Getting your hands on an old Kawasaki means you're going to spend a lot of time thinking about the 650sx engine and how to keep it screaming on the water. If you've ever ridden one of these vintage stand-ups, you know they have a certain personality. They aren't the fastest things by today's standards, but there is something incredibly rewarding about that 635cc twin-cylinder power plant when it's tuned just right. It's a workhorse, plain and simple, and it's the reason so many of these skis are still hitting the lake thirty years after they rolled off the assembly line.
What Makes the 650sx Engine a Classic?
The 650sx engine was really the bridge between the old-school 550s and the more modern, powerful 750s. Back in the late 80s and early 90s, this motor was the king of the "new" generation of Kawasaki Jet Skis. It's a two-stroke, parallel-twin engine that's water-cooled and, honestly, built like a tank. While the 550 was nimble but a bit finicky, the 650 brought a level of reliability and torque that made stand-up riding accessible to more people.
One of the reasons people still love this motor is that it's incredibly simple to work on. There's no fancy electronics or complicated fuel injection systems to worry about. It's just fuel, air, spark, and compression. If you've got a basic set of tools and a little bit of patience, you can pretty much tear the whole thing down and put it back together in a weekend. That simplicity is why you see so many guys still tinker with them in their garages today.
Keeping the Reliability Alive
If you want your 650sx engine to last, you've got to stay on top of the basics. These are old motors, and rubber doesn't last forever. The biggest killer of these engines isn't usually mechanical failure—it's an air leak. If the crank seals get old and brittle, they'll start sucking in air. This leans out the fuel mixture, and before you know it, you've melted a piston. If you just bought a used ski that's been sitting in a shed for five years, do yourself a favor and pressure test the crankcase before you even try to wide-open throttle it.
Another thing to look at is the oil injection. Back in the day, Kawasaki thought it was a great idea to have a separate oil tank and a pump that mixed the oil for you. Most owners nowadays will tell you to rip that pump off and toss it in the trash. Those pumps can fail without warning, and if the engine stops getting oil, it's game over. Most people switch to "pre-mix," where you just mix the oil directly into the gas tank at a 40:1 or 50:1 ratio. It's a little more work at the gas station, but it gives you total peace of mind.
Performance Mods That Actually Work
Let's be real, the stock 650sx engine is a little "doggy" in its original form. It was designed to be safe and reliable for the general public, not to win races. But there is a ton of untapped potential hiding inside that heavy aluminum block.
The first thing most people do is change the exhaust. The stock "pipe" is a heavy, restrictive cast-iron piece that holds back a lot of power. If you can find an old Westcoast, Coffman's, or Factory Pipe, it'll completely transform the ski. You'll feel a massive jump in bottom-end snap, which is exactly what you want when you're trying to pop the nose out of the water or carve a hard turn.
After the exhaust, looking at the cylinder head is the next logical step. You can "mill" the stock head to increase compression, or just buy an aftermarket high-compression head with interchangeable domes. More compression means more "pop," but you have to be careful. If you go too high, you can't run regular pump gas anymore and you'll risk detonation. For a lake boat, keeping it around 150-160 psi of compression is usually the sweet spot for fun and reliability.
The Carburetor Struggle
The carb is another area where the 650sx engine often needs some love. Early models came with a 28mm Keihin carb that was okay, but not great. Later models moved to a 38mm CDK-II, which was better, but many riders eventually swap them out for a Mikuni SBN 38mm or 44mm.
If you're still running the stock carb, make sure it's clean. These old carbs get "gummed up" with modern ethanol fuel really easily. If your ski is bogging when you hit the gas or won't idle right, it's almost always a dirty carb or a bad fuel pump diaphragm. Tuning them is an art form, but once you get the high and low screws dialed in, that 650 will rip.
Why Reed Valves Matter
Inside the intake manifold, you've got reed valves. These are little "flaps" that let the fuel and air into the crankcase but don't let it blow back out. Over time, the stock stainless steel reeds can get chipped or lose their tension. Switching to carbon fiber or fiberglass reeds (like Boyesen reeds) can make the engine feel much more responsive. It won't give you ten more horsepower, but it'll make the throttle feel "crisper," if that makes sense.
Rebuilding vs. Replacing
Eventually, every 650sx engine is going to need a rebuild. Maybe the compression is getting low, or maybe you heard a nasty rattling sound coming from the crank. When that time comes, you've got a choice: do it yourself or buy a remanufactured short block.
DIY rebuilding is a great way to learn. You can buy top-end kits with new pistons, rings, and gaskets for a few hundred bucks. If the cylinders are scratched up, you'll need to take them to a machine shop to get them bored out to the next oversize. It's a fun project, but you have to be meticulous about cleanliness. One little piece of grit in the crank bearings will ruin your whole day.
On the other hand, companies like SBT offer "no-fault" warranty engines. You send them your old broken core, and they send you a freshly rebuilt engine. It's more expensive, but it's a lot faster if you just want to get back on the water before summer ends.
Final Thoughts on the 650sx Experience
At the end of the day, the 650sx engine isn't about being the fastest guy on the lake. It's about that classic two-stroke smell, the sound of a twin-cylinder hitting the powerband, and the physical challenge of riding a stand-up. It's a motor that rewards owners who actually take care of their stuff.
Sure, you could go buy a modern 4-stroke ski that has 300 horsepower and weighs a thousand pounds, but you won't have half as much fun as you do when you're wrestling a 650sx through a wake. These engines have soul. They're loud, they're vibrate-y, and they require you to get your hands dirty every now and then. But when you're out there on a glass-calm morning and that engine is singing, you'll realize why people are still obsessed with these things decades later. Keep the oil mixed, keep the seals tight, and that 650sx will keep you smiling for a long time.