Make and Make Do

By Rob Rohde-Szudy - Madison, Wisconsin - USA

 

Cleaning Up Two-Stroke Outboards

Ah, outboard motoring. The pearlescent sheen of oil on water. The pall of blue-gray smoke. The choking stench of half-burned petroleum. How I love it. Well, I love it except for those parts. Fortunately, I’ve come across some ways to reduce the pollution associated with old two-stroke outboards.

Most of us know why we use these old motors, and outboard guru Max Wawrzyniak has amassed an impressive body of work on this topic in his book Cheap Outboards. I won’t try to duplicate his work except to say that late 1950s through early 1970s OMC outboards are inexpensive, available, very easy to repair, and reliable when properly tended to.

The disadvantages are pollution and inefficiency.

The late Robb White thought two-stroke oil was a relatively innocuous pollutant in moderation, and how the big engines are a much bigger problem than little one-digit-horsepower jobs. While the later is most certainly true, I’m not so sure about the former. Two-stroke engines cough out not only the oil, but also gasoline. If you’ve ever happened to spill any substantial amount while refueling a lawnmower, you know what I mean. Count on years of dead grass in that spot. But old Robb was no fool. He knew perfectly well that cleaner is better, and encouraged me to pursue these ideas.

So it’s no surprise that those of us who are too cheap or poor to use modern outboard motors get a lot of criticism from those who pony up for 4-stroke power. We all know that the two-stroke motors are less efficient and pollute more. But we hear very little about what we can do about it. This is no surprise, of course, since such information doesn’t sell new motors! Since I’m not involved in selling new motors, I spent a bit of time cleaning up the old one I could afford.

And it is an old one indeed. This is a 1954 Johnson 5.5 hp two-stroke outboard. Stylish, isn’t it?

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1954 Johnson

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Baseline Emissions Testing

Stylish or not, something had to be done about the emissions. I ran an informal test to get a baseline and was positively revolted by the results. Here’s the setup.

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I hope you aren’t eating, because here are some photos after idling 5 minutes and running 10 minutes at ¾ throttle – all in neutral.

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This was after dumping the water and a lot of the gunk.

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I don’t know how much ecological harm the above oil slick causes, but I thought it was just too… well… icky, even without the stomach-turning smell. It made me wonder if I could clean this thing up in a meaningful way without spending too much money.

Gaskets and Tune-up

First the obvious. The motor needs to be in good tune so it’s actually vaporizing the correct amount of fuel to begin with, and igniting it efficiently. Again, check out Max’s book. Gaskets are a commonly-neglected problem on these motors too. Check out this oil stain I noticed inside my cowling.

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Exhaust cover leak

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cowling stain

I bet it had been doing it for a while, but I hadn’t really noticed. It was the exhaust port cover gasket, which is hard to find for this motor. Fortunately it’s easy to cut a new one from inexpensive gasket material. Others have dealt with this by adding a little gasket compound to the old gasket, then tightening the screws after the compound cures.

Another commonly-neglected tune-up item is crankcase compression/vacuum. Nothing works quite right if the crankcase is leaky. I made a plate to fit over the intake ports in place of the intake manifold. Two hose barbs allow me to accurately measure the pressure and vacuum in each crankcase. If the readings are very different, new seals are in order. Good thing mine were OK – it’s a relatively major job to get to them.

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Test plate

Now on to some modifications.

Crankcase Bleeders

This modification is relatively easy. In a two-stroke engine, the fuel/oil/air mist is drawn through the crankcase. Some of the oil condenses on the relatively cool crankcase walls and runs down to the bottom. If you leave it in there, it will be blown out whenever it builds up too much, or when you open the throttle further (increasing air flow). This small reservoir of uncontrolled fuel will cause uneven operation and fouled plugs.

So the engineers put in drains to get that oil out of the crankcases. These bleed the oil through check valves so the crankcase doesn’t lose any vacuum. On each compression stroke it blows a little oil through a small reed valve, then the reed valve closes to avoid “diluting” the vacuum or sucking the oil back in.

This is what it looks like:

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The small reed valves on the lower left side of the gasket mating surface are the bleeders. Here’s the side view:

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The top two arrows point to the channels that drain oil from the bottom of each crankcase. These go though the upper and lower sides of the check valve assembly. The lower arrow points to the outlet channel, which runs to the base of the powerhead. There, the wasted oil is discharged into the spent cooling water. That’s right, it just spits the oil into the water. These motors were designed in a very different time. Clearly it would be good if we could clean this up!

Fortunately, it’s easy to do. First, we need a way to add a hose barb to the manifold. Unfortunately there isn’t enough metal to tap into.

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JB Weld to the rescue! A Dremel or similar tool is by far the quickest way to take the paint off this area. You were looking for an excuse to buy one anyway, and it will help you with those fantasies of being a dentist.

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Once the paint is off, wash with a degreaser – I use Spic-n-Span. Rinse and dry, obviously. A final cleaning with acetone is not a bad idea when using epoxy. Then add a little “dam” of masking tape. This holds the JB Weld in place while it cures.

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We will need an 11/32” hole for a 1/8” pipe thread. But just in case, drill a pilot hole first. Do this from the inside, and center punch before drilling. You don’t want any unpleasant surprises, like your hole overlaps the gasket mating surface.

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My hole didn’t leave quite as much metal as I would have liked, so I added more JB Weld. Now it looks like this.

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While you are mixing the JB Weld, you may as well plug the bleeder drain. (Max would say to use hard-setting gasket sealant in case you ever want to reverse the process. But I don’t think it’s much harder to drill JB Weld. And I can’t think why I would want to go back to spitting oil into the lake.) Degrease the metal and put a small dab in the hole. Flatten it under a piece of masking tape and let it cure. When cured, very carefully file it flush with the rest of the surface. You may need to lap the entire surface with a piece of plate glass and Crocus cloth or #00 Emery. Just make sure that your modification doesn’t prevent the gasket from sealing, and even more important, that you don’t cut away too much of the gasket mating surface! (In case you were wondering, the other piece of tape is for Max's "ditching the pressure tank" mod, as seen in the book mentioned above.)

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When all looks well, we enlarge our hole to 11/32” and paint. I used Rustoleum spray primer and enamel. When it dries we can cut threads. Watch carefully so your threading doesn’t cut into the gasket mating surface. Pipe thread is tapered, so you have to start from the outside, and the hole gets wider as you cut deeper. Remember to keep firm pressure on the tap as you’re starting it, or your first couple threads will get stripped.

If your engine really doesn’t have space for 1/8” pipe threading, you could always center bore a 10-32 or even ¼” screw and use it as a hose barb. It is very tedious to end-bore a screw, since you have to stop and add oil often to prevent the tiny drill overheating. It might also be possible to JB Weld on a hose barb, but I think a hole tapped at least partly in metal will resist vibration better. This may be superstition. If you do make a threaded hole, use the yellow Teflon tape – the white stuff isn’t meant to resist petroleum.

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After putting the engine back together I clamped a couple feet of hose to my excuse for a hose barb. This line leads to a catch jar made from an empty pill bottle. I used a green veterinary pill bottle so it would match the engine. Such vanity.

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First I bored the lid to tightly fit two hose barbs. One is double-ended. Once these were fitted, I clamped them in a vice and added JB Weld to lock them in place. (I don’t own stock in JB Weld or anything, by the way. I just find it very convenient.)

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Then I added hoses. This is how they should look. The inlet is the double barb. We add a short section of hose to make sure the waste fuel ends up in the bottom of the bottle. The other barb is the vent. It’s hose is zip-tied into an inverted “U” shape to keep water from easily splashing into the bottle.

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We still need to mount the bottle somewhere. It would be slick to mount it to the leg of the motor, but I went with a simpler solution. It hangs by its hose. Winter is a good time to come up with a better bracket. I’m thinking it will screw to the back of the exhaust “telltale” housing. But if it tilts with the motor, be sure the vent faces aft so you don’t dump the oil when you tilt the motor up!

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No matter how it is mounted, any oil collected in this jar correlates exactly with pollution you prevented. Over a one-hour run I burned less than a half-gallon of fuel, but collected about 20 mL out the bleeders. This may not seem like a lot, but remember this is only one hour. It amounts to maybe a half cup over 6 hours of running. How much would this amount to over an entire season? With a bigger motor? In any case, it makes me feel a lot better about using a two-stroke and it cost hardly anything to implement.

Some people will be thinking of piping this fuel back to the tank. I wouldn’t. seems to take on a little water in the crankcase, and this makes the oil precipitate far too readily. Dispose of it with your waste oil.

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Next time we’ll look at using cleaner fuels to further reduce these emissions.


Rob Rohde-Szudy
Madison, Wisconsin, USA
robrohdeszudy@yahoo.com

A version of this article with fewer photos but further discussion can be found in the October 1, 2006 Issue (Vol 24 - #10) of Messing About In Boats.

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