Jet Boat Work

During Covid, we noticed that our Alumaweld 17' jet boat was struggling to get on plane. I had rebuilt the Evinrude 150 outboard jet motor in 2015. The motor has about 200 hours on it since that rebuild.

The motor is a 1991 Evinrude JetPower 105 outboard Jet motor. (150 HP powerhead)

I did a compression test on the motor this summer and found that the compression was down 20 psi on #2 and down 10 psi on #4 and #6. #1, 3, 5 all showed the recommended compression of 90 psi. I borrowed a bore scope and looked into #2 cylinder. I found the top of the piston had been "peened" from something metal getting into that combustion chamber. 2 and 4 did not show the same peening, but they did show some scratching on the cylinder walls.

I removed the heads and found a deep vertical scratch on the cylinder bore adjacent to an intake port and some damage to the top edge of the piston in #2. I am seeing damage from a foreign object on #2 piston. I am seeing some scratching to the bores on #4 and #6 that look like siezing, possibly from a lean mixture or lack of lubrication. I do not see the same scratching on #1, 3, and 5. I think the damage to the upper circumference of the #2 piston indicaated that the piston overheated at some point. That top square edge of the piston is exposed to the most heat when there is excess heat in the cylinder. I will be looking at the carburator on #1 and #2 cylinders upon re-assembly for proper jetting or other problem that could have led to a lean condition on #2. The top 2 barrell carbutator feeds #1 and #2 cylinder. Each cylinder has its own carburator venturi and jet.

I have contacted a machine shop in Oroville regarding boring the cylinders. Unfortunately, the 80 year old owner is having knee surgery and can't do the work unitl maybe February. I have removed the powerhead and mounted it up on a stand. I wrapped it up and am going to wait until closer to the time I can proceed. I don't want to completely dis-assemble the powerhead too far in advance of the boring.

At this point, I think we will be boring #2,4, and 6 to .030 oversize to remove all the scratches. #2 may require .040 to remove the deep scratch. I will replace all 6 pistons with appropriate sizes and all bearings , seals, and gaskets.

I have been reading about a fixture that is used to position the rod caps on the rods during re-assembly. Apparently it is very important that the caps be EXACTLY centered on the rod. Because the rods use needle bearings, there can be no step where the cap and rod come together. There is a tool that evidently clamps the outside of the rod and cap from the sides to center the cap on the rod when torquing the rod bolts. There must be enough clearance for the rod bolt in the cap to allow the cap to not be aligned perfectly. I have found this tool on Ebay, but it is over $300.00. I am currently looking into that issue. I did not know about this when I rebuilt the engine the first time. Maybe that is why it only ran 200 hours.....