I bought an older, year unknown (Serial Number AVB57574A989 if someone can identify the build) Avon Redcrest 9' Dinghy. Using the stock Avon Transom mount, with a 4 HP (The placard rated hp) Mercury Standard shaft engine, tilted out to last hole, at about 1/4 throttle and higher, the engine, along with the mount, are "tucking under" the boat.

How can I correct this?
Will rigging a higher mount help?
Is it a height of thrust issue?

Thanks