Possibly the valves should be set so each is centred, rather that an "end of stroke" setting?
I should guess that the "strong"exhaust is the wrong 'un. Possibly a problem of overlap? Where the valve permits inlet steam to directly pass into the exhaust as the valve moves? Can you easily remove the valve covers and double check valve port spacing? Particularly the land width between inlet and exhaust? And on the valves, check the centre space and width of the sealing land at each end of each valve? When you are sure everything matches the drawing, reset the valves at mid-stroke to be exactly correct.
Oh, a check before you disturb anything.... apply just a few psi of air to the inlet, not enough to drive the engine, and rotate the crank carefully, to determine the exhaust valve opening and closing times against the crank rotation (even make and fit a degree plate to do this, or stick a circle of card onto the flywheel, and after marking it use a protractor to measure the rotation angles for each event. You should also check that at the change point between the inlet and exhaust strokes it doesn't just blow air straight through the valve. You do need to play with this so you can feel the pressure of the air acting on pistons, without it pushing past any point where friction stops it from turning. Easier on a tight engine than a free-runner?
You may need to do this just one cylinder at a time.
Enjoy,
K2
Post your results
Enjoy,