471 low engine power

"I operate a 4-71 (generat

"I operate a 4-71 (generator service - normally aspirated, Woodward governor) which suffers from low engine power. When you load it up, it looses RPMs but never smokes at all. I bought it recently with 5,000+ hours. Runs very well but with low power - of all the Detroits I have, this is the only one that never smokes even a little. Normally, I would expect clouds of black smoke when it lugs down.
I'm pretty sure it is a fuel problem since it runs so clean. I have fixed:
* Replaced all suction and return lines to tank plus fuel filters.
* Checked fuel pressure and flow both in and out of head - normal (60 psi and 1 gpm overflow, no air in fuel).
* Replaced injectors and made all tune-up adjustments on governor, valves, and injectors.

Is there anything I have missed? Is there something other than fuel problems that could cut the engine power but not produce smoke?
Thanks,
Joel."

"Hi Joel,

It sounds like it

"Hi Joel,

It sounds like it could be is a weak governor weight spring. The reason I say that is that you say it isn't acting as if it's being loaded and asking for fuel. Can you run it with the valve cover off (messy, I know) and watch what the racks do when you load it? It should push them in pretty quickly and then they should glide back out as the weight compresses the spring. If it has an old style control tube without the spring loaded levers, it could be hanging on a sticky injector rack too. I've seen guys overtorque the injector hold down clamps and deform the body enough to cause this.
Good luck."