drilled a hole down the center.
hex bolt with hole drilled through it
I then built the following tool from these plans, slipped the hex bolt into the end of the tube, filled it up with water, and balanced all of the cylinders. This took some trial and error, and I had to be careful that the carbs weren’t too far off or else there would be enough pressure to suck the water into the engine. I started on the right two cylinders, and adjusted the farthest right adjustment screw. I then moved left. When I was done, the problem was fixed, and the motorcycle was running perfectly.
DIY manometer
DIY Manometer
