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Old 09-01-2018, 06:35   #1
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Two-cycle reliability

A unique feature of my 28-foot Rhodes Venture is the auxiliary is a 9.9 hp outboard (25 inch) installed in the lazarette well. Restoration includes deciding whether or not I replace it with a new 4-stroke - which I'm not sure will fit, since I cannot find sizes specified for new motors.

The PO said it runs, and I can feel good compression using the pull-start (it also has electric start). That's all I know. I am a bit prejudiced against two-strokes, having fought with starting them when using lawn equipment.

I am aware of the fuel efficiency penalty, but my question is this: If I did basic maintenance on this unit, could I expect it to be reliable? What kind of experience have folks had with two-strokes?
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Old 09-01-2018, 07:48   #2
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Re: Two-cycle reliability

My grandfather used a 1968 Johnson 18 HP two-stroke as his main fishing motor on a 16' Crestliner from 1968 until his health no longer permitted him to fish in the late 1980s. I ultimately was given the boat and motor and continued to use it until I finally sold it about 10 years ago. While the motor required ongoing maintenance and I think an overhaul at one point it was reasonably reliable. I would guess it had around 1000 hours on it when I sold it.

All the two-strokes I've seen have carburetors. You have to maintain the fuel system. The newer ones are better than the 60s and 70s ones, I think. My experience is that every part of the fuel system will fail over time. Hoses, primer bulbs, gaskets, filters, diaphragms, seals, valves, pumps, clamps. You will have to overhaul the carburetor periodically. Some of this is compatibility problems between older elastomers and newer ethanol-fortified fuel but I think most of it is just the passage of time and the greater opportunity for fuel contamination that the mixing process provides.

Any older motor, 2-stroke or 4-stroke, will be prone to gearcase leaks, and will need new seals at some point.

Take care of all that up front, and you'll have a reliable motor. If the motor is running now, you're probably OK with a little bit of work.
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Old 09-01-2018, 08:00   #3
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Re: Two-cycle reliability

I have made my living mostly as a Mechanic.
A general rule is the more simple something is, the more rugged and easy to fix it will be. Not much is more simple than a two stroke.
However parts availability may be an issue, check on that first.
However the modern four stroke engines are extremely well engineered and very tough to beat, however they are far more complex and not as easy for an inexperienced mechanic to affect repairs.
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Old 09-01-2018, 08:25   #4
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Re: Two-cycle reliability

It largely comes down to the fuel you put in it and the frequency with which you use it.

If you live where you can't get ethanol-free gas, and you don't use your motor that much, you're going to have a problem with both engines in terms of starting and reliability.

I would say that outside of fuel systems the two are equally reliabile, but two-strokes are pretty bullet proof and easily owner serviced. If ethanol is a problem, rebuilding the carb is basically a 1 hour job and pretty straight forward. If it's from the 80's on you should not have a problem getting parts.
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Old 09-01-2018, 08:44   #5
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Re: Two-cycle reliability

There are good and bad ones, of course, but I've had various two stroke outboards (and snowmobiles, and motorcycles) during my life, and the outboards I've had have all been absolutely bulletproof. Extremely high inherent reliability is one of the pluses of two strokes in my opinion. As A64 said, simplicity is a great virtue in machines, and two strokes are far simpler than four strokes. That means there is simply much less that can go wrong with them, and in case something DOES go wrong, it is much easier to fix it.

Two strokes will melt down, though, if something happens to the mixture -- since the lubrication is provided with the fuel. With very low specific output small outboards, this is hardly a problem, but it happens more often with high output motorcycles and snowmobiles. That being said, I have a snowmobile with 600cc Rotax two cylinder two stroke engine (which produces 120 horsepower -- 200 horsepower per liter!!) which has been utterly bulletproof over more than 20,000km, which must be a couple thousand hours. Starts with half a pull each and every time. An amazing machine; on top of all of that it is incredibly tiny and light. Doesn't even require any maintenance (!) other than plugs once a year.

I love two strokes. Two stroke outboards do use quite a bit more fuel, though -- keep that in mind.
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