Testing a ride-on starter motor.

[alchemysa]

Hello Gentlemen. This is the armature from the starter motor of a Stihl Viking ride-on mower with a B&S 18hp Intek single cylinder engine. The starter has always seemed 'weak' but now its unable to rotate the engine past the compression stroke, even when I've connected a fully charged battery directly to the starter with jumper leads.

There was a bit of muck on the bushes and commutator that I cleaned off. Apart from that it looked OK except for some pitting in the hard epoxy stuff between the windings. You can see it in the picture. There's about four pitted areas like this around the armature. The pitting doesn't look black and burnt and it doesnt appear to go right down to the windings. The windings themselves also look clean and healthy. But is the pitting fatal?

Is there some way to test the windings with a multimeter. The starter is here at home, but the mower is at the shack, 300ks away, so I'd like to establish whether this needs to be replaced before I head down there again. Thanks for any advice.

 

[aerostar]

The pitting isn't fatal unless it's evidence of overheating, in which case it is a symptom, not the cause of your problem.

I don't know what the winding resistance of that motor is supposed to be, but I can tell you that the resistance of all the windings should be the same. Try measuring the resistance of each winding by connecting an ohmmeter accross opposite contacts on the commutator. The resistance of all pairs of contacts should be very close to the same. If one is different from the others, you need a new motor, if not, your problem is likely elsewhere.

 

[Iplayfarmer]

The starter has always seemed 'weak' but now its unable to rotate the engine past the compression stroke, even when I've connected a fully charged battery directly to the starter with jumper leads.

Definately test your starter however you'd do it since you already have it off, but it sounds to me more like you have a problem with your valve clearances on the mower. These briggs single cylinder models have a bit of a problem with the compression release mechanism. If you don't keep the valves tuned just right, you won't fully release the compression, and even a brand new starter on a brand new battery won't spin the motor. Someone posted here a while back the valve clearance tolerances.

If the starter will spin the motor at all, it should spin it past the compression stroke if the compression release mechanism is working right.

 

[Tig]

My Briggs 2 cyl has done the same thing since I got it used three years ago. I can't find a problem with the starter other than it is weak. If it gets worse I'll check the valves. Thanks.

 

[zzvyb6]

In a similar manner, my ATV starter was always "weak". It struggled to make the first turn. I changed batteries, cables, hotter plugs, etc. Found a guy on eBay who sold new and remanufactured starters. Came in the mail 2 days later. WOOF ! Now its RUNNING on the first rev. The starter problem also was dropping the voltage to the ignition. I still have the old starter. Your post will make me get it out and check the winding resistance. I also wondered whether the bearings were worn, causing the motor shaft to stick. Brushes were good, as were the windings and epoxy. It just had no torque. Even when I put a wrench on it to feel the torque pulses, it just was weak at every stage in the rotation.

 

[shot_gun]

I agree with everyone else here, test the starter but I seen this alot where the valves need to be adjusted in order for the compression release to work properly. Usually you can tell by putting your hand on the flywheel while turning the key to the crank position to help it. It you can "help " the engine turn over with your palm then it will probably require the adjustment. Note please use a common sense and remove your hand immediately after it comes over compression stroke. We don't need anyone loosing skin now.

 

[alchemysa]

Thanks for the responses. I'd almost given up on getting any.

I used an engraving tool to grind out some of the pitted epoxy. The wires underneath looked OK and were not burnt out as far as I could determine. I guess the pitting was caused by overheating when the starter has been stalled for a few seconds by the compression stroke. But hopefully I havent cooked it enough to actually fuse or melt the wires.

The starter spins like a champion on a bench test but thats under no load so i dont think thats really telling me much. I'll take the starter motor apart again tomorrow and test the resistance as suggested.

One little tip I learned. Its much easier to re-assemble the starter if you pin the bushes back with thin nails before pulling off the end cap (or whatever its called). In this instance it took 4 nails to hold back the 4 bushes. I then held the nails in place with a metal ring - a round blade from a hole saw.

The valve clearance thing is also a good suggestion. I'll see if I can find that thread. I wonder also if that could explain why the engine has never revved as high as I would like?

 

[alchemysa]

Found the thread that iplayfarmer mentioned. Very interesting. Thanks

http://www.tractorbynet.com/forums/lawn-garden/123770-decompression-valve.html

 

[Tig]

I think I'll check my valve clearance this weekend.

 

[Iplayfarmer]

I think I'll check my valve clearance this weekend.

I only have done it with the one engine, but the difference was amazing!! Not only did it start better, but it ran better too.