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This is a tecumseh engine. The starter turns over but is weak. I take it out, use a jumper to start it (not from battery, but from a box). It turns over. However, when put into the engine, it would not be able to turn the engine more than 1/4 turn. The engine can be turned by hand easily.
I took the starter apart, and I don't see any suspicious. Do you think replace the brushes would help solve this problem? The shaft can turn freely, and there is not free play.

When I put in the engine, I also use the jumper, not the battery. Also, the jumper would shut down due to the starter stoppage (safety). Is it normal for the starter to exhibit "shorted" behavior when it's stopped? It seems normal (electric engine stopped due to overload force would trip electric fuse).
 

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It is hard to understand you are trying to say.
If the starter is turning the brushes are ok. It sounds like engine problems.
Stay tuned and the experts will find your problems.
Frustrating I know.
 

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st,

The starter is basically only a heavy duty, high torque direct current motor. As with any DC motor it can have minor problems that aren't obvious in inspection or testing, specifically bearing (or bushing) problems and poor conduction from the commutator, the place on the armature where the brushes contact it.

I would start by checking the commutator. If the pads are black it's dirty. They may also be glazed where they have a nonconducting coating from the brushes. Polish it with a piece of emery cloth or sand paper until it is a bright brass color. Make sure to clean the sanding residue off completely with air or alcohol or the small filings of copper may short out the commutator. If you have an ohm meter your can test the continuity of the coils on the armature by touching the pads on opposite sides of the commutator. Each set should have very low, near zero resistance.

Another problem is the bearings or bushings. When the starter engages, the armature it is pushed away from the gear it drives increasing the wear on the bushings, especially the drive end bushing/bearing. It may be more worn than it appears. Replace if possible. If not, get some graphite grease to lubricate them. (Usually, the bushings are very thin and impregnated with patches of graphite. You don't push them out to remove them, you deform them by bending them with a small shart object between the bushing and its holder to provide a grip for long nosed pliers to pull them out. The new thin bushing is best inserted by putting the bushing on the appropriate part of the armature shaft and gently inserting the shaft into it's end of the starter. The thicker bushings or bearings require pressing out or pulling out with a special tool. Some require replacement of the part holding the bearing.)

The reason the bushing wear is critical is because to get the high torque out of the starter motor, the armature must be very close to the field coils/magnets. When they are so close it doesn't take much bearing wear for the armature to start draging on the field coils. (You might also check for any loose field coils which would throw the field coil pads into the armature.)

Unless there is something significant wrong (burned out armature or field coil winding, or damaged field magnets), these two fixes will fix most starter problems. The other main problem, which you don't seem to have, is corroded starter solenoid contacts which will cause a clicking noise and not engage the starter with a fully charged battery. (To fix this problem, you must carefully take the solenoid apart and clean off the contacts, generally with a small file.)

Try cleaing the commutator first, it may solve your problem by itself. The bearing fix will require money.

Runningbare
 

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Is your battery fully charged? Also, perhaps the "box" can't provide enough current to run the starter under load. Try jumping it from a known good battery. Also, make sure your grounds are good.
 

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Check the engagement of the starter to the ring gear on the flywheel, it may be not engaging properly or is wedging itself into the gear.
 

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Discussion Starter #6
Is your battery fully charged? Also, perhaps the "box" can't provide enough current to run the starter under load. Try jumping it from a known good battery. Also, make sure your grounds are good.
The box has 55 AMP current, which should be good enough. I use this jumper box many times before without any problem. The battery is charged, but disconnected, so it should not play into the equation here.
 

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Discussion Starter #7
Check the engagement of the starter to the ring gear on the flywheel, it may be not engaging properly or is wedging itself into the gear.
Good point about this, it appears the engagement is fine. When running outside of the gear (for a few seconds), the spring loaded gear of the starter doesn't popup sometimes. This shows a weak starter. If the torque is strong, then the round metal piece will have a hard time catching up, since pushed the gear up. Also, without holding the starter, I was able to run it (usually, it would dance around if not holding it down).

If you have an ohm meter your can test the continuity of the coils on the armature by touching the pads on opposite sides of the commutator. Each set should have very low, near zero resistance.
I use an ohm meter and measure the opposite side, and there is 0 resistance. Interestingly, I measured from one point, to all other points around the pads of commutator, and there is still 0 resistance. This means it's shorted somehow. I wonder how it turns over in the first place. Is it correct to conclude it's shorted because of 0 resistance from any point to other point around the commutator pads?
 

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st,

You're getting a reading indicating a short or multiple shorts. Each pad is supposed to connect to only one other pad through one winding that is wound around at least two of the core towers on the armature. I suspect the problem is that the pads on the commutator are shorting from the brush (graphite/carbon) debris, but the entire armature can possibly be shorted. Even so, the battery/charger could be providing enough current to activate the correct winding with enough electomotive force to overcome the stray force from the shorted windings. That's why it still works but is weak.

You need to check to see if there is a short between the commutator pads and the actual metal of the armature. Measure the resistance between any of the pads and the actual armature. If it is shorted, the starter may have overheated and destroyed the insulation on the wires. If so, you will need another armature.

If the short is isolated to the commutator pads, clean the pads taking particular care to clean the spaces between the pads to eliminate graphite buildup.

RB
 
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