Small Engines (Lawn Mowers, etc.): Lawnflite Garden Tractor, wont turn over., voltage drop calculator, voltage drop test

Question
Hello I have a Lawnflite garden tractor I am having trouble with. model#139-642-092/L158C

My problem is, it won't turn over.

When I turn the key, it acts like a drained battery, sometimes it will turn completely over but it groans as it does it.

I have replaced the starter, (bad armature) solenoid, (no continuity) and battery (didn't need to).

I have as much voltage to the starter as I do the solenoid,(11.45) and when the mower is running it runs great.

I have cleaned every contact I can think of, including grounds, and have 100% ground continuity from the  (-)batt to the starter, although I understand that doesn't necessarily mean a good connection.   

My next step is to run a jumper straight from the (-)batt to the starter in hopes of a better connection.

I guess I am at the limit of my small engine repair knowledge, without tearing into the motor itself.

I am about to start swearing, so if you have any suggestions I would be greatful.

Answer
While rare, I have had a couple of bad battery cables and once Snapper had a mower where the solenoid wire was to small to carry enough current for the starter solenoid to function properly.  Performing a voltage drop across each cable is a very good method to determine their condition.

http://www.aa1car.com/library/voltage_drop_testing.htm

Voltage Drop Calculator:

http://www.altronix.com/app_notes/calc.php

Video:

http://www.chacha.com/video/158075314/how-to-voltage-drop-test-your-car

I have also had a couple of older engines with bad/work starter pinion gears cause the starter not to turn the engine.  Once I had an engine with excessive carbon build up in the cylinder that would cause the engine not to fully spin.  

While all three of these situations are rare they are easy to check.  A voltage drop test only takes a few minutes to do so I'd start here, then make sure the starter pinion gear is not binding.  If you take the spark plug out will the starter turn the engine?

Can you send me the engine model, type and code number so I can look up the parts list? Some engines are prone to certain issues and I want to be sure your engine is not prone to compression release issues.

Eric