Vibration Problems on a Homemade SPecial

[Fastjeff57]

Spent all winter (and many $$) building this beast and so far it’s a total loser. To wit:

Merc 850 (Big Four) power head mounted on a Mark 55 type tower and LU. Made an adapter that lines up within a 1/16 inch, and my buddy made the 8 spline to 11 spline drive shaft adapter. Though the motor runs beautiful at low speeds, the combination develops a HORRENDOUS vibration starting at 4,000 rpms. It’s smooth as silk below that, and a bit less nasty above that (though way too harsh for parts to survive).

I thought the driveshaft may be bent, or the drive shaft adapter running out. To see if that's it, I powered the prop shaft with a 3/8 drill (spinning the drive shaft at least 800 rpms) and, no run out whatsoever. So, what/ where/ why/ how am I getting that destructive vibration?

I’m asking here since many of you have adapted various power heads to racing towers and MAY have experienced something like this.

Any advice will be heavily appreciated, for the project is dead in the water (so to speak) at this point.

Jeff