I have an MTD Yard Machine powered by a B&S 12.5 hp code 9311064E type 0154-01. The engine cranks slowly and in spurts approximating the four cycles. When I remove the spark plug the engine spins normally. I had the battery checked at a battery shop and it was pronounced fine after a load test. A lawn mower shop told me that it was not the battery. The battery itself looks like a motorcycle battery, six inches long, 3.5 inches wide, and 6.5 inches high. It is a Sears model 28-9617. The MTD manual gives a battery part number of 725-1635. A Google search only turned up a replacement battery for this, Rocket 31-7724. It was longer (eight inches) and stated the cranking amps at 275 (seemed like a lot to me). A motorcycle shop also told me that it was not the battery.
My question is: What is wrong.
Additional Information
Cranking speed does not appreciably change with a jump start from a Wheel Horse with a known good battery - standard size.
I changed battery cables from 8 gauge to 6. No change.
I am thinking that if there is a compression release valve it is not doing its job but from the engine parts manual I cannot find the compression release valve.
Any help will be appreciated.

Id have the starter tested next. OHV or flat head engine?

Starter? Never really considered that. What's involved with a starter test? Is it on machine or bench test?

The engine is a flat head, run of the mill B&S. The label on the engine is I C, industrial commercial.

Basically just taking it off and hooking it directly to a battery and making sure it turns over fast.. if you can measure the rpms (never had to myself) it should spin the engine over 300 rpms or more.

You may also check the drag on the mower belts. If the mower is not releasing belt tension enough, it may turn over fine without compression, but bog down with the spark plug in.

You may also check the drag on the mower belts. If the mower is not releasing belt tension enough, it may turn over fine without compression, but bog down with the spark plug in.
Yes that will produce a drag... but in most cases the starter is still strong enough to start the engine, with the trans in neutral but clutch not depressed...
Even stron enough to start in gear, and deck engaged in some cases... but this is with safety switches removed...

I pulled off the started and two things caught my eye. First the starter is not painted Briggs & Stratton red. It is bear metal. There was no B&S numbering or label on it. Since I brought the tractor from a private party my guess was that there had been starter problems previously and the guy put anything that fit on it.
Second the bottom plate was deformed and cracked. It is some type of plastic. The armature turned but not smoothly. Bench test verified that the starter is not working the way it should.
I have ordered a new B&S starter. It should be here by the end of the week. I'll keep you posted. Thanks for the aid.

I have an MTD Yard Machine powered by a B&S 12.5 hp code 9311064E type 0154-01. The engine cranks slowly and in spurts approximating the four cycles. When I remove the spark plug the engine spins normally. I had the battery checked at a battery shop and it was pronounced fine after a load test. A lawn mower shop told me that it was not the battery. The battery itself looks like a motorcycle battery, six inches long, 3.5 inches wide, and 6.5 inches high. It is a Sears model 28-9617. The MTD manual gives a battery part number of 725-1635. A Google search only turned up a replacement battery for this, Rocket 31-7724. It was longer (eight inches) and stated the cranking amps at 275 (seemed like a lot to me). A motorcycle shop also told me that it was not the battery.
My question is: What is wrong.
Additional Information
Cranking speed does not appreciably change with a jump start from a Wheel Horse with a known good battery - standard size.
I changed battery cables from 8 gauge to 6. No change.
I am thinking that if there is a compression release valve it is not doing its job but from the engine parts manual I cannot find the compression release valve.
Any help will be appreciated.
Brig Stratton engine crank over but no spark in spark plug

brig Stratton engine crank over but no spark in spark plug34

It has been about three weeks and the new starter with the old battery on the otherwise unaltered engine has started the engine every time it was asked.

Thank you for your help.

No problem... glad to help!

brig Stratton engine crank over but no spark in spark plug

Check gap in plug/ check magneto-points with an ammeter.