Diagnosing if a cylinder miss is due to electrics or fuel

Very often one will have a situation where only one cylinder will run badly, or not run at all. The first step in diagnosing the problem is to determine if the problem is of fuel delivery or of electric origin.

As a rule of thumb-

- Fuel problems usually give erratic and problematic running, but don't stop the cylinder from running completely.

- Electrical problems usually stop things dead.

But this is not always true, and sometimes it is necessary to isolate the problem to one or the other system. Fortunately, as the twin cylinder XS650 motor is virtually two completely separate single cylinder motors side by side, this is easy to do.

Step One: Test the plugs.

Swap the sparkplugs between the cylinders, and start the bike. If the problem changes sides, it is the plug at fault and you've found the immediate problem. (However you do need to check to see why the plug stopped working, it may be an indication of other root causes, such as too rich a carburettor setting or poor rings and valve guides.)

If the problem does not change sides, it is not the sparkplug; go to Step Two.

Step Two: Test the firing systems

Swap the firing system between cylinders. This is simple to do, and is done as follows for the two different designs:

Electronic ignition (1980 to 1983)

Just swap the high tension leads between spark plugs by crossing them over behind the cylinder. That's all that is needed; the electronic ignition wildcat sparks. That means there is a spark in both cylinders every revolution. As the non-firing cylinder is at the top of the exhaust stroke this does not bother anything.

Points ignition (1969 to 1979)

A little trickier, but not much. Swap the high-tension leads as described above. Then take the cover plate off both the points and the auto advance unit. Remove the nut on the auto advance unit, and take off the cam mounted on the advance shaft that the weights engage. Be careful not to lose the little locating pin in the shaft, and leave this little pin in place. From the other side, put a wrench on the nut that holds the points cam in place, and rotate the points cam and advance shaft 180 degrees. Then reinstall the cam in the advance unit, being sure to engage the weights. The locating pin will still be in its original position in the shaft, but it will have rotated 180 degrees. As the cam turns once for every two crankshaft revolutions, you have just changed the firing phase by 360 degrees, and the spark will now come at the correct time for the crossed high tension leads.

Now start the bike. If the problem changes sides, then it is the electrical system. If it doesn't, then it is not electrical, but most likely carburetion or fuel. It could also be mechanical, but mechanical problems usually give a warning, are a steady deterioration, and are usually not erratic. They are also usually also accompanied by very unpleasant grinding, clunking and banging noises.