Small Engines (Lawn Mowers, etc.): Craftsman Lawn Tractor cranking difficutly, hard to start briggs

Question
I have a Craftsman 18.5 HP, 42" Mower with electric start and 6 speed manual transaxle, model number 917.273634. The engine has a very hard time cranking over on its own. The starter engages once, makes a single click noise and stops, or cranks the engine about a ¼ to ½ turn and stops. I have replaced the battery, starter solenoid, muffler, spark plug, air filter, and adjusted the valves. I have bench tested the starter and it seems to work ok on a bench. If I turn the engine by hand about ¼ to ½ turn, it is somewhat difficult to a certain point, then once it passes that point in the rotation, it is easy. After doing this, I get back on the tractor, re-attempt to start it, and guess what, it cranks over just fine and starts. It is as if there is too much pressure in the cylinder or something, causing an increased strain on the cranking process. As a test, I removed the plug and cranked the engine just to see what would happen, and it spins quickly with no issues, of course there is no compression either to slow it down. Is there some type of system that releases pressure to assist in cranking that could be bad, or could this be something else such as a bad starter? Any suggestions or tips would be greatly appreciated?

Answer
This has the Briggs engine doesn't it?  If you're certain that the electrics are good.  Ie: good starter, cables, connections and grounds, then move to the valvetrain and combustion chamber.

If you adjusted the valves at 1/4 Inch past TDC and you're satisfied they're right, then you may have a cam that's a bit worn. The compression release is a slight raising of the cam profile on the exhaust side.  Sometimes you can tighten up the lash on the valves a thousandth or two and solve the problem.

The problem you're experiencing is pretty common on Briggs singles with 100 plus hours.  Often all that's needed is to use SeaFoam in the gas.  This can help rid the combustion chamber of excess carbon that may be increasing the effective compression ratio.  We'll pull the heads after trying several times to adjust the valves.  Usually we'll find a good bit of carbon that needs to be cleaned off.  

Other things to look for are a bent exhaust pushrod and worn rocker or other valvetrain damage or wear.

When the valves are adjusted at 1/4" past TDC and adjusted right, I'd say that 1/2 the time it's built up carbon and the other half a problem with the cam or valvetrain.  I recommend using SeaFoam or Techron in the fuel to help prevent this problem once you do get it fixed.