4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Resto-Mod Formula – Kawasaki 650 SX

Overview – Our customer, Rob, is a long time stand-up enthusiast who rides an SXR 800. He wanted to share the sport he loves with his two teenage sons (and their friends), but (understandably) didn’t feel like buying them both $6000 SXRs. From an economic standpoint, he wanted boats that were cheap to buy, cheap to build-up, and cheap to buy parts for. From a functional standpoint, he wanted boats for his boys that had great fuel range, were as reliable as rocks (on 91 octane), easy to tune, and quick enough “stay near” his SXR. In addition, he wanted them to have the potential to upgrade these boats to run the same speeds as his SXR (in the event that the boys “got serious” about riding at the same pace as dad).

Being that his boys were both very different personalities, they did not want the same brand boats. Rob finally opted to buy a 1990 Kawasaki 650SX, and a 1990 Yamaha Super Jet, and he asked us to build them up to fit his needs. This document will cover the build-up of the Kawasaki 650SX. Our “Resto-Mod” section has the document that has the same write up on the Yamaha 650 Super-Jet.

At the same time that Rob was having us build his stock-piped F650, we were building our own in-house version that was being fitted with an aftermarket exhaust-pipe (in this case the Factory Pipe Products pipe). The final performance results of both boats was very impressive…. especially when you consider that each boat for about half the price of a new SXR.

The Finished Boats

F650SX “REC” – Stock Pipe, Sleeper Engine Mod (38 carb), OEM Ignition, Single Input Cooling

This boat is the ideal recreational high-performance workhorse. It has acceleration and top speeds within a hair breath of a stock SXR. It offers responsive acceleration at every throttle setting, and smooth cruising at all rpms from planeing up to peak speed. This boat is manageable enough for any novice to ride, yet quick enough to be plenty of fun for more experienced riders. Our hull setup virtually eliminated all porpoising, and had rock solid straight-line control.

F650SX “Comp” – FPP Pipe, Sleeper Top-End Mod (44 carb), Dale’s Ignition, Dual Input Cooling

This single carb “pocket rocket” will blow the doors off any stock SXR, while consuming about 40% less fuel than an SXR. The strong and instant acceleration is ideal for wave jumping and buoy riding, yet the boat maintains excellent mid-range” cruising” abilities. At the 51mph peak speed, is has great straight-line stability, and minimal porpoising. The relatively low peak rpm assures great long-term reliability, and excellent long-term engine wear.

Kawasaki 650 SX



Fuel Octane



38-39 mph


87 Octane


F650 “REC” – (Stock Pipe)

45-46 mph


91 Octane


F650 “COMP” – (FPP Pipe)

50-51 mph


91 Octane


“Recreational” Mod vs. “Competition” Mod

Nearly every rider will say that they want their watercraft to have the “maximum power” allowed by pump gas… But there is more to it than that. Group K separates high performance setups into two categories … “Recreation” and “Competition” levels of tuning. The big difference between the two is the power delivery.

91-octane “Recreation” setups have a very linear acceleration rate that allows steady cruising at any rpm between planeing-speed and peak-speed. These setups are very quick, but still very easy to control. In the case of the Kawasaki 650SX, getting this “Rec” power delivery meant retaining the stock exhaust pipe. If you are interested in a responsive machine that gets the best possible fuel range and the best comfort during cruising, the “Rec” mod is the way to go.

91-octane “Competition” setups typically turn a lot more rpms and make considerably more power than the recreational setups (usually a function of an aftermarket exhaust pipe). The down side of these “Comp” setups is that they have higher fuel consumption, and they have an area in the lower rpm range where the exhaust-pipe suddenly comes into its high-efficiency window. When this happens, there is a very sudden increase into the higher rpm range that many riders refer to as an engine “coming onto the pipe” or having a “low-range hit”. When riding a “Comp” F650sx on smooth water with a digital tach, you find that the boat runs smoothly from planning speed up to about 5100rpm, and smoothly from 5600rpm to the peak of 7000+. However this sudden surge of mid-range power in between makes it virtually impossible to “cruise” between 5100 and 5600 rpm. A pwc with this kind of power delivery is great for more experienced expert level riders, but it can become tiring for some novice level riders.

Technical Goals – Given Rob’s requests, we decided do his initial build-up as the “Recreation” version that retained the entire stock exhaust pipe. Based on our long running experience building our “Sleeper” kits, we knew that the 650SX could produce plenty more rpms and speed with the stock pipe still in place. This would help greatly to keep down the cost of the initial build-up. If Rob decides later to upgrade the speed of this machine, the simple addition of an aftermarket pipe (and some carb jetting) would easily net the big increase in rpms and speed to get this boat to run the speeds of the stock SXR (and then some).

The used 650SX that Rob bought was completely stock, and in great looking condition (obviously a fresh-water boat it’s entire life). The boat started well, idled okay, and ran like a typical stocker (weak acceleration & plenty of cavitation). This boat was not going to need a “complete” rebuild… but there were several bases that needed to be covered. Since this boat was expected to be a long-time reliable runner, we decided that a complete teardown of hull and engine components (for inspection) would be in order. It’s a good thing we went this route because we found several detail items that would have caused problems down the road. These engines are so simple in their construction, a complete engine removal and teardown can be done in a very short amount of time with just basic (metric tools) and a basic mechanical understanding. It’s one of the qualities that makes these boats so darn practical to own.

Why not go to 750 instead of staying 650?

The main reason that 650s fell from favor among pwc enthusiasts in the early 1990s was that the bigger 750s were better suited for the high speeds of professional racing. The 650s were (and continue to be) reliable, fuel efficient, fun and fast PWCs. For non-racing recreational applications, a 650SX is still a fantastic boat.

One common desire of many 650 owners is increase displacement, or upgrade to 750cc. Before upgrading your 650 to 750, there are a few important things to consider. The bore and stroke displacements are as follows:

OEM Stroke

OEM Displacement

1 mm Over

2 MM Over

3 MM Over

650 – 70 mm

76 mm – 635cc

77 mm – 657cc

78 mm – 668cc

Not Available

750 – 74 mm

80 mm – 743cc

81 mm – 760cc

82 mm – 779cc

83 mm – 795cc

Big boring a 650 cylinder to anything near 750cc is obviously not doable. Installing the longer stroke 750 crank in the 650 cases is also not doable (won’t fit), and installing the 750 cylinder on 650 cases is a setup wrought with lots of undesirable technical compromises. Many 650 owners in the past have fitted entire 750cc engines into their 650 hulls, but the end result is questionably worth the cost and effort…here is why.

The 650 crankshaft is considerably smaller and lighter than the 750 crank, allowing for very quick rpm acceleration. On top of that, the stock 650 ignition flywheel is about half the weight of the stock 750 flywheel, which further benefits the acceleration of the 650 motor. Given all this, a well prepared 650SX motor can easily match the acceleration of most 750s. The difference is that the 750 engine has the ability to drive the hull to a much higher peak-speed (close to 60mph). Unfortunately, even the best prepared 650SX hull will become “very” hard to control at speeds over 50mph….and almost impossible to control at speeds over 55mph. In our minds, if you want the speeds of a 750 motor, it’s wise to get the hull to match. However for a large number of stand-up enthusiasts, a lightweight, quick accelerating, stone-reliable 46-51mph boat is exactly what they want. From a more technical standpoint, there are also some important reliability reasons for keeping the 650cc motor.

The shorter stroke of the 650 crank subjects all the lower end bearings (and rods) to far less wear and stress than a 750 crank turning the same rpms. This is one of the reasons that 650Kawasaki lower ends have that “last forever” reputation…. Even when modified.

The head-bolt patterns of the 650 and 750 Kawasaki engines are the same. This means that the smaller bore 650 cylinder gets a much wider sealing surface between the bore diameters and the head bolts. This added surface area makes it much easier for the 650 motor to maintain a long term head gasket seal…especially when higher compression ratios are used.

There is “no” 750 that can come close to the long fuel range of a single carb 650. This is a very important quality for recreational riders interested in longer outings (and lower fuel costs).

These qualities, along with others, makes the 650SX a very practical and affordable platform that easily matches the “fun factor” of a 750 in a high-performance recreational application.

Engine Lower End

The areas of every older boat that merit the most inspection are the rubber parts that are most prone to deterioration. In the case of this 650SX, that meant motor mounts, crank seals, the rubber parts in the carb, and the crankcase drain mechanism. As expected, we found problems in all those areas on our 650s.

When removing the motor from Rob’s hull, we found that 3 of the 4 rubber motor mounts were de-vulcanized, and not attaching the engine to the hull. This was not evident on the first visual inspection or short initial ride…. It’s an area worth double-checking on every 650SX.

No matter how you cut it, the crank seals in this engine were 15+ year-old rubber. As expected, when we got the engine all apart, the front and rear crank seals fell apart in our hands. You would think that an engine with blown crank seals would not start, idle, or run well (as Rob’s did), but we see it often. Since there are no rubber center crank-seals on this engine, we simply replaced the two rear crank-seals and the single front seal. We made sure to mount them with the spring-side facing “outwards”.

All 650 and 750 Kawasakis come stock with a “crankcase drain”. The original intent of this drain was to allow for quick and easy draining of the water inside the motor in the event that the boat is completely submerged. This drain employs a spring loaded tapered rubber plug that is released with a pull knob on the exhaust manifold. On most 650/750 high rpm setups (running an aftermarket pipe), this drain is removed and blocked because it presents a common airleak risk. However if you are building a boat for use/abuse by teenagers, having this drain may not be a bad idea…. Rob had us leave the drain on his stock-piped Formula 650SX.

Cylinder & Head

While “big boring” the 650 cylinder is not a big performance benefit, there is a big benefit to having straight fresh bores. Rob’s 650 had some very worn stock bores that we bored to 1mm oversize.

To get the most power from this 650, we performed our 91-octane cylinder porting and decking modification. The main intent of this porting is to maximize bottom end and mid-range acceleration. As it happens, the porting also allows for a nice increase in peak rpm’s as well. Included in our porting modification is exhaust manifold matching, and cylinder decking that properly sets up the squish clearances between the head domes and piston crowns.

There were many aftermarket heads sold for the 650 motors over the years. Some were very good parts…and many were not. In short, the best aftermarket heads were the ones that had a volume between 28-30cc, and did not employ “O” rings for head sealing.

Rob’s 650 had a stock head, which is a very good part that is easily modified to get the compression that our Formula 650 would need. At Group K, we typically prepare the ported cylinder and modified cylinder-head as a matched pair to assure that the deck heights and compression ratio is correct for the fuel octane being used (in this case 91octane).

In addition to the compression modification, we added a second water outlet fitting to the cylinder-head. This fitting has a two-fold purpose. Firstly, it allows more even cooling of both cylinders, as well as increasing the ability to exchange away a lot more heat. Secondly, it reduces the amount of water pressure delivering water into the exhaust pipe interior. Reducing this water input (within limits) allows for better overall performance from both the stock pipe and most aftermarket pipes.

Carburetor & Inlet Options

All 1986-1990 650s came stock with a 28mm Kiehin carb that is fed by a remote fuel pump. The 1991 and later 650s all came stock with a 38mm Kiehin carb fed by a built-in fuel pump. Group K did (and still does) carb boring and re-jetting of these stock Kiehin carbs for our popular “Sleeper” Engine kits. However when a higher revving aftermarket pipe was installed, the stock Kiehin carbs struggled to supply enough fuel. “Back in the day”, performance shops sold many different single and dual carb kits. The many aftermarket twin-carb setups were very problematic and temperamental to tuning, and we do not recommend any of them for any application.

With hindsight being 20/20, the very best performing of these carb setups were a single 44 SBN Mikuni carb on an “opened up” stock inlet manifold, (or on an aftermarket inlet manifold).

Rob’s 650 (like all ’86-’90 models) came stock with the 28mm Keihin. The later ’91-’93 models all came with a 38mm Kiehin. Rob had a 38 carb from a ’92 model, and wanted to upgrade to the 38mm Keihin with our “True Boring modification. This conversion is a snap since the stock 28carb and 38carb have the same bolt pattern, and the stock throttle cable hooks right up to the bigger 38. We simply bored the stock inlet manifold to 38mm for the new carb. Even the stock flame arrestor case matched up perfectly to the new 38…. And the True Bore 38 offered a clear improvement in throttle response over the tiny stock 28mm carb.

The used 650SX we had purchased for our own in-house “Factory Piped” version project had the “post 1988” version of the 28mm manifold and carb. We opted to replace the stock 28 carb with a newer “square pump Mikuni SBN carb (it’s the same carb that comes on all 1990-1995 Super Jets). We Installed a 44 Mikuni adaptor plate to our stock manifold, and then bored out the stock manifold to the 44mm internal diameter. Some finish “blending needed to be done with a die grinder, but the finished part perfectly accommodated our new 44mm SBN. With the wide range of jets available for the SBN-44, fine-tuning was a snap. The 44 SBN Mikuni is a perfect match for the fuel needs of the FPP pipe on our “Comp” F650SX.

For both machines, we opted to eliminate the choke butterfly, and install a “primer” kit that allows for a primer pump to inject a short burst of raw fuel into the carb for cold starting.

Fuel System Issues

While our F650SX “Comp” consumes far less fuel than a full out race boat, it does consume considerably more than a stock 650SX. Given that, the fuel system needed some modification.

All 650SXs come with plastic mesh filters molded on to the fuel pickup tubes. Over time, parts of this fine plastic mesh can become obstructed with various kinds of fine debris. The end result is that a stock fuel stem may not be able to draw fuel as fast as the “Comp” F650SX needs to pull it….thus leading to fuel deficit at high rpms. To eliminate this possibility, we cut the mesh filter completely off our “reserve” pickup tube, and replaced it with a length of fuel line and a free flowing metal filter that could lay on the bottom of the fuel tank. During casual cruising, we could safely run our F650 on the “regular” fuel feed with the stock mesh filter. However anytime we planned to run the F800 hard at higher rpms, we simply switched over to the freer-flowing modified reserve pick-up tube.

With the fuel pick-up tube attended to, we though all of our fuel-supply problems were solved … but we had another lesson yet to learn.

While testing props and nozzles one morning, we were making repeated, long, smooth-water passes turning consistently 7050 rpm. About 2 hours into that testing, the test rider encountered a few ripples at peak speed that set the nose bouncing a bit…. But he still maintained full rpm. Suddenly, the boat surged a couple of times (obviously from air bubbles getting in the fuel pickup of a 1/3 full tank)… but he still maintained full throttle. After about 2-3 seconds of the intermittent surging, the engine shut down. Back at the shop we found a scored rear piston…. obviously from air bubbles entering the fuel lines while running the engine at full rpm.

We realized that we needed to re-define the term “pump gas safe”. As long as our “Comp F650SX had a full tank of fuel, it was totally “pump-gas safe”. However as soon as the tank got under half full, the occasional air in the fuel line, from rough water riding, could easily create piston-killing detonation in mere moments. For any closed-course race boat, it is impossible to keep air from entering the fuel pickup tube…the only thing you can do is eliminate those air bubbles before they reach the carb….so we did.

The solution was to install a pulse-pump fuel-air separator. With the separator mounted, we were able to run the tank to less than a half gallon at peak rpm with no detonation or surging at all. The only down side of the separator is that when you run out of gas…. you are “out of gas”… there is no notice. Just the same, we figured this inconvenience is better than a scored piston. We made a simple aluminum strap bracket to mount the remote pump & chamber on top of the battery (worked like a charm). For any modified closed course or freestyle “pump-gas” boats (like our F650SX), we strongly recommend a fuel/air separator to avoid damaging a piston when fuel levels get low.

Reeds – The reed cages on the Kawasaki 650s are considerably smaller than the ones used on the 750s, however they are a good match for the fuel/air demands of a recreationally modified 650 engine. The 86-90 models all came with stainless steel reed petals that would sometimes break off, and cause significant damage to the engine internals. The ’91 and later 650s all used composite “fibre” reed petals that performed as well as the steel petals, but damaged nothing if they happened to eventually break. Both Rob and our 650SX came to us with the metal reed petals, so we just updated to the fibre ones.

Some performance shops advocated removing the reed petal stopper plates mounted over the top of the reed petals. We strongly recommend to “not” do that. Doing so can dramatically shorten the life of the reed petal, and has a very questionable impact on performance….we do not recommend it.

Flame Arrestor

There are a wide array of nice aftermarket flame arrestors available for the Kaw 650s. During the late 80s & early ‘90s it was common for owners to use “fabric” filter K&N style flame arrestors. Sadly, this fabric became easily soaked with water which serious hurt performance. All the newer arrestor elements use only metal screen elements that are not effected by water exposure.

It bears noting that the design of the 650 engine naturally locates the carb (and air intake) relatively low in the engine compartment. For boats that will be used for frequent freestyle maneuvers, this can be a recipe that exposes the flame-arrestor/air-inlet to a significant amount of water spray from the floor of the engine compartment. One way to reduce this risk is to use an extended aftermarket adaptor that holds the arrestor element higher up in the engine compartment, thus reducing water exposure.

Ignition System

Stock Kaw 650s turned about 6200rpm, and the stock limiter is at 7000rpms. This stock limiter is easily high enough to accommodate the 6600rpm of Rob’s stock-pipe F650. In fact this limit is also okay for most of the older aftermarket pipes (Westcoast, Mariner, Coffman, etc). However the Factory Pipe exhaust on our in-house F650 project has a peak between 7100-7200 rpm, and so a limiter mod was needed. By far the best limiter modification available is from Dale’s Jetsports in Texas. Dales sells a complete cdi box, with built-in coil, that eliminated the limiter altogether. We have found the Dales cdi to offer the best long term reliably of any of the mods we tried.

As mentioned above, the stock ignition flywheel is a very lightweight part, but it can be lighten even further. Group K does a lighten modification for these flywheels that reduces weight by about.3 pounds. The weight reduction is not huge, but it has a noticeable effect because we remove the weight from the outer parameter of the flywheel where it hurts the inertia the most. Rob opted to keep his flywheel stock, while we lighted the flywheel on our in-house F650.

The Kaw 650SX models never came equipped with a lanyard “dead-man” switch that came on all future 750 stand-up models. Since these 650s were both going to run considerably faster than a stock, we decided to get lanyard-style 750 hand-switches for both of theses 650s. We found these switches at a mail order salvage shop for a lot less that the $128 that the Kawasaki shop wanted for a new one. The installation was easy, and the added safety margin is well worth the cost.

Cooling System

The stock Kawasaki 650 comes with a single input cooling system that uses pressurized water from the pump run through the top-end and pipe to perform engine cooling. This stock systems capacity can be easily upgraded by drilling all the brass fittings out to 17/64” (.265”). Doing so bring in enough extra water so that a second “water-bypass” outlet fitting can be installed in the cylinder head. This added fitting helps provide more even cooling for the two cylinders.

Our in-house “Comp F650” would be turning a lot more rpms than Rob’s stock-pipe version, so we opted to fit it with a “dual-input” cooling system. The stock 650SX hull is fitted with a stock “drain-tube” that runs from the engine compartment to the pump area. We simply drilled our pump for a second water input fitting, and used that abandoned drain tube to deliver the second supply of water to the engine. The aftermarket Westcoast exhaust manifold we got for our “Comp” boat already had the fittings for two water inlets, so the plumbing was a snap. Since we were bringing so much additional water to the engine, we needed to add two extra water “bypass-outlets” to increase water flow. We added one extra bypass into the line between the cylinder and pipe, and one between the head-pipe and stinger of our FPP pipe.

Bilge System – The stock bilge system on the 650SX is a “siphon” type that uses thrust column passing through the exit nozzle to literally “siphon” water from the water pickup located in the bottom rear of the engine compartment. This siphon works great as long as the line is clear. To help the bilge keep the engine compartment as dry as possible, we recommend to glue the bilge pick-up to the floor of the engine compartment with silicone sealer.

Exhaust System

Stock Pipe Setups – While the stock exhaust pipe is by no means optimum, it does have the ability to deliver a very controllable and liner acceleration curve that is very desirable for novice level riders. In addition, it can be modified to offer some easy power gains. For Rob’s stock-pipe F650, we added a large water bypass to the cylinder-head. This bypass has the two-fold effect of reducing water pressure being delivered to the interior of the pipe, and also more evenly cooling the two cylinders and head domes.

The stock 650 pipe has a stainless steel stinger-cone that is easily removed. Shortening the large diameter part of this cone by ¾” allows the stock-pipe F650 to turn about 100rpms higher. This mod does slightly weaken low-end acceleration, but it well worth the loss for riders putting in a “higher-speed” day on smooth waters. Rob wanted the option to run his 650 either way, and simply bought a second stinger-cone and hose on Ebay. Changing the cones (even at the beach) takes a screwdriver and about 3 minutes to do.

Aftermarket Exhaust Pipes – Over the years, there were many aftermarket shops that sold pipes for the 650SXs. The cast pipes from Westcoast and Coffman’s were favorites of many riders because of their decent performance and great durability. However, the latest development on the 650SX exhausts has been done by Factory Pipe Products. Their pipe delivers excellent overall power, as well as adjustable water inputs ion the head-pipe. Since the FPP pipe is the only 650SX aftermarket pipe that can still be purchased new, we opted to use it on our “Comp” F650sx…. It worked great.

Exhaust Manifolds – In the 80s & 90s, many shops offered aftermarket exhaust manifolds that had a tall “deflector” bridge between the two exhaust passages in the manifold. In 1989, Kawasaki adopted that same design for all their stock 650 exhaust manifolds…and those manifolds (Called TS manifolds because they first appeared on that model) perform much better. When preparing a high-performance 650, it’s important to have one of these “TS” manifolds, or one of the similar aftermarket manifolds. For Rob’s stock piped “Rec” 650, we used a “TS” exhaust manifold. For our “Comp”, we found an aftermarket “Westcoast” manifold that had slightly larger exhaust passages as well as the much needed deflector.

Waterbox Silencer – All 650SX models came stock with a round stainless steel waterbox that sits in the nose of the boat. These waterboxes offered exceptionally good silencing characteristics as well as very low restriction to power. Contrary to what some folks believe, the back-pressure offered by the stock waterbox is very important to maintaining good low-end power. In the 1980s, many shops sold aftermarket waterboxes that were unquestionably “a lot” louder than stock, and very questionably better (for power). We strongly recommend “only” the stock waterbox on all 650SXs. The power increase of an aftermarket waterbox is minimal, and the big increase in sound level is annoying to everyone.

The 1991 and later 650SXs came stock with a rear exhaust that helped to make the boats quieter, as well as get the exhaust gases out of the riders face. Both our boats were the older versions that had the annoying forward mounted exhaust exit. Rather than go to all the trouble and expense of installing a rear exit exhaust, we fitted both boats with an easy and inexpensive “half-rear” exhaust outlet. In short, we relocated the stock exhaust outlet the rear of the engine compartment. This required an extra length of exhaust hose that easily fit in both boats, and blocking off the old outlet hole. Rob opted to fiberglass his up…we just fabricated an aluminum plate and attached it with rivets & lots of silicone sealer….worked like a charm. The end result was great. The exhaust gases were completely out of our face at all times, and the boat was noticeably quieter from the shoreline. We consider this a must do for every front exit 650SX.

Fuel & Oil

All 650SXs came stock with an oil injection system. While these systems worked “okay”, the oil pump is always at risk of failing at any time, and causing serious engine damage. The oil pump should be replaced with an “oil-pump block-off” plate, requiring that all fuel and oil be pre-mixed.

The amount of oil needed in the premix is a function of average operating rpms. If a boat is operated by a novice rider, that seldom runs at full throttle, 50:1 could safely be used. For Rob’s 6600rpm F650 a 40:1 would be needed…and our 7100rpm version would need a 32:1 premix.

All of our “Sleeper” Kits and F650 kits are normally setup to use 91-octane pump gas. The local marina where Rob rides only has 89-octane, so we reduced the compression on his F650 to accommodate that octane. This compression reduction would allow safe use of 89-octane so long as the boat was never operated for an extended run (2+ miles) at peak rpm. This sat fine with Rob and his boys. For our FPP piped version, there is no 89 octane specification. A 650SX that turns 7000+ rpm “must be run on 91+ octane fuel. In situations where our 7100rpm version would need to make extended peak rpm runs, adding a 20-30% mix of av gas or race gas would be adequate to control temperatures.

Pump & Impeller

The stock impeller on all 650SXs is aluminum…and not useful for any high-performance application. In the 1980s there were several stainless steel aftermarket impellers offered for the 650SX, and they clearly worked better than the stock prop. However since that time, there has been much progress in impeller technology. Most importantly is that modern day impellers use very small contour hub diameters, and thin blades with progressive pitch bends. The end result is a prop that offers more bottom-end “and” more top-end, because it actually processes more water per revolution than any of the props from the 80s era.

Of all the new design impellers, the one best suited for the 650SX is the Solas “I” pitch impeller. As these props come out of the box, they are pitched too steep for the 650SX, however with re-pitching this prop out performs any other prop we tested.

Unlike all the 750 models, all 650s use a pump case that must be installed using an abundant amount of industrial grade silicone sealer (we use only Permitex “66” industrial clear) in order to seal the pump-case to the hull. This brings in a small element of assembly “art” that is fundamentally important. If this sealing assembly is not done well, the best prop in the world will experience cavitation. It bears noting that any “good” sealer should be allowed at least 18 hours to fully cure. If the machine is run before full curing, the high pressures will just blast the silicone away.

For 650 owners who want to get every possible advantage available, we recommend pump “blueprinting”. Blueprinting involves removing all the interruptions and casting drafts from the vane body interior. This blueprinting offers much better hook-up when the boat is run at high speed in rough water, however it will not increase peak speeds on smooth water.

Handling Parts

Ride Plates – Having a ride plate that is longer than stock helps to greatly reduce porpoising, and improves high speed stability. In the 1980s when 650SXs were routinely raced, the IJSBA rules only allowed the ride plate to be 1.5 inches longer than stock…. And all the ride-plates from that era are at that 1.5” limit. More recently, IJSBA has modified this rule to allow ride-plates that are 100mm (3.9”) longer than stock. That additional 2.4” of ride-plate length would have been a huge help to the handling of all 650SXs….Unfortunately, most after-marketers had stopped production of 650SX plates by that time, and never made the longer ride-plates for the 650. To amend this problem, Rob simply welded on a 4’ long extension to the back his stock ride plate. Our 650SX came with a 1.5” longer “Westcoast” ride-plate, so we welded on a 2.4” extension to get our extra length…. Wow what a difference. Both of our 650SXs handled worlds better at high speed with the extended plates.

One of the older style 1.5 longer ride-plates that deserves an honorable mention is the “concave” Jet Dynamics ride plate. This plate is still available today, and offers great handling for riders who do most of their riding in the surf. It offers very quick turning as well as excellent holding in turns.

Scoop Grates – Unlike the ride plates, there are modern technology ride plates available for the 650SX. Worx Racing offers a modern design “top Loader” scoop grate for the 650SX that offers a big increase in rough water hook-up without harming smooth water peak speeds (much). We recommend this Worx grate for any 650SX that will be ridden in rough waters.

Hull Extensions – Another handling device allowed by IJSBA in recent years is “hull extensions”. These extensions come in many different designs and configurations. The main intent is to add on an extension to the rear of the hull that helps high speed stability and straight line control. We fabricated our own hull extension from a piece of 2” angle aluminum extrusion. No shop makes these extensions for the 650SX, so you’ll need to fabricate your own… but the results are worth the effort. We attached our extensions with a few sheet metal screws and lots of clear silicone.

Tiny Tach

Like all stand-up watercrafts, the 650SX has no digital tachometer, and no hour-meter. We consider both of these items to be of great importance for the fine tuning and monitoring of a high performance stand-up like our “Comp” F650sx (for all the obvious reasons).

The Tiny-Tach is a waterproof and very durable tachometer/hour meter that is easily installed on any pwc. The hour meter registers any time that current is moving through the spark plug wires. The digital tach readout (which reads the voltage pulses from a plug wire) offers rpm data that is updated every two seconds. This means that meaningful tachometer testing can only be done on long runs of smooth water were rpms can be sustained long enough for accurate measurement. For our initial testing, we mounted the Tiny-Tach on the handlepole where it was easily seen during full speed runs. After our detail tach testing was done, we moved the tachometer to the interior of the engine compartment where it served nicely as an hour meter in the relative protection from water spray.


Handlebars – The first thing that any serious 650SX owner needs to change is the handlebars. Replacing the back-swept stock bars with “straight” bars makes a huge improvement in steering control at higher speeds.

Quick Steer – In the 1980s, it was common for 650SX owners to install “quick-steering” plates or modifications. These plates moved the steering cable mount point in such a way that the steering nozzle gets more movement with each degree of handlebar movement (compared to stock). It bears noting that just increasing the sheer thrust of a boat serves to make it steer more easily and “quicker”. Rob’s stock-piped F650 had a standard steering plate that didn’t “feel” like it needed “a lot” of additional quickness in the steering. We suspect that the increase of thrust from the new-design impeller, and the extra power of the engine modifications, helped to make the boat much more responsive to steering input from the bars.

Hull Preparation – Our F650s ran faster speeds than any 650SXs we had ever tested with. With that, we experienced handling issues we had not seen before on these hulls. One expected problem was that it became increasingly difficult to steer the boat perfectly straight during radar testing on glass water. The other problem was the boat “pulling” to one side at higher speeds on smooth water. The installation of handling parts (ride plates, grates, etc) seemed to make little difference with this pulling to one side. With that, we tried a different approach. Tests we had conducted with other machines showed very positive results from using 30-40 grit sandpaper to create deep full-length “scratches in the hull’s bottom surface. All the sanding strokes should be front to back (continuous) for the full length of the hull. Properly done, the deep, full length, scratches should eventually eliminate any part of shiny surface. While this preparation may not look attractive, these scratches will act as thousands of small rudders that will make the hull track “a lot” straighter in all water conditions (especially at high speeds). These scratches can also allow for much better surface holding in high-speed turns. This hull sanding prep made a big improvement of the straight-line control. We consider this preparation to be mandatory for any modified 650SX.

Package / Upgrade / Parts

Group K Price

F650 Top End Modification –  Includes:

  • Cylinder “Rec-Finish” Porting

  • Decking

  • Cylinder Head Modification and Exhaust Manifold Matching

  • Cooling System Upgrade

  • (SEND: Head, Cylinder, & Exhaust Manifold)


Additional Engine Upgrades

“Competition” Port Finishing Option


Cylinder Head Modification only


Carb “True Boring” Modification (28mm or 38mm Carb)


Intake Manifold Modification to Accommodate 38 Kiehin Carb


Intake Manifold Modification to Accommodate 44SBN Mikuni Carb


Ignition Flywheel Modification (-.3 lb.)


Engine Parts

“Half-Rear” Exhaust Hose Extension


Factory Pipe Products Exhaust Pipe


“TS” or Aftermarket Exhaust Manifold


Group K “Pulse” Fuel/Air Separator


Pre-Jetted 44mm Mikuni SBN Carburetor


Primer Kit


Pod Flame Arrestor with Adaptor (for Mikuni 44)


OEM Fibre Reed Petals


Dale’s “Bad Attitude” Ignition Coil Modification


Driveline Labor

Pump Blueprinting & Scoop Grate Matching


Pump Nozzle Taper Boring


Driveline Parts

Solas “I” Pitch Impeller (Re-Pitched to F650 Specification)


Worx “Top-Loader” Scoop Grate


Ride Plate Extension Modification


Jet Dynamics “Concave” Ride-Plate



Tiny Tach” Digital Tachometer/Hour-Meter


Aluminum Hull Extensions


0’ Handlebars


Aluminum Trigger Throttle


Cylinder Boring, Hone, & Chamfering


Complete Gasket and Seal Set (no Exhaust Body Gaskets)


Rebuild Crankshaft with Crank Bearings (exchange)


Wiseco Pistons with Rings, Wrist Pins, and Clips


Wrist Pin Bearings


*prices subject to change based on manufactures pricing


GROUP K • 4597 CALLE DEL MEDIA • FORT MOHAVE, AZ. 86426 • (928) 763-7600

GETTING THE WORK DONE – Most customers send GROUP K the parts needed for modification via UPS, and then do the engine assembly work themselves. We also do complete engine and pump assemblies for customers who want a finished unit ready for installation. The 150-lb. UPS weight limit makes engine shipping practical and affordable. NOTE: Group K will bill an additional $25.00 handling charge for complete engine assemblies. All orders prepaid with a cashiers check or money order will be returned freight free via ups ground service anywhere in the continental United States. All other orders will be billed to a visa/master card or sent freight collect cod cash. If you would like to pay additional for 3 day, 2 day, or 1 day return shipment, please specify your preference in a cover letter with your parts. Be sure to include your return address and day phone information in case we have any questions regarding your order. PACK YOUR PARTS CAREFULLY !!