1977 Evinrude 140 stutter/speed issue - Giving me fits.

[ghostwave]

Would sincerely appreciate some thoughts on where I should go looking to fix an issue that's giving me fits.

1977 Rude 140 on a 1981 Hydrapsports 2100 cc.
Newly Replaced:
Fuel Pump
Powerpack
Coils
Plugs
Rectifier
Rebuilt Carb
New four blade 15 pitch Solus aluminum propellor.
120PSI all cylinders.
Original Stator and Timer Base. Mostly original wiring (harness, etc).
SE Sport 300 Hydrafoil (new) that definitely helped it plane out and turn more smoothly.

The motor idles smooth and pulls strongly up to around 4300 RPMs and just puts the boat on a plane - I haven't GPS'd it, but reckon this happens at around 19-21MPH. Sometimes I can coax it up to 4500 - and another MPH or two, if I advance the throttle very slowly, but any higher than that, the Rude starts to stumble. Seems like an ignition issue. But then when I push the throttle up all the way, the stumbling disappears and RPMs go to around 5000 and not much higher. The motor then seems to bog a bit at those speeds (but no stumbles) - and even tho it's running at higher RPMs, the boat actually slows down by a few miles per hour over the speed it had at at 4300RPMs.

Could my timing not be advancing properly? Could the stumbles be the start of cavitation and then when the motor gets faster than the low-mid 4000's the prop starts aerating and thus the slowdown in speed? I'm reasonably sure that even tho it's mounted on a plate that lowered it some, the motor is still a bit too high for this boat (if I lower it anymore w/the plate, I can't trim it). It originally had a Johnson 235.

Professional - or unprofessional opinions WELCOME!!
Here is a photo of the bottom of the boat in relation to the propellor/lower unit.

Thx

 

[Daviet]

Re: 1977 Evinrude 140 stutter/speed issue - Giving me fits.

A couple of thoughts.
Use an inductive timing light and have someone run the boat for you, check each plug wire while it is stumbling. Check for a constant steady flash of the timing light, if you have an ignition problem you might be able to see it.
By the first picture the engine is mounted to high. The cavation plate should be even or just a little above the bottom of the boat. You can use a stright 2X4 and a jack to hold the 2X4 against the bottom of the boat and let it extend back to the lower unit. If the tips of the prop or the water intake is higher than the top of the board, you need to lower the engine.