What do you see out the exhaust when the big motor being cranked?
- If exhaust is clear it's not getting fuel. Open the throttle fully and crank try again. If it's still clear suspect plugged filter(s) or air in the injection lines.
- If exhaust white it's getting fuel but not igniting. Make sure to close the compression release soon as the pony is turning the big motor. Otherwise problem is low compression from worn or improperly adjusted valves or compression release mechanism, worn rings, etc.
- Finally if exhaust is black it's starving for air. Disconnect the air cleaner and check for a mouse nest, etc.