Small Engines (Lawn Mowers, etc.): B&S Starter Weak, using jumper cables, negative cable

Question
My mower engine is 12.5 HP, B&S Model 28U707, Type 1113-E1. The starter won't turn the engine unless the spark plug is removed, thereby eliminating all compression. I test the starter using jumper cables to directly connect my car's battery to the starter. I connect the negative cable to the starter flange. I touch the positive cable to the nut on the starter where the red wire from the solenoid is attached. I think I'm making good connections. I've used this method successfully on other mowers before.

I took the starter off the mower and removed the end plate. The brushes look good with lots of material left. Everything in the starter looks good. It runs fine when removed from the engine. It just doesn't seem to have enough torque to turn the engine with the spark plug installed. The mower is several years old but hasn't had a lot of use. Do these starter's get weak? Or is something else going on?


Answer
Two areas to check, is the battery the correct size for the mower, in the stores they will sometimes have 4 different batteries for a mower, they will all look the same, be the same size, the difference is in the cranking amps. Most people just by the cheapest one, but they lac the cranking amps for their mower. Second, there is what is called a compression release, it keeps the valve open a little to help relieve the compression during starting purposes. it is located in the oil pan on the lower portion of the engine. And yes the starter could be weak.........Tom