1922 Super 6 Max Engine Revs

Unknown

Hi fellow owners,
Having now more or less sorted the clutch problem, am now finding, after running in my rebuilt engine for about 1000 miles, that the engine will not exceed about 2000 revs in top gear, which with my back axle ratio and wheel size gives about 50 mph. Although 50 mph is probably enough, there seems to be something wrong that my foot is on the floor at that speed. I remember having read that a Super Six should have a top speed of 70mph, ie about 3000 revs.
My rear wheels have a diameter of 38 in and the rear axle ratio is standard at 4-5/11 to 1. The plugs are all new as is the ht wiring, points, etc, ignition timing spot on, advance weights work.
Valves all ground in with correct clearances, piston in carb free to go up and down.
What else could be wrong?
Any suggestions would be most helpful
Robert Frankcom