So Your Bike Chokes Off At Stoplights

The by far most common request for help that I get goes something like this:

"I recently bought an XS650 (or had my XS650 serviced), and it ran fine for about a week. Then it started conking out on me at stoplights, but would start immediately at the touch of the starter button. Shortly afterwards it would no longer start on the electric starter, but would start on the kick start. I also noticed the turn signals would come on, but not blink. Now it wonít start at all. Whatís wrong?"

The Likely Cause:

These are the classic symptoms of a failing alternator rotor, the XS650ís Achilles heel. When the bike is bought, or serviced, it starts off with a fully charged battery. This fully charged battery will initially supply enough power to work the headlight, the ignition, and the turn signals, but because the battery is not getting a proper charge, it drops in voltage. In addition, the failing alternator rotor will only generate whatever charge itís still delivering at speed, and not at idle. So when the battery voltage is too low to support the ignition by itself, and the bike is at idle, the motor quits. Initially there is still enough voltage in the battery to turn the starter motor, and the bike will start and run when the revs are kept up a little, but as the battery voltage drops further there is not enough voltage to also serve the ignition while the starter motor is drawing power. So at that point it will only start on kick start. At about that time there is also not enough charge left in the battery to activate the turn signal flasher, so the flashers stop blinking. Finally the bike wonít start at all, as the battery charge drops below that necessary to trigger the ignition.

Confirmation:

Check as follows:

(1) If you rev your bike to 3000 RPM and the lights donít visibly brighten from what they are at idle, you can go straight to step (2), your alternator is not providing a charge. If your lights only brighten very marginally, check charging output in volts by putting a multimeter across the battery terminals. Make sure you have a fully charged and known good battery when checking charging output, otherwise all measurements are meaningless.  Fully charged means still at 12.6 volts at least 1/2 hour after you have disconnected the charger.  75 % charged at 12.4 volts is still OK.  The system could be putting out 14.5 volts, but if it is putting it into a battery only charged to, or capable of, 11.9 volts, it will give a low reading.

 

(2) If the bike is charging low (below 13.5 volts) on a fully charged battery, or not charging at all, the first thing to check is the alternator rotor.  This is most likely the cause, and if it has not yet been changed out, it is certainly time for the original to fail. 

 

(3) Remove the cover plate from the alternator on the left side of the engine (two screws). You will see the brushes in their plastic mountings. 

 

(4) Remove the brushes.  If you are lucky, they will be worn and need replacement, and that will be the only problem. But itís not likely.  There is a mark on the brush showing the wear limit, the brush has to be at least 7mm (.277") long.

 

(5) Measure the resistance across the rotor windings by taking the ohm reading between the two copper slip-rings on the face of the rotor. Be sure the brushes are removed when taking the reading, or your reading will be affected by the circuits feeding the brushes, and be sure the reading is taken directly on the slip-rings, NOT THROUGH THE BRUSHES. To reach the inner ring you will have to pass the probe of the multimeter through the inner brush mounting slot anyway. A stock OEM Yamaha alternator rotor when new would measure between 5.5 and 7 ohms, measured between the slip-rings. After market rotors are wound with heavier wire and with thicker insulation, to avoid the failures described above, which are caused by insulation breakdown. Because of this aftermarket rotors, even when new, may test as low as 4.2 ohms, but they are still good rotors. Any rotor will work satisfactorily down to about 3.5 ohms, especially if you ride the bike at speed for reasonable periods. However, in town riding with slow speeds, lots of stopping and idling, your battery will not maintain a charge on a 3.5 ohm rotor. In any case, it is failing. Once you get down to below 3.5 ohms, it has failed beyond service, and instead of providing current to the battery, it is drawing current, as much as the headlight is drawing current. Often they will get down to 2 ohms or less, at that reading they will kill the battery in minutes.

Note: There are other reasons that could cause the alternator to fail, but diagnosing and repairing them are beyond the scope of this article. The fact is, every single alternator problem Iíve ever been exposed to on an XS650, and there have been many, with one exception, were invariably due to a failed rotor. If it walks like a duck, quacks like a duck, and looks like a duck, it probably is a duck.

Temporary measure to get you home

Pull the headlight fuse, and remove the leads from the brushes. Tape the ends of the leads to prevent them shorting against anything, and replace the inspection side plate. Kick start the bike. If there is not enough life in the battery for the bike to start on the kick start, connect it to a car with jumper cables, and let the car charge it for ten minutes. Then try again, with the jumper cables still attached. Once it starts remove the jumper cables.

With the headlight load off and the load of the failed rotor removed, the bike will run about three hours on a fully charged battery. BUT KNOW THAT RUNNING WITH THE HEADLIGHT OFF IS AGAINST THE LAW AND CONTRARY TO YOUR OWN SAFETY IN EVERY STATE IN THE UNION; I AM NOT RECOMMENDING YOU DO IT, I AM JUST TELLING YOU HOW TO KEEP A BIKE RUNNING FOR A WHILE AFTER THE ROTOR HAS FAILED.

 

Repair alternatives.

Alternative (1): Used rotor

You may be able to pick up a used rotor, but if itís an original Yamaha rotor it needs to read 5 ohms or better. Chances of finding one are slim, these are desired items because of the high failure rate on bikes which are now more than 25 to 40 years old. If you do find one chances are the cost will not be much less than a rebuilt one, which is a much better alternative. Note a rotor off an 80 through an 84 bike will serve in any year XS650, but rotors off bikes from 70 through 79 will not serve in 80 through 84 bikes with stock Yamaha ignition, which must have the imbedded trigger in the rotor for the stock Yamaha electronic ignition.

See

www.650central.com/electrical.htm

for details of the alternatives (2) & (3) described below.

Alternative (2): Rewound rotor

This is the alternative most people will choose, particularly those with 80 through 84 bikes with stock Yamaha electronic ignition. A rewound rotor is about $175, free loan of a puller to remove the old rotor included. Do not try to remove the old rotor without using this special single purpose Yamaha puller, no matter how many universal pullers you own, nor how skilled a mechanic you are. The best you will do is fail, the worst you will do is crack open the crankcase.

Alternative (2) Permanent magnet rotor

An ideal solution available is to replace the alternator rotor with a permanent magnet unit, such as the Sparx unit, which does away with rotor windings and brushes altogether. This solution, long sought after by XS650 owners, is now available. Downside is the cost, just over $455 on 1970 through 1979 bikes, and an additional $275 for a Boyer Brandsen or other aftermarket electronic ignition unit needed on the 80 through 84, because the Sparx rotor does not have the trigger required for the stock Yamaha electronic unit.

 

Farrell Hope