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First thing to do is to measure between the case and the positive lead on resistace to see if you get any reading.
If there is no reading, turn the pinion a 1/4 turn and try again.
If you then get a reading, the commutator probably has dead segments and it could well be a pattern motor.
You can now take the motor apart.
First, be careful with the three Philips screws.
They can be very tight and they are soft.
Put lots of pressure down on the screws as you remove them.
If they start to round off, stop and use a small pair of pipe grips or good pliers on the head.
Once the screws are out, grasp the pinion very firmly and then pull off the cover.
You will be left with the armatue in the end housing.
check the brush braids, the positive terminal will hopefully be broken.
carefully pull the armature out and be ready to catch the brush springs.
Clean up the commutator with a Scotchbrite or very fine emery if it is really contaminated.
Replace the brushes and now get another pair of hands and a couple of small screwdrivers.
Note that the springs on a genuine Roatax motor are of a rectangular section, nor round.
Put the new springs in one at a time and fit the brush in after it and hold in place with a screwdriver.
Fit the other spring and brush and hold accordingly.
slide the armatgure in and release the brushes once they are past the commutator.
Put a single drop of oil in the bearing in the cover and holding the pinion FIRMLY, slide the cover back on.
test for resistance between the cover and positive terminal and turn the pinion while you measure.
the resistance should vary but not drop completely to O/C.
Put the screws back and run the motor on a 12volt battery.
It will run either way round so don't worry about polarity, just don't jump when it sparks, as it draws quite a bit of initial current.
Good luck.
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