Engine Overhaul


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Well-known member
Apr 18, 2021
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Washburn Co, WI
We have an '84 Alumacraft Pro 17 that we bought when we retired in 2019. We had bought a lake home on 640 acres with a 1/2 mile of lake frontage and we went looking for a boat that we could leave on the lake 7 months of the year. Although when not using the boat we have it in our ShoreStation boat lift. We looked at all sorts of new boats, but my wife found this '84 in the dealer's used showroom and she immediately liked the deck layout. I do to, it has bass boat style seating with storage under seats, where all the new ones had pedestal seats. Raised fishing deck platforms both bow and stern. Enclosed bilge. It's a really nice boat and was Alumacraft's "Luxury Series" back in the day.

But it needed a lot of work. The transom was rotted. So I had to replace that. The wiring was a cobbled rat's nest. Since I had the rivets drilled and the transom out of it, I pulled all the rest of the decks and completely rewired the boat and put new decks and carpets in it.

A year later we had a really nice boat that we liked, and now comes the part about the outboard.

The boat came with an old Chrysler Force 50 on it. The engine ran good and it pushed this heavy boat at 24.7 mph GPS. But I wanted a Mercury on it. And the one I wanted was a early to mid-80's 50hp four-cylinder two-stroke. It took a year to find one and when I found it it was not in very good shape. The previous owner couldn't get it to run. He had "rebuilt" the carbs and screwed those all up. During the process of trying to get it to run he had replaced the stator, trigger and ignition switchbox. He put a new water pump in it, but when he put the lower unit back on he got the shift shaft out of time so it wouldn't go in reverse. In the process of trying to get it running he ran it without water and burned his new water pump up, but he didn't know the difference. And then he figured he finally had 'er - it was running on only two cylinders but he didn't know the difference on that either. So he painted the lower unit and leg Caterpillar yellow, he painted the cowling and top end John Deere green - these were the color of spray bombs he had laying around the garage. Then he took it out on the lake and on two cylinders it would barely get his 15ft Alumacraft up on plane, which he thought was really going until it locked up.

Let's just say this guy was not a marine mechanic.

So he advertised the engine on craigslist for $500 for parts. Doesn't run. This is where I found it and I gladly paid the $500 for it because all the ignition parts he had put on it were worth that.

I tore it down. It had a broken reed block on the #2 main bearing - there's no way it could run on the top two cylinders. I bored the block .020 over - factory pistons used to come in .015 and .030. I put .020 Wiseco pistons in it. I did some mild port work on the transfers and exhaust ports - not full racing port timing, but enough to let it breathe a little better on the top end and still idle smooth at 650 rpm. New reed blocks and two-stage racing reeds. Went thru the carbs and re-worked them for E15 ethanol blended fuel (premium 88). It's got more compression with the overbore and it needs good gas - it will ping on non-ethanol "recreational gas". With E15 I can run 34 degrees of timing advance and it's a cleaner, slower and cooler burning fuel than straight gasoline.

Ran it and broke it in in the test tank. After breakin I put it on the old Taylor marine dyno and it put out 67.8hp at the prop shaft @ 5,800 rpm on the powerhead. So I bought a new Solas 17" pitch prop for it.

In the end, May 2021, got a new boat, new outboard, all for under $6,000 (but a lot of work). And it pushes our Pro 17 at 38.0mph @ 6,000 rpm. It could use a bit more prop but I'm happy with the hole shot with the Solas 17. It jumps right up out of the water and on plane inside 5 seconds. Couldn't find any of the old blue-stripe decals. So I re-decal'd it as a 60 with the later red and silver stripe decals. It is the only 60 four-cylinder in existence built on a 44 cube powerhead. It's actually closer to a 65 or 70, but the red stripe 60 decals were readily available.

A few photos I snapped during the rebuild and engine swap, and the final product. The boat has a CMC PT-130 power tilt and trim and jackplate. So the original power tilt and trim was removed from the engine as it was no longer functional anyway and parts for the hydraulic pump are impossible to find anymore.





Thanks, we really like this boat. That hull later morphed into the Trophy-series and it's not a high-speed hull. The boat is placarded for 70hp and they were supposed to go 34-36 mph with a 70 on it, so it's right in the ballpark. I'm getting a bit of a boost with the engine set back from the transom too, with the PT-130. It allows me to ventilate the prop on the top end and gets more of the lower unit out of the water with the outboard set higher that it could be if was bolted direct to the transom. The clamp bolts on the transom bracket are about even with the top of the transom with the jack plate on it. That alone is good for an extra 4 mph and lets you turn a bigger prop than you could otherwise. The prop is only 11.2" diameter but I'm pretty sure it would turn a 19" (pitch) with no problem, and 19-21" pitch is usually what you'll find on a 150, although 14" diameter.

With the port work and running more timing that is possible with straight gas, the engine is pretty strong in the mid-range and top end, and no reed flutter @ 6,000 rpm with the lightweight carbon fiber reeds in it.

The boat is so heavy with two people and the livewell full that it won't stay on plane below 17 mph, even with the outboard trimmed all the way down.
HI pappy!
That is actually a problem. I use all the up trim to get 38 mph.

Part of the problem (I think) is the way the boat is set up and balanced. The captain's station is considerably aft on the Pro 17, and of source so is the passenger seat and livewell. I got two Group 31 trolling motor batteries under the bow deck hatch, instead of one there and one in the stern as it was originally designed. And that helped the balance some. But it's still stern heavy so if we slow down to <17 mph the bow gradually rises and the boat goes off plane.

I can't say it's a bad setup. It's about perfectly trimmed for planing speeds above 17. We don't very often use the top speed because the engine burns 5 1/2 gal/hr at WOT. But if we're 19 or 20 miles up the lake and a storm is coming that we want to try to beat to the dock, I'll run it flat out.
Nice post. I wish I had your talents. I’d be able to do more fixing up my 1992 40 Yamaha. But I got it doing good on my jon boat. I just keep wanting a little more and a little more you know. Lol
I worked with a technician at a boat shop many moons ago and I watched him set up mostly bass boats for tournament fisherman and he knew the science. He took his own 21’ Triton with a 235 Mercury and changed out the what I’m going to call computer with one for a 300hp. Installed high performance heads , bored the exhaust, and many other things that he did. It sounded like a dragster. I got in it with him and held a gps in my hand and we got up to 92 mph. I looked back and the entire boat was balanced on the little 12x18 running pad just in front of the motor. I liked to go fast then and he had mine hitting 70 but I never wanted to ride it his again after that.
I run my little 40 hp now wot and it’s about as fast as I want to go nowadays. Like you I wouldn’t mind having a little extra if a storm is coming.
Thanks again for posting I enjoyed reading.
Have you made anymore changes?
Nothing recently. The boat works good for us and we enjoy it. Right now, though, we're waiting for snow and ice to melt up here and the boat is on the trailer.
Nothing recently. The boat works good for us and we enjoy it. Right now, though, we're waiting for snow and ice to melt up here and the boat is on the trailer.
That’s the only problem I have with living in the cold. Here in Texas I fish year round.
Glad you’re still running good. I like to hear when someone gets the reward of enjoying what they built.
Well, I must say we enjoy the change in seasons up here. Plenty of stuff to do outdoors in the winter time too, we like snowmobiling. We had 8ft of snow this year so the snowmobiling was really good.

The lake ice here is still holding up but we're starting to see pools of water collecting on it from the melting snow in the sun. The ice always goes out within a couple days every year no matter what type of season it was. Then we'll get the boat launched again for the season.

The reason I wanted that old inline-four two-stroke Merc is because they run so smooth. If we go out trolling for walleye, that outboard will idle at 600-650 rpm for hours on end and never complain, and doesn't sit back there jerking back and forth on the transom when it's idling. It's a little stronger than a stock 44 cube powerhead, and it'll turn a few more rpm with the racing reeds and port work than a stock one will. But with the carbs jetted for E15 it burns quite a bit of gas at wide open throttle. So I rarely run it that hard. At 20 mph cruise speed we can make it from one end of the lake to the other in an hour and still got enough fuel to get back home. If I run it at full throttle we can make it from one end of the lake to other in 35 minutes. But the tank will go dry before we get home if I do that.

But even so, those old Carl Kiekhaefer-designed inline-four and inline-six two-stroke Mercs were and are one of the best outboard engines ever built by the human race, in my opinion. It's kind of a shame so that many of those old 44 cube inline-four powerheads got snapped up by D-mod hydroplane racers and they're hard to find anymore for a recreational boat. In these hydroplanes they're turning them over 12,000 rpm and pushing 100+ hp and the bottom end of those engines take it.


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