Only system cookies are currently stored. Tracking of activities and analytics features are turned off.

At Jetski Army, we have exciting offers and news about products and services that we hope you'd like to hear about. To present you the offers that suit you the most, we need to know a few personal details about you. We will gather some of your activities on our website (such as which pages you've visited, etc.) and use them to personalize the website content and improve analytics about our visitors. In addition, we will store small piecies of data in your browser cookies. We promise we will treat your data with respect, store it in a secured storage, and won't release it to any third parties.

Submit Article

If you have something to say then share it! Articles are the best way to share knowledge.

Articles / Standup / From The Beginning - The Jetski Motor

From The Beginning - The Jetski Motor

15 November 2019
Article From The Beginning - The Jetski Motor
After WW2 - Kawasaki Heavy Industries rose to prominence creating ships - airplanes and steel products. To this day - the biggest part of a considerable corporation. A very small division of that conglomerate made motorcycles for the world and followed Honda VERY closely into the USA market. When Ronald Reagan enacted a protective tariff against larger bore motorcycles - Kawasaki Motors USA reacted by building an assembly plant in Lincoln Nebraska. This plant was operational by the early 70’s and still in operation today.

Those ships that KHI ( Kawasaki Heavy Industries ) built - and others like them - needed a way to maneuver at low speeds - like in a port of call. The accepted way to accomplish this was to put a jet stream motor into a “tube” and push the bow or aft end sideways. The tube was small and did not affect the ship while it was under power from the huge propeller that pushed these monsters. SO - KHI designed and manufactured a small 398cc twin cylinder - 2-stroke motor to power these individual “thrusters” . Usually 4 per ship or more. They were started and controlled remotely and were usually fired up and blasted full throttle for a minute or two - and then shut off for another month. These motors also found their way to a use as irrigation motors to drive water pumps in Asian rice paddies. Several of my friends who had been in Nam - said they saw those very motors screaming their hearts out - pushing irrigation water.

At the start of the 70’s - after KMC ( Kawasaki Motors Corp ) had decided to take a leap and build Clayton Jacobsen’s little invention to be called the Jet Ski - they made a few hundred hulls with hand laid fiberglass and a couple different designs and dropped these little steering motors into them and hooked up a jet pump to drive them forward - a moveable hand pole for steering - and shipped them to the USA. And waited a few years for the response to the attempt. A small group of KMC employees and hangers on went about learning how to use them and showing them and all became enthusiasts and still are to this day. SO - KMC took the ultimate leap of faith and purchased the SMC hull making equipment and moved production to Lincoln Nebraska USA. An American invention - built in America - by Americans.

For this article we will only concern ourselves with the 398cc motor and it’s evolution -
The 400cc motor had an alloy cylinder and thick steel sleeves.- conventional Piston Porting ( PP ) and a design for fairly low RPM power output. A single 38mm Mikuni pumper carb was used with the required ( Coast Guard ) flame arrestor on top. A 6 piece pressed together crankshaft with 5 support bearings ( where there should have always been six - or at least a stouter one at the front ( flywheel ) end. The cylinder was bullet proof as it had been proven to be for many years before. The crankshaft was another matter. The crank used the accepted in the day method of a smaller diameter crank pin where it went into the crank halves and a larger diameter where the rod bearing lived. This method allowed for fast assembly - as you simply pressed the crank half on until it stopped up against the ridge of the larger center part. Quick and easy. BUT - the forces of the pump entering and exiting the water caused the crank pins to break right there.

What the “factory” may not have known was that these motors that broke the crankshafts had H-2 pistons in them - upping the displacement to 474cc’s - our American need for speed and bigger is better upgrades in action. This motors use was ended in 1976.

These failures - along with the desire for more power prompted the new and improved 440cc jet ski motor. The cylinder was basically just a bored out to 68mm cylinder that was almost identical to the 400 cylinder. The crankshaft - however - was much larger design with rod pins the same diameter all the way across. A newer and larger flywheel was also installed to fit the larger diameter crankshaft. The electrics included on the handlebars start and stop buttons and a separate electrical box and motor connection. So the motor could be removed while the Ebox stayed put. The 1977 Jet Ski was not just painted a patriotic red white and blue - but - it WAS a vast improvement. This model year also sported a spigot mount carb - with no support bracket at the top - so - it shook wildly and fell off in the ocean surf when a hard landing was experienced. The new separate start and stop buttons allowed water to enter and created many problems. In 1978 the carb was back to being flange mount - WITH a top bracket - the carb was essentially the same as the pre-77’s with minor improvements. There were no cooling deficiencies with these motors - but - they were pitifully slow accelerating and everyone wanted more. EVERYONE.

1980 was a “crossover” year - some skis were simply 79’s badged as 1980 - some had alloy couplers ( between the motor and the pump shaft ) and a newer lighter “T” crank but still the same alloy cylinder. Seems like they were cleaning house.

1981 was the all new motor and electrics and exhaust - but - it looked the same. Gone was the full circle crankshaft and the heavy metal couplers and the alloy cylinder. In their place was a much lighter “T” crank ( some say half circle ) - with alloy couplers. This weight saving was very substantial. The new electrical box was smaller and contained combined pieces in place of individual items ( ie - coil/CDI combo in place of separate coil and CDI units ) - and - a new start stop switch that solved the earlier problems for good. The cylinder no longer had any alloy - but was all iron. The head was the same - except missing 2 studs that were deemed not needed and the holes plugged. This cylinder had a considerable amount more transfer port area - a much needed improvement as these transfer ports ARE the weak link. In addition - the transfer port shape right at the top - was vastly larger and directed the fuel straight across the top of the piston - instead of directing it upwards toward the head. This is a great improvement for quickness. BUT - they were all iron. SO - the heat dissipation qualities of the alloy were gone. In stock form with 110 PSI - it is no big deal - but - when you start making them run - it becomes a problem. Over all - a vast improvement in quickness of acceleration and electrical reliability. NONE of this - however was done to please the horsepower freaks - it was ALL done to save money.
This 440 motor and Ebox remained almost exactly the same until the final production run in 1992. The exhaust was finally routed out the back starting in 1990.

When our first 81 - 440 arrived at Jetco - as was our habit - it was torn down before it ever was serviced or started. Some would question this policy - but - as one who has seen many a “factory defect” - I will stick with it. Like the 82 - 440 with the sand still in the intake manifold from the casting - starting that one would have been interesting - or the 83 that had 440 graphics on one side and 550 graphics on the other ( it was a 440 ) - with both unit’s the factory denied any possibility of these “mistakes” getting past “ quality control “ - uh huh. When we discovered what was “inside” this new unit - our plans to make one a 474 were dashed - as the new cylinder would be paper thin - if bored that much. The weight reduction in the rotating mass was exciting - as were the single ring pistons. So we opted for a “super stock” mod to start with.

Ported - as far as I dared and the pistons cut ( 440 pistons are backwards - the skirts block the transfer ports - not wanting to relocate the ring pin - I just cut the pistons to match and did my intake mod to the pistons also ) - head was milled to .120 and a 2.5 float valve installed with a larger main jet mod. The power was remarkable ! Very quick and really fast. The much larger transfer port CFM’s were truly strutting their stuff. BUT - they did not dissipate heat worth a crap - and piston seizures were inevitable. One night - for some reason - I dreamed of having dual water lines - not the first “wet” dream I ever had - but - it was there. I went to the shop that morning and told everyone to just leave me alone - pretend I am not here - as I went about finding a way to make this “dream” a reality. I had a friend weld up a drill bit - so - it was 3 feet long - and drilled another hole through the hull - installed a piece of copper pipe ( it was what the factory used - so ? ) - drilled and tapped the pump - and the same with the L&S exhaust manifold. Figuring out exactly where everything needed to go took some time - but - by the end of the day - I had a working dual water line set up. Off to the river to test went Ripper - while I cleaned up the substantial mess I had made.

The smile on his face when he returned - told the story before he ever said a word. Success. Over the next month we “dialed in” the water works. There was SO much water going through the exhaust now that a substantial loss of power was noticed - when the water flooded the system. SO - I made up a water by pass - and routed MOST of the water overboard - only burned up a couple of water boxes and several hoses getting it right. Derek Anderson took this mod to the highest possible point years later - he really got the water OUT. With a cool running motor and a dryer exhaust and a LOT of compression we tried a super screw on one. This was the fore-runner of the Skat Trak props that Ken Stuart would develop years later. They were heavy - not balanced - and unrefined. But MAN did they ever push the water. These were made by a great guy from the Midwest - 18 degree stainless - and this was the first guy who tried and failed to keep up with the demand - we needed many and he tried - but - could only produce a few - I think he still hates me for calling him and pleading for MORE !!! That fall - Larry won the modified at world finals ( on his super stock ) and Tony Corry won the super stock and was second in the modified - on his super stock. Which - by the way - had the stock exhaust system on it - with the water by pass. Obviously - the money saving modifications that KMC made - worked - as long as one was willing to “work” with what was there.

There was never a doubt that KMC was planning a larger faster Jet Ski. Pictures of a 600 had appeared before - and - surely they heard the clamoring of “ gimme more !!! “ We did not have to wait long as the very next year KMC introduced the 1982 - 550. Back with a yellow paint scheme to get’er started. We got one of the first ones at Jetco and we were less than impressed - to say the least. The cylinder was paper thin - even though they had opened up the casting to allow a really tight fit within the studs - and - that took away the transfer port gains that the 440 iron cylinder had. The exhaust system was a joke. Really. I ported a cylinder - installed water lines and did the best I could with it. But the pump just would NOT allow the motor to really “work”. With a cool manifold and exhaust that ported motor sounded truly bad ass out of the water - but - the pump just killed it. Brian Bendix rode that one to a respectable finish at world finals - but was beaten by our super stocks.- racing in the modified class.

The next year we became a Jet Ski dealer and ordered a freight car load of 1983 - 440’s and sold them all before June first. We did not order any 550’s. The next year in 1984 we tripled our order of 440’s and still did not order any 550’s. And - we were not alone. There was a handful of Jet Ski ONLY dealerships back then - probably 20 total - and we collectively sold the vast majority of Jet Skis. The motorcycle shops were slow to “warm” to these things with no clutch’s or brakes or tires and the tech was SO different. Motors with a clutch can be SO much different than one with a direct drive propeller. This is why folks like the Stockdale clan from Texas did so well with their mods. Being airplane folks - they understood the needs and nuances of a motor that has to drive a prop. Good people there - and they are still at it.

Jetco’s lack of 550 sales did not go un-noticed by KMC and in late 1984 - we had a visit from three Japanese engineers and an interpreter/driver from KHI. They wanted to know why we did not sell any 550’s. We explained and showed them that with some simple mods a 440 would be quicker ( and that IS what people wanted - even when they said they wanted faster ) and more reliable - and handle better - and most of all - the finished 440 product COST less than a stock 550. They had been told about Brian’s finish at worlds on that 82 - 550 and wanted to know what we had done to make it work - at least that well - AND - finish the race. I have never hidden things I do so I was happy to explain. The cylinder off that unit was in storage now - the entire ski long gone with newer stuff on it. I gave the engineers that ported cylinder - they offered to pay and I declined. I expressed my concern over the thin wall cylinder and begged them to design something more stout. My water by-pass was discussed at length as were the dual water lines. The 85 - 550 was changed very little - and - after threats from KMC - I ordered 5 along with 295 - 440’s for 300 units total - and we still had 2 of those - 85 -550’s on the floor the next year.

In 1986 - the 550 motor was re-designed. Gone were the thick ring pistons and in it’s place was a very thin ring piston. The exhaust was dry ( bread box ) with water by pass. The cylinder was Jetco porting - nearly identical to that cylinder I had given them - stock. The cylinder walls were VERY smooth - almost polished like. ( probably a 1200 grit hone stone ) They were truly a LOT faster - not much quicker ( that was all on the pump ) - but - surely faster. I feel that MOST of this extra speed was the copy of Bob Zantos’s Westcoast exhaust system - now referred to as the bread box exhaust. Sure the carb helped ( 44 now ) as did the porting - but - that dry pipe made it all come together.

BUT - all was not well...

The ultra smooth hone job and the thin rings combined to wear through the cylinder walls in very short order. The few we did sell over the next 4 year period - we pulled apart - ran a 300 grit hone stone just enough to cross hatch and re-assembled - and they would last for awhile. Some other dealers did this too. KMC would not warranty this work - as nothing had “failed”. A stock from the factory 86 through 90 - 550 with 80 hours on the clock would have about 80 PSI - no matter what kind of oil you used. They really did wear out THAT fast. Most motorcycle shops - unlike the Jet Ski only shops - sold mostly 550’s - and - not knowing any better they just sent them out as they got them. No mods. Sometime within the warranty period ( 1 year ) almost every single one came back to the shop with the complaint. “It won’t start”. The motorcycle guy would try and when it would not start - not knowing any better - he would replace the starter. The new starter would turn the motor over just a bit faster - and the ski would start. About a month later the unit was back and another starter would be installed. Usually - the discovery of the motor being already bored out - by the thin rings and smooth bore - beyond the first over size - was made AFTER the warranty expired. No manufacturer will “warranty” a worn out motor - there HAS to be a “failure” - ie broken part. Many people were less than happy - even less happy when the bore would not “clean” at first oversize.

In late 1987 the same three engineers and a different interpreter/driver were back in my shop. This time - it was all about why Jetco did not have any 550 starter warranties and everyone else in the world had nothing but. I explained the problem and was basically dismissed - politely. The driver did an eye roll that was still the best I have ever seen. They took some older starters ( denso’s ) with them that had been water damaged. It seems that KMC was telling Denso that their starters were crap - and Denso was saying “why now ? - they worked for 10 years” . KHI switched to Hitachi starters over this SNAFU and damaged their relationship with Denso in the process. NO changes were made to the 550 motor until 1991. To do anything would have opened KMC up to a flood of warranty requests - as that would have been an admission that they screwed up. That is NOT a “dig” on KMC - it is a FACT - of the manufacturing world - everyone does the same thing. NEVER admit a mistake. Like the 300 crank bearings. ( another story )

Simply put - the 550 PP - iron cylinder is a colossal mistake - way too thin - too smooth a bore - way too small of transfers - and they warp by merely installing the manifolds and the head - but - mostly from the pressures of the torque between the head and the cases.

IF - you have one of these and you want to run it - and run it hard - you will need to bore the cylinder to clean. Chamfer the ports REAL good and smooth. Put the cylinder in a honing jig and torque it down as you would with a normal install - install the intake and exhaust manifolds - and use a 300 grit gear drive hone - and hone until the drill quits trying to jump out of your hands - or until you get .004 piston to ring clearance. .006 if using forged pistons. Install your dual water lines with the water inlets being just above the intake manifold and below the head on either side. One exit at the base of the exhaust manifold ( the old intake ) and one out the front of the head. Keep the compression BELOW 180 PSI - and - IF you port it - use a 400 pump and bulkhead brace for best results. Leaving the ports alone is probably best for most folks IF you will be using a 550 pump.
From 1991 to 1995 - KMC built the SX-550 - an alloy cylinder - with a reed valves motor and floating drive shaft - and finally “fixed” the 550 - once and for all. The next year the 750 hit the market and made this a moot point. For those who prefer the handling and lightness of the JS hull - I always recommend finding one of these - leave it alone - and ride it. And buy a 440 base unit and go nuts - for the madness in us all. And you KNOW we are

This is MY story as I lived it - others in the industry may tell a slightly different tale - but - this is my story and I am sticking to it .
Cliff Jones


Jul 2021
Great article. Had no idea.
Nov 2019
This is very interesting. It's like the whole damn history! Thanks Cliff great read...

Related products