Originally Posted by
Petrol & Powder
That engine has an automatic compression release mechanism that holds the exhaust valve slightly open at very low rpm's (like when first starting to rotate the crank). If that mechanism fails, the starter motor must overcome the full compression of the engine and it will not spin fast enough to start.
So if you eliminate all of electrical problems (battery, cables, starter motor, etc.) the next place to look for slow cranking is the automatic compression release.