I forgot to mention a stall test also.
Bstar makes a good point. The sprag clutch in your torque convertor could be slipping. This happened to me the first time on an MA60. Perkins 4.236, low power. Machine had about 10,000 hours on it. The owner/ my boss was sure it needed an engine. He allowed me to contact White and discuss the issue. The support tech told me to go into tilt relief and if the engine could overcome the pump and speed up to call him back. When I called h again he had me stall the truck against a pipe in the concrete and compare the stall speed against a sister machine. There was an obvious difference. The convertor was locking up about 800RPM, much too low to make any power. We replaced the convertor at customer location and the machine pulled great! Man, do I NOT miss those trucks!
Steve
If all of your engine checks (fuel, timing, etc.) all check OK. Take pressure reading on the toque converter. I have seen it many times. Does the engine rev up to full power and RPM's in neutral, when you put it in gear it takes off like a slug and gradually builds about three quarters power and RPM's. If so, definitely take pressure readings.
If it is Impco you need 1.5-2 psi @ test port. Also check your L-R inlet valve @ carb, you may need to set it more rich.
Steve
What fuel system do you have on this truck?