When we start the PW50 in the START position it will start, holding full throttle. When we switch it into the RUN position, it will only stay running when applying FULL THROTTLE. It stalls if you let off. {Jeremiah C., MI, USA - 2001 Yamaha PW50}
I am assuming that you have already checked the throttle cable and installed a clean spark plug. There is a small chance that you could have a
dirty carburetor. Use the Search Box below to search our site for more references to this.
It is more likely that one of the electronic components has failed. Please refer to the PW50 Wiring Diagram. We will reference this page below.
If your carb is good then let’s move on to testing the PW50 electronics. Starting with the ignition control switch. The ignition control switch is number 1 on the wiring diagram. The odds of this being the problem are pretty small, but it is better to test it now rather than find out hundreds of dollars later that it was only a faulty switch.
As with most electrical tests, the results are given for testing at room temperature or approximately 68°F, or 20°C. If you are working on your PW50 in the sun on a 95°F summer day, you may receive a reading that indicates the component is no good, when in fact it is. Please try to perform tests as close to room temperature as you can. You will need your multi-function electrical meter with a Ω or Ohms setting, or multi-meter for short, for the following tests. When it says insert your lead it can also mean hold the tip against, stab a wire or clamp on with an alligator clip.
Unplug the ignition switch from the wire harness. Set your meter to Ω. Insert the positive {+} lead into the black with red tracer {B/R} wire. Insert the negative lead into the black {B} wire. With the switch set to OFF you should have continuity. If you have no continuity in the OFF position, replace the ignition control switch. With the switch set to Start/Run you should not have continuity. If you have continuity in the Start/Run position, replace the ignition control switch.
If the ignition switch tests good, move on to the ignition coil, number 2 on the wiring diagram to test for primary coil resistance. Set your meter to Ω. Insert the positive {+} lead into the orange {O} lead or wire. Insert the negative lead into the black {B} lead. You should receive a resistance reading of 0.32 to 0.48 Ω. Anything outside of this means the coil should be replaced.
Next, test the ignition coil for secondary resistance. Set your meter to Ω x 1k or k Ω. Insert the positive {+} lead into the spark plug lead {or boot}. Insert the negative lead {-}into the orange {O} lead. You should receive a resistance reading of 5.68 to 8.62 kΩ. Anything outside of this {higher or lower} means the coil should be replaced.
Now you can test the PW50 CDI magneto, number 6 on the wiring diagram, for pick up coil resistance. Unplug it and set your meter to Ω x 10 or 10 Ω. Insert the positive {+} lead into the white with red tracer {W/R} wire or plug opening. This is the larger plug on the CDI magneto, held with the clip on top, right side opening. Insert the negative lead {-} into the black {B} wire or plug opening. This is in the smaller plug, held with the clip on top, right side. You need a reading of 18 to 22 Ω . Anything outside of this {higher or lower} means it should be replaced.
The last test you can do yourself is the CDI magneto for charging coil resistance. Set your meter to Ω x 100 or 100 Ω. Move your positive lead from the right side of the larger plug, to the left side, or the black with red tracer {B/R} wire. Leave your negative lead where it is. You should receive a resistance reading of 297 to 363 Ω. Once again, anything outside of this {higher or lower} means it should be replaced.
If all of these electrical components tests good, you either need a CDI box or a Control Unit, numbers 5 and 3 respectively. You may be able to have these tested at your local Yamaha dealership.
We had a similar problem with our 1999 PW50. We ended up replacing the control unit.
