Bax and all who commented,
Today I verified the loose nipple was the problem. A month ago my oldest son's table saw died. I asked for the steel stand it was on. Today I was up in the attic and found the Easy Just motor test stand I bought while a teenager. Price on the bottom was $1.30. It is made ofhard maple wood. I have never ran any thing bigger than a 35 in it. The FS91 fit and by starting it at lowthrottle, and immediately getting behind the prop,I thought it would be worth a try. I bolted it to the table saw stand and lashed the standto a 17 inch Green Ash tree.
I had tightened the loose nipple about a full turn.
First run - Evolution
15 x 6 prop - 7500 RPM. Last time it peaked at 7000 RPM. Did not touch the needle valve. Loosened the nipple 1/2 turn.
Second run - 7000 RPM. Re tightened the vent nipple.
Thirdrun 14 x 6 APC prop - 9,100 RPM. Saturday on a 14 x 6 APC prop It could not power through the wind like I needed it to. There was no 9000+ RPM available!! This is the highest RPM I have seen on the Tach on this engine. Started to adjust the needle to see how much more was available when it started to die, the two ounces of fuel in the test stand tank were gone. End of testing.
It is important to understand the key definition of how this engine operates to visualize the problem I was having.Note: there are TWOnipples connected directly to the crankcase. The first is the common breather vent which on this engine isplumbed to the intake manifold. The second one ismounted in the center of the backplate and is connected to the Fuel Pump.
"The varying pressure in thelower crankcaseactuates the diaphragmin the Fuel Pump allowing it toprovide sufficient fuel, under pressure, to the Pressure Regulator to allow it to provide the required amount of fuel, at a constant pressure, to the Carb."
The loose crankcase vent nipple was causing a loss of varying crankcase pressure preventing theFuel Pumpfromproviding sufficient fuel to the Pressure Regulator to feed the engine at high power settings. It was acting like a Governor!!.