An Ode To Two Strokes

20
1971
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Regulars here know I’m in the process of restoring an old Kawasaki motorcycle – a 1975 S1C Triple. I love the bike’s lines and looks – but I also admire its engineering. Which is startlingly simple.

Because it’s a two-stroke.

There is much less to the engine – parts and physical size-wise. The entire valvetrain is eliminated. No camshaft(s), no valves, no pushrods or chains; no keepers or shims or buckets to bother with. The “top end” of the S1’s engine consists of  three one-piece cylinder head castings – each little more than a small block of aluminum with a cast-in combustion chamber on the underside and a threaded hole in the center for the spark plug. There is nothing to rebuild. Ever. Provided the block of aluminum is not physically damaged (e.g. warped, or a cooling fin broken off or the spark plug hole’s threads ruined)  it is a “forever part” that you can use and re-use almost indefinitely. You can clean it occasionally, if you’re concerned about looks. But function-wise, it never wears out.

The rest of the S1’s “top end” consists of the cylinder barrels, into which are machined the ports that replace the valves you’d find in a four-stroke engine – or rather, on top of a four-stroke engine (usually).  These ports are just fixed holes that are covered and uncovered by the piston as it travels up and down and perform  the same function as opening and closing valves – only with no moving parts (other than the pistons themselves, of course.)

Other than de-carboning them during the rebuild, they, too, are essentially maintenance free. The upper bore may occasionally need to be machined oversize (and oversize pistons fitted) but otherwise, that’s it. The whole thing, top end-wise.

There’s no oil pan – and no oil pump in a sump. A two-stroke gets its lubricating oil mixed in with the gas it burns. Scratch the need for at least one more part (the oil pump ) and one more routine maintenance job to perform (oil changes).

The bottom end of the engine is just a pair of aluminum cases that hold the crankshaft and transmission and onto which the cylinder barrels are bolted.

With a four-stroke engine, there’s a lot more work, weight, size – and money – involved. The cylinder head(s) is an assembly, not just a single part. There are valves and springs and rocker arms and shims. Usually, machine work is required before worn valvetrain components can be replaced. The resultant package (the engine) is also physically larger because there’s more stuff inside. The S1’s engine is tiny compared to the four-strokes in my other bikes. The complete assembly – the entire engine – weighs less than 100 pounds.

It also packs more punch – on a per cc basis, at least.

That’s because a two-stroke engine produces power twice as often – once for every full rotation of the crank. The piston in a two-stroke is always working, never merely going up and down. As it’s compressing the air/fuel charge on the upstroke, it is also simultaneously drawing in (by negative pressure) the air-fuel charge for the next combustion cycle. Then, as the pistons travels downward, the exhaust port is uncovered, allowing the spent exhaust gases to escape. The next air-fuel charge  also enters the combustion chamber at this time, and the cycle is ready to repeat.

Result? A lot of power from not much engine.

My S1’s engine is only a 250 but it produces almost 30 hp. For some perspective, the same-size but four-stroke 250 in one of my other bikes only produces about half as much horsepower. I also have a much larger (nearly 700 cc) four-stroke in another bike that produces about 60 hp, only twice the S1’s output from nearly three times the total displacement.

So, if two-strokes are so marvelous – what happened?

The Smog Police happened.

Because they burn oil along with gas, the resultant exhaust is more politically incorrect than Lawn Darts – and just as illegal. On paved roads, anyhow. You can still buy off-road motorcycles with two-stroke engines and chainsaws and other outdoor power equipment still have them, too.

But they’ve been off the table for on-road use since the ’80s.

That, however, may change. Engineers are working on the exhaust issue and if that can be dealt with, two-stroke engines could make a comeback.

As a fan of the elegantly simple, I hope they succeed.

Throw it in the Woods?

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20 COMMENTS

  1. i too had a Yamaha RD 350. i had expansion chambers on it, racing style footpegs on it and added 1 extra tooth on the rear wheel sprocket and minus a tooth on the main sprocket. it wouldn’t go over 80-90 mph but it got their FAST!!!!

    • Excellent! I am weighing whether to stick with my stock pipes, which look cool – or go with aftermarket (Denco reproduction) chambers, which of course, sound cool….

      • They not only sound cool, but they will significantly increase your performance in their power band (with proper jetting of course or you’ll scorch your pistons). I had a good friend that had a 1975 Kawasaki H2 when I had my RD350. He had been stationed out at Davis – Monthan AFB and found a set of chambers the hawker claimed were team Kawasaki. They had no baffles and definitely weren’t street legal, but he had them chromed and put them on his bike. From behind, it sounded like you were riding into three mini-guns and from the seat it sounded like a diesel semi engine. You could roll the throttle in 3rd on that old triple 750 and she would stand straight up! My RD was a fast blast to ride, but that H2 was downright spooky. I lived down the street from the Yamaha shop and every time they had the service bay door open, he’d wheelie past the mechanics and they’d flip him off. Those were the days….

  2. Dammit Eric! Why did you have to write this article? Now I want my ’75 RD350 Yamaha back. I was on the Mobile bay bridge-tunnel in 1980 running along side a 280Z at 124 MPH. The driver kept looking over at me like “there’s no way the 350 on that side plate is right!” Another time a “stud” in a Mako kit bodied big block Vette tried to run me light to light in Ft. Walton Bch., FL. I embarassed him so bad with that little bike, he wouldn’t even look at me when he pulled up by me at the second light. There’s just nothing like an oil burner, nothing. I guess I’m going to have to start checking Craigslist on a regular basis now….

    • My pleasure!

      They really are a lot of fun. And – I know where you can get a nice, complete (and purple) stock RD350, ready to restore, for very nice price… ping me if interested.

    • Yeah, really, Eric, you rotten S.O.B.!

      Waving around bike goodness in the face of “recovering” (hint: no such thing) bike addicts. Might as well put a vial of smack in front of a heroin junkie.

      The question I have to ask myself is: how long will it take to convince my wife? For, unlike wise men, I forsook my bike before I met my betrothed.

      Maybe I should have a bike affair…keep her stashed in a U-Stor-It…she’d never have to find out…hide the gasoline smell with cologne…yeah…

      • I love being an enabler!

        For real, my friend Scott – who owns a small bike shop – has this purple RD350 out back. I’ve seen it and it’s all there and appears to be in overall pretty good shape. Looks like an easy resto, or just get it running and ride the snot out of it. If you want me to hook you up, send me an e-mail and I will put you in touch with Scott.

  3. I am thinking of taking my MSF course within the next two years.
    It would be great if more options were available.

    Do you need to mix the oil into an oil/gas (1:40 or 1:50) mixture similar to other 2-stroke engines?

    Would a 2-stroke engine be more inexpensive than a similar 4-stroke engine? (ie 2 same cc engines)

    • “I am thinking of taking my MSF course within the next two years.”

      Do it! Riding is fun – and cheap. It’s a genuine two-fer.

      You can pick up a very presentable used street bike for $3,000 or less; most will give you at least 40 MPG and some a lot more than that (I have a 250 cc dual sport that’ll approach 70 MPG).

      Probably, though, you’ll want a four-stroke for your first bike. Here’s why:

      They haven’t made two stroke street bikes in 30 years so you’d have to buy an antique bike, which is fine if you’re experienced and know how to deal with old bike quirks/maintenance – but not so fine if you are just starting out. You want something newer that’s not going to require much from you beyond basic tune-ups and so on and for which it is still very easy to find parts as well as people willing/able to work on them.

      The MSF course will start you out with something like a 250 cc Honda Rebel (street bike) or 250 cc dual sport (Suzuki DR series, etc.) These are great bikes to learn on – and learn on without actually buying. Once you’ve learned how to ride and are comfortable you can move up to something a little larger (250 cc bikes are ok for putting around but marginal on the highway, etc.)

      I can make some specific recommendations on bikes if you like – just let me know.

      • A friend of mine named Dan was describing riding to my other friend Dan one time. He said “riding a bike is the most fun you can have with your clothes on.” Guess he never rides naked! j/k

        • “riding a bike is the most fun you can have with your clothes on.”

          Almost. I’d have to rate flying an airplane as even more fun, based on personal experience.

          • JdL I would agree that airplanes are more fun, but bikes are something a lot more of us can actually afford. The other cool thing is when a bike’s engine quits, you don’t end in pieces on the ground (as a general rule). 😉

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