Do you have a multi-meter? If so you can check the ohms of your rotor and stator. You can start by pulling your brushes and measuring them. The book calls for at least 1/4 inch, I like 3/8 inch better. This could be your problem.
While you have the brushes out is a good time to check the rotor. Set your meter to it's lowest ohm setting. Touch the probes together. Whatever you get as a reading remember it. You will need to subtract this reading from your test readings.
Touch a probe to each slip ring, the places the brushes touch the rotor.
Lets say that when you touched the probes together you got a .4 reading. When you touched the slip rings you got a 5.7 reading. Subtract the .4 from 5.7 gives you 5.3 ohms. Your rotor should be 5 ohms +or- 10% so 4.5 to 5.5 is good.
Your reading is 5.3 well with in specs.
Also check from the slip rings to ground, should be infinity to ground.
You can check the stator about the same way. If you follow the wires up from the stator you will find a large plug with 6 wires in it and a single plug with a yellow wire. unplug both plugs.
Now locate the three white wires in the plug from the stator. Touch your probes to each pair of white wires. You should get a reading of about .9 ohms, remember to subtract the reading you get when touching the probes together. So lets say you get the .4 used earlier. You get 1.4 ohms - .4 ohms= 1 ohm on all three pairs of white wires. The .9 ohms is a +or- 10% too. So anything from .8 to 1 is good. You may get a different reading, some stators spec .46 ohms. As long as they all match and read infinity to ground its ok.
If your parts fail any of these checks they may need to be replaced.
Do the checks and report back your results.
Do you have a repair manual? I would suggest you get one. They can be gotten several places. I like the Yamaha Factory manuals best, After that the Haynes and Clymer are about even. They both have good and bad points. Biker.net has them as downloads.
