4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Yamaha GPR-XLL-XLT 1390 – Big Bore

Group K Modifications

Overview – From it’s release in 2000, the GPR1200 has been a very popular platform for performance minded PWC owners looking to build a recreational “hot rod”. From 2000-2002, the majority of our GPR performance testing and development was centered around the endurance race machines that we built. Unlike closed course race machines, these endurance machines hold reliability and longevity as important a priority as speed.

For any GPR owner/racer that was seeking sheer speed and power, going to a triple pipe exhaust was mandatory. While the triple pipes do generate huge increases in rpm, power and speed, they also introduced maintenance schedules and maintenance costs that many owners were uncomfortable with. In addition, most triple pipe setups mandated the use of expensive racing gasolines (as if regular gas wasn’t expensive enough). These conditions separated performance minded GPR owners into two distinct groups …. Single-Pipe, and Triple Pipe. But the machine everyone wanted was a machine with the simplicity and reliability of a single pipe setup that could run the speeds of a triple pipe setup …. On 91 octane pump gas.

After years of testing on single pipe and triple pipe endurance racing machines, we were able to make single pipe 1200s run much faster than stock, with great reliability. However as these single pipe setups became more highly developed, they were no longer pump gas safe, and they began to suffer from “side effect” reliability issues. In short, there was only so much we could get from a race legal 1200 single pipe GPR.

While most of our development centered around race machines that had to comply with various rules and regulations, the bulk of our customers were high performance recreational riders who didn’t care about rules or regulations … just reliable power. With this, we set out to develop a big bore modification for the GPR that would net the reliability and speeds these customers were asking for, and at the same time resolve other reliability problems of the GPR. In short, these 1390cc Big Bore packages have been developed to run very reliably on pump gas….”and blow the doors off any pump-gas boat on the water”.


1390 Big Bore “87” (octane) – This is the strongest accelerating and fastest 87octane-safe PWC package available from anyone …anywhere. Use of the stock pipe, stock head, stock carbs, stock ignition, and mild compression ratio allows this boat to start and idle just like a stocker …. And offer excellent reliability. The “87” has vicious acceleration at any speed … any rpm.

1390 Big Bore “91” (octane) – The same kit as the “87” with higher compression. Accelerates even harder… runs slightly faster …. Leaves RXP owners speechless.

The following table shows the performance results of this project. Note that the same mods can be made to the XLL and XLT models, we will post the data as we obtain it.

The Numbers:


Cold RPM’s

Speed (Cold)

Hot RPM’s

Hot RPM’s

Stock GPR 1200






Group K 1390 Big Bore “87”





Solas 14/20

Group K 1390 Big Bore “87”





Solas 14/20

Group K 1390 Big Bore “91”





Solas 14/20

Group K 1390 Big Bore “91”





Solas 14/20

*conditions 450 ft. alt., 70-80’ f , 20-30% humidity, smooth water, 200 lb rider.

About Cold Numbers and Hot Numbers – One hard reality of high performance 2-cycle pwc engines running on pump gas is the slight loss of peak rpm ability as the engine becomes “heat soaked” (this applies to all stock machines as well). We focus on this because it is a significant performance issue (particularly on 87 octane versions). During our testing, we refer to these different rpm numbers as “cold numbers” and “hot numbers”

For any well tuned two-cycle pwc, the fastest run of the day will always be the very first one…when the engine is at it’s coolest. After that first run, peak rpms will decline slightly, and then stabilize. This rpm loss from heat-soak takes place for a number of technical reasons. As a course of logic, it would seem that simply increasing cooling system ability would be the easy solution. Unfortunately, it doesn’t work out that way. Even if you doubled the exchange of water through the existing water jacket, it would only have a minor effect on the “hot numbers” loss.

The act of combustion, in a low octane engine platform, generates heat much faster than “any” cooling system can exchange that heat away. The best tactic to avoiding big losses in hot-numbers is to take measures to generate less heat in the first place. We spent a great deal of time during our development maximizing all our opportunities to reduce excessive combustion heat, and with that reducing hot numbers losses. The one tactic that worked best, was just going to a higher octane fuel. The higher octane fuel allows for good power output, but generates “alot” less heat… and better hot numbers. The table above shows the change in hot numbers when octane is increased.

We realize that PWC owners who will buy our 1390 kits will want to do everything possible to get the most out of them …. Upgrading octane is the best way.

About Octane – We are constantly approached by owners who are trying to craft a way that can allow them to get race gas horsepower, without having to run expensive race fuel. Sadly, it just can’t be done.

Normal “octane boosters” are capable of making really poor gas into decent gas, but they cannot make good gas into race gas. That is, normal boosters can make 86 octane to 89. However the same booster can only make 91 into 91.5 or 92. Keep in mind that most octane boosters do not turn bad gas into high performance fuel … they make bad gas into a fuel that is “less likely” to harm your engine. We discourage the use of octane booster except in emergencies (even in 87 octane 1390s).

One alternative used by many owners is to run a mix of aviation fuel (aka “av gas) with their 91 octane pump gas. Av gas is usually around 100 octane, but av gas has other properties that are important.

One of the main qualities that makes av gas desirable for aircraft use is that it is actually lighter (per gallon) than conventional gasolines. This reduced weight is important for small underpowered planes that need to be as light as possible to take off. Normal gasoline molecules have two “ends”. The “light” end of the molecule ignites easily, and burns with a quick low temperature flame (as a piece of newspaper would burn). The “heavy” end of the molecule is harder to ignite, but burns with a much hotter flame (as a log would burn). To make av gas lighter, the heavy end of the molecule is omitted to reduce the weight of each gallon. Since most aviation engines are low rpm motors, they experience little power loss. However if 100% av gas is used in a high output pwc engine made for race gas, there is a visible loss of power because the “heavy” ends are not there to contribute to the making of power. All that said, we consider it a great idea to mix 50/50 av/91oct fuel for 91 octane 1390 owners who want a little extra temperature protection during exceptionally hard operation. A 50/50 mix still contains enough “heavy” ends” to allow for good overall power and has a much-improved octane (about 96).

About “Normal Use” – There is no clear definition of “normal use” for high performance machines, but there is a definition for “heavy use”.

“Nobody” runs their automobile, street bike, or dirt bike at peak rpm in high gear, for 10-20 minutes nonstop, and calls that “normal use”. This kind of use is regarded as something between “extreme” and “abusive”. Whichever word you choose, it applies to the 1390 in the same way it applies to your car, bike, etc. The 1390 kits can easily endure occasional long drag race passes and aggressive riding on the fuel octanes specified. But if you plan to run you 1390 in a way that might be considered extreme or abusive, you simply need to feed it a better octane of fuel to get reliable operation. No machine you own is completely indestructible under long term abusive use conditions … the 1390 is no different.

About “Cruising Speeds” – The 1390 offers clean carburetion and smooth operation at “most” speeds, but it does have a cruising limitation. Because of the port timing layout, and the absence of moving valves, the 1390 can cruise steadily up to 4600 rpm, and above 6200 rpm. However no amount of throttle control can allow cruising between 4700 and 6100 rpm (at 4700 it jumps instantly to 6200+). For sport riders, this is never a problem of any kind. However for riders that engage in towing-sports that require steady median cruising speeds, the 1390 may not be the ideal choice.

About Cylinder Valves – Owners of GPRs have a love / hate relationship with the valve mechanisms on their machines. Technically speaking, the valve system offers the best of both worlds .. good bottom end power and good top end power. Without this valve system, the high port timings of the GPR1200 cylinders would give horrible low speed performance.

However, in the real world, the GPR valve system has been the source of countless malfunctions, not to mention plenty of expensive engine failures. A majority of these problems can be resolved on the 1200cc engines by replacing the entire valve system with the aftermarket “pressure-driven” non-mechanical valves. This ($650) upgrade eliminates all the electrical drives, as well as the valve actuating shafts.

We wanted to completely eliminate all the “valve problems” in our 1390 kits, so we started out with the pressure-operated valves. However as time went on, we realized that our 1390 makes so much sheer horsepower at all speeds that no valve system is required at all. Eliminating the use of moving valves would mean that we could not use radical port timings to net extremely high peak rpms. But excessively large exhaust ports also contributed to a big loss in hot-numbers on both the 87 and 91 octane platforms. In the end, choosing a more conservative set of porting specifications gave us all the acceleration and torque we needed to operate without valves, while maintaining the heat reduction we needed to get better hot-numbers.

Steel Sleeve vs. Nikasil Plating – The stock aluminum Yamaha 1200 cylinder bores are plated with a very tough nickel and silicon alloy, hence the term Nikasil. This thin plating has great long term wear, and allows a swift heat path to the aluminum water-jacketing of the cylinders. The down side of plated bore cylinders is that they cannot be over bored when worn or damaged (as a steel sleeved cylinder can be).

We produce a 1350cc big bore kit for Kawasaki 1200s that employs the use of steel sleeves, so we first tested our Yamaha 1390s with steel sleeve bores… and encountered 2 significant problems. First and foremost, the Yamaha 1200 cylinder casting is not nearly as “beefy” as the Kawasaki 1200. When we bored the Yamaha cylinder large enough to install a reasonable thickness sleeve, we could see that the remaining cast aluminum part had some precarious “thin spots” that might easily lead to future cylinder fracturing (especially under the heavier horsepower loads of the 1390). Our second problem was cooling. The steel sleeve made for less efficient heat exchange to the water jacket … and a greater loss in hot-numbers. Since neither of these issues were “minor” we opted to bore only enough material to accept the 1390 piston, and the thin Nikasil bore plating. The result is a cylinder casting that still has very good strength and integrity, “and” offers excellent heat exchange to the water jacket. It bears noting that Nikasil bores also have a lower friction coefficient than steel bores. This lower friction results in a noticeable increase in piston acceleration, as well as a further reduction in heat (improving hot numbers).

About GP800 Cylinders – The GP800 cylinders are structurally the same as GP1200 cylinders, however have a very different porting configuration. The higher exhaust port timing of the 800 cylinders is not an advantage in the 1390 format. In fact, the larger exhaust port area elevates operating temperatures to the point where 800 cylinders cannot be used for 87 octane platforms.

About Pistons – The key to our kit is the special pistons we had built for this application. There are a few aftermarket GPR big bore kits available in other diameters (84mm, 85mm) however to get the sheer gains in torque that would be needed to operate with fixed valve cylinders, we opted to build an 87mm piston. This diameter still offers much better cylinder strength than a steel-sleeved 84mm cylinder, as well as better overall torque. Another nice side effect of using the larger pistons is that the exhaust ports become “narrower” in the sense that their width is now a smaller percentage of bore diameter. This means that the rings can pass much more easily over the exhaust port, and overall ring wear is reduced. In addition to this, our 1390 pistons have a nearly flat crown, which helps greatly to reduce crown temperatures, and improves hot-numbers. Lastly, our 1390 pistons utilize a “boost” port on the intake side. This port has the two-fold benefit of bringing more mixture into the cylinder, and, more importantly, moving cool gases across the underneath of the piston crown to reduce piston crown temperatures (another measure taken to improve hot-numbers).

About Cylinder Heads & Compression – We have designed our 1390 kits to generate excellent overall power without having to rely on high compression ratios. The use of these conservative compression ratios helps to reduce operating temperatures, and minimizes the loss in hot-numbers. We cut new larger domes into the stock cylinder head to net the squish angle and clearance to match the new 1390 pistons. The stock head easily has enough material to accommodate this modification, and offers very good cooling and sealing characteristics for the 1390. We tested several means of sealing the head to the cylinders. In the end, a stock 3-layer metal head gasket, bored to the new piston diameters, worked best. Like all the other gaskets of the 1390, we used 3-Bond 1211 to assure a good long-term seal.

A big question that is on many owners minds is “what’s the compression of the 1390 (in psi)?”. Sadly, a compression gauge does not give good relative compression or “psi” data for two reasons. The first is that the starter motor does not spin the 1390 the same cranking speeds as a 1200 because it’s working a lot harder (this by itself yields lower readings). During our testing, we prototyped many heads of different head volumes. Volume changes that would have yielded 40 psi difference on a 1200 gauge-test often yielded only a 10 psi difference on the 1390. The reason for the small change in indicated readings was that the starter was having to work harder yet to turn the motor over with the higher compression heads, and the motor spun that much slower. The psi data became so worthless to us that we stopped taking the measurements altogether. We specify all our 1390 compression/head data by volume and dimension only …. And we do not disclose those dome dimensions to anyone for any reason … please don’t ask.

Inlet signal Part 1 – One area of constant concern on the GPR1200s we built in the past was the slight lean condition the stock carbs had at 70% throttle when ever a free breathing aftermarket flame arrestor was installed. The stock 44mm Mikuni “I” series carbs on the 1200 were actually designed to have weak inlet “signal” at 70% throttle because that is the throttle setting where most emissions measurements are taken. On a stock machine, the (somewhat restrictive) stock flame arrestor makes for a strong negative pressure (ie. vacuum) in the carbs. With the stock flame arrestor in place, this 70% throttle lean condition is very manageable. However installing free-breathing aftermarket arrestors on a 1200 will reduce the strength of the negative pressure wave to a point where there is a significant lean spot at 70% throttle that cannot be jetted out.

Inlet Signal Part 2 – Many 2-cycle pwc engines (Yamaha 1200s included) employ the use of intake manifold cross-over channels to improve the smooth-ness of low speed carburetion. These cross-overs work very effectively for stock engine platforms. However on a setup built for high rpm output, these cross-overs can significantly reduce inlet signal at high rpm. This reduction in high rpm signal is not a problem for stock engines, however it can cause uneven “full throttle” fuel metering on a high rpm modified platform. In testing we’ve done on other modified pwcs, we have found that simply blocking off these inlet manifold cross-over channels can yield a big improvement in the accuracy of fuel metering during extended full throttle passes.

Inlet Signal Part 3 – All of our testing with big bores on other pwc platforms has shown us a pattern of one very positive quality. Big boring any 2-cycle pwc engine will dramatically increase the inlet tract signal (a result of the larger pistons making a larger negative pressure wave in the inlet tracts). Our 1390 Yamaha kits have followed suit with all the other big bore pwc engine mods in this regard. The end result is that between the increased inlet signal of the blocked inlet crossovers, and the increased signal from the larger cylinder bore diameters, our 1390 kits are able to utilize a free-breathing aftermarket flame arrestor, on the stock 44mm carbs, without experiencing any lean conditions at 70% throttle. As a result, we have calibrated the jetting of the stock carbs (with chokes) for the 87 and 91 octane 1390 kits to be used with aftermarket arrestors and a “blocked cross-over” inlet manifold.

With the exceptionally strong inlet signal of the 1390, we were able to calibrate fuel metering (jetting) with much greater accuracy than a 1200cc platform would permit. The end result is throttle response that is instant at any rpm along with the ability to cover long 5mph zones with no loading up. Our test riders likened the clean response of the 1390 to the best fuel injected boats they had ridden.

Flame Arrestors – As per the previous text, we feel that aftermarket flame arrestors are a big benefit to the 1390. On all our test boats we ran individual “pod” type filters with good results. We presume that “most” 1390 customers will be smooth water “grudge racers” that will not expose the engine compartment to significant water exposure. However if you are the kind of rider that does frequent closed-course or surf-riding type operation, you will be exposing your engine compartment to lots of added water. This is a big concern because the air intake of the carbs is so close to the floor of the engine compartment (where they are subject to water splashing upward). For these owners, we recommend slip-on “Outer-Wears” to help protest the filters from water exposure, and the engine from water ingestion.

Carburetor Tuning – We have spent many testing hours getting the correct carb setting for these kits. That said, we understand that there are some owners that may still wish to do their own carb fine-tuning. If you are one of these owners, we recommend that you purchase a set of Pro Tec “T” Handle adjuster screws for your carbs. These screws allow good access (by hand and/or long screwdriver) to all 3 high-speed adjuster screws. Accessing the 3 low speed adjusters(on the underneath side of the carb rack) is “less easy”. In any case, if you like doing your own tuning, these T-handle adjusters will be well worth the money.

Aftermarket Carburetors – The 1390 can gain a significant amount of acceleration and speed from the use of aftermarket carburetors. We tested with the 48mm Novi carburetors on the 1390. We mounted these carbs over the stock reed cages on a billet aluminum manifold plate that employed no crossover passages. We used flange-mount type carbs (each carb held in place by 2 bolts) as oppose to carbs mounted in rubber spigot holders. The manifold with the rubber spigots offered little clearance for flame arrestors, and has increased internal volume that can slightly reduce inlet signal. Installing the flange-mount carbs was not easy or fun … but the performance results were impressive. One down side of the 48 Novis was that the oil injection cable no longer fits, and the injector had to be abandoned in favor of fuel premix. Another down side was a clear increase in fuel consumption over the stock carbs.

About Reeds – Many aftermarket reed cages and petals are available for the GRP 1200 engine. We tested the 1390 with a few of them, and found performance increases that were questionably measurable. Along with this, the stock reed cages and petals appeared to offer the best long term life. Hence, we are recommending stock reed cages and petals above all others for the 1390s.

About Oil Injection – Because of the increased temperatures and RPMs, the 1390 does need a bit more oil than the standard 1200. For recreation use 1390s, the stock oil injection can be adjusted higher to feed the additional oil needed. However 1390s that will be run primarily at high rpms, an additional premix of 2 ounces per gallon would be required to offer proper lubrication protection. For owners who wish to remove the oil injection all together, we recommend a 32:1 oil premix.

About the Lower End – Most of the smaller diameter “big bore” kits on the market can bolt onto the stock lower end with no machining at all. However the skirt bottoms of the 1390s 87mm pistons “can” make minor contact with casting flash on the crankcases toward the intake side. Because of the convenient location of this contact area, we easily shielded the lower end and reed areas with paper towels, and quickly removed the material needed with a small electric Dremel tool. Photos and diagrams are supplied with each kit.

Besides this minor material removal, the lower end of a GPR does not need to be modified in any way to accept the 1390 (even stock base gaskets are used). Some owners may wish to true and weld their crankshafts … and that is great. However crankshaft truing and welding is not a mandatory upgrade unless you opt to use a 12/14 vane stainless steel racing pump.

Cooling System – The stock GPR cooling system is easily up to the task of keeping the 1390 kits cool. The only risk of overheating would be a result of reduced water access to the cooling system while running at peak rpm. The most common scenario that causes this kind of overheating is high-speed runs across very rough water (with lots of air time at full throttle). Since the cooling system only gets water when the pump is in the water, this kind of operation definitely yields higher temperatures. If this is you, octane must be increased to control temperatures. The other common scenario is doing extended non-stop spinouts that keep the pump cavitating, and the engine at peak rpm. This is likely the quickest possible way to “cook” a high output pwc engine (and/or break a crankshaft) … don’t do it.

Exhaust System Modifications – Most GPR / XLL 1200 owners are familiar with the stainless steel “D” plate that replaces the stock catalytic converter. The “D” plate (named after the large “D” shaped opening it allows in the pipe) does not actually increase power, but it does slightly reduce operating temperatures. When using the “D” plate, we recommend to install the Riva resistor into the exhaust body temp sensor to “fool” the ignition into thinking that the same high temperatures yielded by the cat-con are still happening. Without the resistor, performance will still be fine, but the temp sensor beeper will repeatedly come on because it’s not getting the desired temp data. In any case, we strongly recommend the use of a “D” plate and resistor with all 1390 kits.

NOTE: Many aftermarket companies make and sell “D” plates…but they are not all equal. Some aftermarket shops have made their “D” plate with a smaller opening to increase back-pressure, and there-by improve peak rpm power. Unfortunately the higher back-pressure of these smaller spec “D” plates will increase operating temperatures, and decrease hot-numbers. The “D” plates sold by Group K are identical to the stock OEM Yamaha European spec “D” plates, and offer the correct back-pressure for the 1390 kits.

As of late, there are several versions of “pressure valve” stinger kits for the GPR1200s. The basic premise of these kits is to modify the plumbing of the stinger tip so that it only receives water input when rpms (and pump pressure) are high. This keeps water from entering the water-box at low rpms… and improves low speed acceleration under many riding circumstances. We fitted these systems to our 1390 test boats, and they appeared to work well. It bears noting that the 1390s have so much low speed acceleration, that it’s hard to define the measure of the benefit. However it bears noting that these system definitely do not hurt the 1390 in any way, and may be a slight benefit in preserving better hot-numbers.

About Triple Pipes – To date, we have not tested the 1390 with triple pipes, and have no tuning or piston/cylinder life data . For any customer considering such a setup we would mandate the following mods. Trued and welded crankshaft, lightweight ignition flywheel, and girdle type cylinder-head. All the Yamaha 1200 girdle heads we have seen utilize an “O” ring sealing system. Unfortunately, the diameters of these “O” rings would not permit for sealing around the new larger bores of the 1390. Given this, we would only recommend a girdle head that has no “O” rings, and can be used with a conventional head gasket for sealing (likely a custom part). Group K can cut the 1390 domes into any such head.

About Ignition – The stock Yamaha ignition has an advance/retard curve that is very favorable to the 1390 Big Bore. In addition, the stock rev limiter is at 7400-7450 rpm, which is higher than the 1390 will ever need to rev. Given all this, we have chosen to retain the stock ignition for our 1390s.

About Impellers – The added torque of the 1390s can easily pull the relatively steep Solas 14/20 Concord impeller. Like other props from Solas, there is a broad range of tolerance with regards to “out of the box” blade pitch that can cause rpm variations up to 100 rpm off target. Given this, all Solas props sold by Group K for the 1390s will be inspected and “spec-pitched” to exactly the same specs as the props used during all our testing.

About Pump Modifications – The 1390 makes tons of torque, and so, puts a much heavier load on the pump bearings. The stock GPR pump has two large pump bearings that support the drive-shaft. Unfortunately the pump is designed in a way that delivers “all” the impeller thrust load to only the rear bearing in the pump. For our 1390s, we offer a chromoly “thrush bushing” that fits between the two bearings to allow the thrust load to be shared by both of the bearings in the pump case, and greatly increase bearing life. While the thrust-bushing is a simple part, proper installation is not at all easy or simple. Installing the bushing should be done by a shop with a hydraulic press, and the fixtures to properly seat the bearings in the pump case … a mallet is not gonna do it.

Many customers wish to optimize the performance of their stock pump. For them, we offer “pump-blueprinting”. Blueprinting consists of removing all the casting drafts and interruptions in the stock pump housing. This blueprinting does not increase peak speeds in smooth water, but it can increase speed and hook-up in rougher water conditions.

R&D offers an angled rear pump cone that fits perfectly centered in the 5 degree angled exit nozzle of the stock GPR pump. This pump cone does not make a huge increase (.5mph), however it is an all-gain/no-lose bolt-on modification that can benefit any 1390…. A nice add on.

Some customers may opt to install an all stainless steel 12/14 vane aftermarket pump for the added hook-up they offer. These pumps do, in fact, offer a huge increase in hook up in rough water conditions, however in smooth water they do not seem to offer the 1390 any peak speed benefit. We liken them to putting big sticky slicks on a street car. That is, they are only an advantage if excessive wheel slip (or prop cavitation) is a big problem. For recreational use, “grudge racing” GPRs, we find that the Solas prop in a blueprinted oem pump is more than adequate. For owners that choose the stainless steel pumps anyway, we advise increasing octane by 3+ points to contend with the extra heat caused by the added engine loading of the improved pump hook-up.

Handling Components – Handling is often a subjective evaluation, and so it is impossible for anyone to say what handling parts may be “best”. That said, we feel that the R&D ride plate and standard R&D Aqua-Vane scoop grate are top choices that should be considered. Our previous GPR documents have referred to a “nose” steering issue that randomly takes place on both the GPR and XLL hulls. Installing the R&D grate completely eliminates this nose steering problem, and so we consider this part to be an absolutely mandatory bolt-on for any 1390 kit.

The R&D ride plate, like any mass produced parts, has subtle production variations. On our test 1390s, we were able to machine the mounting faces of the R&D ride plate to net a slight peak waterspeed gain (.5 mph) in smooth water conditions, without inducing added nose bouncing (also known as “porpoising”). This mod would be ideal for smooth-water grudge racers, however might not be a good choice for owners who spend most of their time in 1 foot or bigger chop.

All our test 1390s were equipped with stock sponsons. We have tested many sponsons in the past on 1200 race boats. Most of these sponsons offered better turning precision, but also scrub off smooth water speeds. If sheer peak speed is your interest, retain the stock sponsons.

Hull Preparation – Our GPR test boats ran faster speeds than any other single pipe machine we had ever tested with. With that, we experienced handling issues we had not seen on other recreational GPRs. The installation of handling parts (ride plates, grates, etc) helps some … but only some. With that, we tried a different approach. Tests we had conducted with other machines showed very positive results from “hull sanding” using 20-40 grit sand paper 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, these deep, full-length, scratches should eventually eliminate any part of shiny hull 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 in the straight-line control of our 1390s. We consider this preparation to be mandatory for any 1390 used for grudge racing.

About Tuning and Instrumentation – We have spent many hours of testing with all these single pipe kits to assure the best possible performance and reliability. Despite that, there can be some subtle variations in the stock machines themselves that results in slight variations in performance with the kits installed. For the average recreational user, these variations are of minor importance. However there are many “enthusiast owners” intent on getting the most from their machines. For these owners, accurate instrumentation will be necessary to assure the benefit of adjustments and tuning. In particular the tracking of rpms, and peak speeds.

There are two popular digital tachometers used to track rpms: the $59 “Tiny-Tach”, and the $308 P.E.T. tachometers. The Tiny-Tach is very durable and affordable, however it has limitations. The Tiny-Tach update every 2 seconds, making for difficulty seeing accurate increases in rpm during acceleration. Once at speed, the T/T can offer accurate data, so long as the rpms are being sustained on smooth water. In rough water, the TT is incapable of yielding data that is accurate or meaningful.

While much more accurate, the PET tachs are less forgiving to heavy vibration and heavy water exposure. We consider them more as a tool for testing rather than a full time on board item. The PET tachs update every 1/2 second, allowing for very good tracking of rpm acceleration trends and immediate display of peak rpms. Like the T/T, the PET tach can only give meaningful data in smooth water conditions.

Radar testing has long been the standard for accurate speed measurement of pwcs, however the accuracy and durability of current GPS devices makes them a very effective “relative” speed measuring tool for watercraft. We recommend a GPS that can show real time speed to the tenth of a mile per hour (they are generally more accurate). When using GPS for speed measurement, it is essential to maintain the straightest possible path. Some GPS’s will show false reductions in terminal speeds when the machine is traveling on a radius or arc.

About Constructing Your 1390 – Most customers will send their parts for modification and assemble their 1390 themselves. The assembly requires some mechanical ability, and basic metric tools. We recommend that any owner doing their own assembly should also purchase a Factory Yamaha shop manual for their model machine. Each kit comes with assembly instructions, and the 3-Bond 1211 gasket sealer that we use to assemble 1390s in our shop.

For owners who wish to have their 1390s built, we can construct your engine, or complete boat in our shop. Contact us directly for freight info and labor costs for your 1390 project.

Package / Upgrade / Parts

Group K Price

Group K 1390cc Big Bore Kit (specify 87-octane or 91-octane)
Includes: Fixed valve modification, Cylinder Boring, porting, & Nikasil plating, Head Gasket Boring, Cylinder head modification, Carburetor jetting inlet manifold block off, 3 Pistons with rings and clips, and a tube of 3-Bond 1211 gasket sealer. (Send cylinders, exhaust valves, cylinder head, head gasket, & carb rack)


Pod Flame Arrestors


“D” Plate


Riva Exhaust Resistor


Riva “Free Flow Kit”


Jet Works Exhaust Pressure Valve Kit


Pump / Hull

Group K Price

R&D Ride Plate


Optional Ride Plate Modification


R&D Aqua Vane Scoop Grate


Solas 14/20 Impeller (pitch inspected)


Group K Pump “Thrust Bushing”


Pump Blue-Printing (send vane body and impeller housing)



Group K Price

1390 Piston Set (piston, rings, clips)


1390 Head Gasket


1390 Head Modification



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 !!