8. Troubleshooting

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See also: 1.06. Foibles

When it won't start

Before going through any effort to remove anything (but after the obvious checks of petcock and choke position), I always simply pull the spark plug boots of the plugs (they just slide right off), stick my finger in the boot, and give it a little kick start. If you get a mild shock, like a static shock but with a bit more tingle, you know that that coil is working fine (the 650s have one coil for each cyclinder). Try it with the other boot to test that coil and lead, and if both give a mild jolt, then it's time to move one and take out the plugs. If you are not getting jolts, your problem is in the ignition system (the timing or coils). If the plugs are filthy and black, clean them off. If you are using basic cheap plugs, clean them off with a wirebrush (or buy new ones.) If you are using plugs coated in gold, silver, iridium, platinum, or yttrium, clean them off with a toothbrush (not one you plan to use on your teeth) and naphtha or alcohol so you won't damage the coating. Be sure to determine why they are dirty: either they are really old, or your fuel mixture is too rich, or you're trying to ride with the choke on. Reconnect the boots to the plugs without putting them back in the cylinder, and let them rest against the engine to ground it. Give another kick start, and you should see little bright blue flashes come from the spark tips. If you don't, time to get new plugs. If you do see the sparks, the problem is fuel related, not electrical. If the bike is giving you a lot of problems starting, but eventually runs fine, your carbuerators could be dirty. I had extensive problems with this, where the bike could take 10 minutes or more of kick-starting before it would finally catch. The fix was simple: a bottle of "fuel injector and carbuerator cleaner" for $1 in the fuel tank cleared it all up (once I was finally able to get it going).


Diagnosing if a cylinder miss is due to electrics or fuel

Farrell Hope

Farrell's articles


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.


Easier way...After swapping tension leads on the plugs, lift the tank, follow the wire loom from the points up to their connectors, and swap at that point...(also lets you check cleanliness of the the connectors)

Chris Tulloch


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