I have a Pearson 10M that I just repowered with a M25 diesel. I agree with the suggestion above: the friction bolt will fix your issue.
Remove the Compass, being careful in removing the wires for the compass light, and look at the screws/bolts on the throttle shaft side. There will be two of them. One that is a smaller bolt that keeps the throttle lever in place horizontally. The second, which may be a hex head bolt will tighten the throttle lever when you tighten it. It is easy to adjust and check in real time by moving the lever. Adjust it and take the boat out (you can motor w/o compass for a while) and check the friction at various RPMs. Close up the compass after final adjustment.
Hope that helps.