86 Johnson 28 Surging

[RVPastorKevin]

‘86 Johnson 28 on a Sundance 16’ fiberglass center console skiff. I just purchased this boat, and the original owner repowered it with this engine, after the first engine blew.
Now, With just me on the boat, and the engine trimmed to the second closest trim hole, it will top out at 15mph. However, in a turn, or in any other trim hole, the engine surges.

With me and my wife on the boat, the boat will only go 9mph, and anything past 3/4 throttle will make the engine surge.
The engine has new coils, I rebuilt and cleaned the carb, new prop, new doel-fin on the cavitation plate, new plugs, new fuel pump, new fuel line and bulb. Cylinders have 120 (top) and 122 (bottom) lbs compression.
I am almost thinking the guy before me mounted the engine too high or too low? Any ideas?
A neighbor has the same engine (although newer) on a 17’ aluminum boat, and he tops out at 27mph, so something is off with this setup.
Any help is appreciated!

 

[tphoyt]

Sounds like your prop is slipping.
Or you may be right about it not being mounted at the correct hight.
The cavitation plate should be inline with the lowest point if the hull. If it is then check for a spun hub in the prop.

 

[RVPastorKevin]

I thought that too….. brand new prop on it as well.

 

[Crosbyman]

my snow blower surges to and has no prop must be something else

 

[tphoyt]

I’m not sure how I missed that. You did say it has a new prop. So how about the mounting hight? It does sound like it’s catching air and that could be because it mounted to high.

 

[CaptnKingfisher]

I can't decide now if electrical or fuel.

 

[oldboat1]

sounds like prop ventilation -- prop is picking up air on turns. Mount with the AV plate even with the bottom of the hull.

 

[flyingscott]

What happens if you pump the primer ball? Has the carb been worked on recently? Fuel pump worked on? Put up a pic of the motor and transom. Are you running a shortshaft on a longshaft transom?

 

[RVPastorKevin]

Brand New fuel pump. Brand new prop. (I purchased a new prop thinking the old one was spun too…. Still doing same thing) Cavitation plate even with bottom of boat. Fuel bulb stays firm throughout…. Even during the surging….
Below is a link to another video…. With me by myself, wot, 15mph. Does the wake/rooster tail show any tell-tell signs of improper motor height? It seems to me the doel-fin is right at the water line….. at wot. The last picture is the boat at the dock, you can see the trim of the motor feels set too much to plow, but that is the only trim hole that doesn’t surge, and gets the boat the fastest…..)
With another person on the boat, or in a turn, or trimmed in any other hole, I get the surging. The surging goes away when I back the throttle Down to 1/2 or so. By myself it feels like it never really planes off well, and I am running 15mph. With someone else on board, certainly no plane, feels bow high even though the trim fees like it is set to plow, max speed is 9mph.

 

[RVPastorKevin]

A

 

[racerone]

How high is the transom?----Actual measurement ?----Model # of the motor ?

 

[F_R]

That thing is ventilating. Just for kicks, remove the doel fin and see what happens.

 

[RVPastorKevin]

thanks to all y’all for your suggestions. I would have sworn I needed a jack plate to lower the motor…… but…. It’s fixed!!!
Apparently, the Michigan match prop on the motor (with a 12 pitch) was just to aggressive for the hull. For grins, I purchased a 10 pitch, and she is running like a champ now. Today it did not surge with 2 people on the boat, and hit 20 mph, which is just fine for me. Thank you all who watched the video and helped, I sure did appreciate the help!!