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  1. #1
    philip_gpr's Avatar
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    Dec 2005

    STEP by STEP - One alternative to building a fast rec ski.

    This post includes complete instructions on how to put together a fast rec ski. There are many people that have contributed to this project and I would like to thank them all for their contribution.

    Assembly instructions

    Every modification that has been done had one specific purpose. I didn't use anyone's Stage I or II packages. Instead I assembled a ski that is very much in line with the philosophy that Carl at Island Racing promotes for his custmers. I would summarize this philosophy as:

    "If you want to go fast simply add go fast parts and don't add any go slow parts."

    Building the motor - Lowell Horning

    Lowell Horning basically built a complete motor for me. We used a 84 mm bore using Pro-X pistons.

    Lowell's web site can by found by searching for " 2-stroke-porting ".

    Starting with 3 very beat up 1200 cylinders, Lowell had them bored and then Lowell ported them to his specs and then they were plated. We both felt it was important to use Nikasil plated cylinders.

    The Porting Specs used were specifically intended for a single pipe application. The porting was designed to maximize peak HP. My instructions to Lowell were simple, I told him "I want to go fast".

    The porting specs Lowell uses are his own. He has looked at what the best in the PWC industry are doing and he has developed his own specs for the GPR motor. Lowell has a wealth of experience building snowmobile motors and it was a relatively easy transition for him to build jetski motors.

    Lowell believes for maximum peak rpm the finish of the ports is important and that is why he spends a lot of time finishing the ports. I think this is why others have commented positively on look of his work.

    In my discussions with Lowell he felt there were some gains to be had by porting the cases so that has also been done.

    Squish Lowell has tried to explain to me about the significance of sequish. Here is my simple interpretation of this conversation.

    Around the outside of the dome on the head is a squish band - when the piston rises, the squish band creates air turbulence, and this helps mix the charge - too much squish clearance is not a good thing, and likewise, not enough squish clearance can cause contact with the piston
    & detonation. The ideal squish band (width & clearance) moves the charge to the combustion chamber away from the cylinder walls and piston edges - this is a good thing. For squish velocity MSV (maximum squish velocity) there are two measurements that are important 1) the width of the squish band and 2) the actual squish clearance; which is the gap between the piston and the head at TDC (top dead center).

    On my motor Lowell used an OEM GP1200R head that was been machined to get the squish specs Lowell wanted.

    You are able to fine tune the squish and compression numbers by changing how many layers of the head gasket we use. We can use 1 or 2 layers of the OEM 1300 gasket. (Note: When splitting apart a 1200 gasket and using using only 1 or 2 layers it is important that these layers only be either of the two outside layers of the OEM gasket. These layers have a coating on them and the inside layer of the PEM gasket is not coated.) Both layers of the 1300 gasket arer coated. To help the preserve the gasket coating it will be smeared with a light layer of water proof grease. I used one layer of the 1300 gasket.

    A good target squish measurement would be around .050 inch. (though you can go as low as .040 inch)

    For a single pipe ski a target compression might be 150 lbs. (This depends on timing, fuel octane, squish, etc. I have run as high as 170 lbs I was starting to see detonation using Sunoco 94 fuel.)

    For the crank I had my OEM crank true and welded by Phil at Crankworks. I installed a RAD total loss flywheel. With the higher HP and higher rpm these modified motors are developing I believe both of these steps are good preventive measures. In my case because I often ride in real rough water I felt both of these steps should be done. The loading and unloading of the pump (fact) and the flexing of the hull (theory) you get in rough water is very hard on the crank.

    For the Power valves I used a set of Riva gas valves. A cheaper alternative would have been to use a set of OEM 1300 valves. Lowell was able to get the PVs to fit the exhaust port like a glove - great work!

    When you use the gas valves on a single pipe ski you have to remove some material from the bottom of the cover for valve #2 in order for the valve to fit. If you don't remove the material the valve cover will hit the exhaust manifold. After you have cut the cover down be sure to check the PV can still open all the way.

    When setting up the valves first remove the spring to ensure the valve can fully open and record you rpm at WOT. Then close the valve down until your low and mid is as good as possible. Then check to ensure your rpms at WOT are the same as with the original open position.

    Pressure Test The engine was pressure tested using a pressure test kit that I purchased from Randy at Watcon.XXX

    Fuel Sender The black trap door was removed. The main sender feed line was extended to match the reserve line. The fuel selector switch was removed. Both of the fuel sender's intake lines feed the carbs.

    Removing trap door on sender

    Primer Install

    1) Disconnect the fuel selector switch from the fuel circuit

    2) If you are only using a single feed line for the carbs use the reserve feed siphon tube from the fuel sender. The reserve siphon tube is longer and provides more consistent fuel delivery.

    3) If you are using 2 feed lines for the carbs splice the primer feed into the return line. (after the restrictor jet if you are running Novi's) If you are only using 1 feed line to supply the carbs feed the primer with what was previously the main siphon tube from the fuel sender.

    4) For the stock carbs if you have disconnected the accelerator pump you can use those fittings to supply the fuel to cylinders 1 and 3. For a US ski you will need to add a fitting for cylinder number 2. For a European ski the carbs have 3 accelerator pump fittings. For Novi's the carbs come with primer fittings.

    5) Mount the primer plunger in the spot the fuel selector switch was in.


    I am using an old style R&D GP1200R ride plate. Other good choices are the old style Riva GP1200R plate or the new style R&D GP1300R plate.

    Rideplate mods I do:

    - machine (or grind) on the rear mounts to increase the angle
    - grind notches in the plate so that it will fit around the bottom of the pump extension and pump
    - the riding surfaces of the plate are not very flat (crappy quality control) ideally they should be trued flat

    For those of you not able to you own rideplate mods there are a couple shops equipped to provide this service.

    In order for the rideplate to fit the bottom of the pump and pump nozzle both have to have some material removed to provide additional clearance.

    Compared to a stock rideplate, a modifed rideplate will add appox 4 mph. This is the single most effective speed mod available for the GPR.

    When installing the plate I will be positioning it so the center section of the plate is on a 5 degree angle to the hull. At the same time ensuring the sides of the ride plate don't sit lower than the pump shoe and hull. It is OK to have a small step up from the shoe to the center flat section of the rideplate.

    Why put the rideplate on a 5 degree angle? The pump nozzle is on a 5 degree angle and at WOT the GPR hull is on approximately a 5 degree angle in the water. I will do some testing on this angle this year - but I am convinced 5 degrees will be close to the sweet spot and that will be my starting point for all testing.

    Skegs versus No Skegss

    In the past I have always had the skegs machined off, this year I have a couple plates I am using. One with skegs and a 2nd without.

    The plate without skegs will be marginally faster than a plate with skegs. The speed difference isn't as much as you would expect - the theory is the skegs provide lift which raises the rear of the ski out of the water which reduces drag which then provides higher top speed numbers.

    I have found the ride to be very different when I compare my skegged R&D plate to a R&D plate that has had the skegs machined off. The plate with skegs tracks much more firmly. This can best be described as, the rear end is planted to the water and it doesn't want to wash out when cornering. On the other hand with the plate without skegs the ride is much looser (from side to side) and the ski feels like it is skidding or sliding around a corner.

    Rideplate angle theory

    Rideplate Discussion

    Hull Angle Discussion

    Trim Tabs

    The set-up of the trim tabs can play a very significant role in reducing the skis tendency to porpoise. Don't think of this as the tabs have to be set up in one exact way but rather think of it as a sliding scale. The more you want to reduce the skis tendency to porpoise the more you want to shim the trim tabs down.

    For most set-ups there is a small speed loss when you shim the trim tabs down but I have always felt the improved handling was worth this trade-off. On flat water with the stock trim tabs 1 washer reduces top speed .3 mph, 2 washers .6 mph, etc.

    There is also a conflicting theory that shimming the trim tabs down actually increases speed. This is because the shimmed trim tabs create lift that raises the rear of the ski out of the water.

    For flat water (my water is never truly flat) with an IR modded plate I use the stock trim tabs with 2 washers under each bolt.

    When shimming the stock trim tabs use the same number of washers under all 8 mounting bolts. If you use more than 2 washers get longer bolts.

    The washers I use are 2mm thick.

    The same concept can be used with aftermarket trim tabs. The Riva and R&D tabs are both very similar in design. The difference between the two of them is the Riva has channels in them similar to the stock tabs and the R&D are completely flat. I have tested both back to back and I couldn't detect any difference between the two. When comparing the aftermarket tabs to the stock tabs the aftermarket are thicker and are angled. The aftermarket tabs do a better job of matching the hull angles. When shimming aftermarket tabs the sides of the trim tabs will hit the sponsons so the sides of the tabs will require grinding to provide clearance. Another mod you can do to the aftermarket tabs is to cut the rear section off. This will make the aftermarket tabs the same length as the stock trim tabs. I have no test results on this mod but others have posted on it.

    If you are fortunate enough to have really flat water you could also try no trim tabs. My testing has shown there is a potential 1 - 2 mph gain if no tabs are used - but ride quality deteriorates significantly. Make sure you get some round headed bolts to fill in the trim tab mounting holes if you try this.

    For those of you that ride in rough water this table might help on what rideplate, trim tab and nozzle set-up I would recommend:

    In water with 0 – 9 inches chop
    - use modified R&D plate
    - GPR 5 degree pump nozzle
    - Trim tabs with 1 or 2 washers under all 8 bolts

    In water 9 to 18 inches
    - Use stock plate
    - GPR 5 degree pump nozzle
    - Trim tabs with 1 or 2 washers under all 8 bolts

    More than 18 in chop
    - Use stock plate
    - XL 3 degree pump nozzle
    - Trim tabs with 2 washers under all 8 bolts

    Filling Pump Shoe and Rideplate holes with putty


    OEM in the up position for flat water speed runs.

    Stepped OEM sponsons if I need to settle the ski dowm even more. To step the sponsons you mount them in the up position at the front and in the down position in the back and you cut out the plastic between the 2 middle holes and create a new neutral position for the middle mounting bolt. In flat flat stepped sponsons can slow the ski down up to .7 mph so I will only step the sponsons if the water conditions require it to be done (or if my rideplate set-up requires it to be done).

    Stepped OEM Sponsons - 1

    Stepped OEM Sponsons - 2

    For everyday riding most of the aftermarket sponsons work fine. I have used Riva and Beach House and have liked both. The R&D sponsons also get good reviews. I use a set of Bullet sponsons for day to day riding. (Only because the Bullet ones look so cool!)

    The aftermarket sponsons can't be stepped so this means if you have to reduce the skis tendency to porpoise you will need to increase the shimms under the trim tabs or to reduce the pump nozzle angle.

    Bullet Sponsons

    Pump Nozzle

    With a 2004 FX HO pump I am using a 87 mm GP1200R nozzle. I used the same nozzle with my 2000 GPR pump.

    Pump nozzle testing

    When I want to further reduce the skis tendency to porpoise or bounce in rough water I use an XL 1200 nozzle. This is because it has less angle on the thrust and it helps keep the front end planted down. When I use the XL nozzle I also use the stock rideplate and I have the trim tabs shimmed down with at least 2 washers.

    Pump Plug Kit

    After installing a PPK you should take a long straight edge and see if the pump shoe is sitting down below the hull line. If it is, it needs to come off and be reinstalled. When I install the PPK I do not use the rubber pieces that sit in the holes in the bottom of the shoe. (this way I never have this problem) I focus my effort on ensuring the large gap between the pump shoe and the transom is filled with 3M fast cure 4200.


    Free Flow The free flow reduces back pressure in the exhaust system. If you are using triples or very high compression it should be used.

    For most rec skis I am not sure if a free flow is needed. Another forum member has done quite a bit of testing with a stock 1300 and a stage II 1300. In both cases adding the free flow lowered the top speed.

    Using a free flow makes the ski much noisier, especially in the 3,000 to 5,000 rpm range.

    D plate or Cat? First off, all D plates are not the same. There are 2 common sizes, R&D and Riva, (Yamaha's D plate is identical to Riva's D plate) The R&D has a smaller opening and for a 1200 motor that is the D plate I recommend. For a 1300 or larger motor I would suggest you use the Riva D plate.

    From what I can tell on a ported BB they are no differences in the max peak rpm from one D plate to another.

    I no longer suggest anyone use a Catalytic Converter. Even though Yamaha came out with a new and improved Cat in 2003 there is still too high a chance the Cat will fail to warrant its use in any ski.

    Jetworks Mod

    For the time being I have removed the Jetworks mod form my ski. It's primary role is to help help shot and that is not a priority for me right now.

    For a no holds barred single pipe ski this was one of the best mods from last year that was made popular by Fernando. For someone interested in this mod Island Racing sells a kit with the parts required. Here are some comments that describe how I implemented this mod with some of my own improvements.

    - plug up the water spout hole in the pump nozzle with silicone

    - remove the siphon tube in the pump nozzle (it is press fit in) and plug that hole up with JB weld. If it won't come out easily you might just shorten it with a hacksaw. While you are at it grind on the bottom of the pump nozzle to provide more clearance for the angled rideplate

    Even if you don't do the Jetworks mod adding an electric bilge pump and removing the siphon tube is a worthwhile mod - your rpms will go up

    - add an electric bilge pump

    - add a dual cooling block to the pump strainer - that is your water source - you can make one by drilling and tapping the strainer cover or you can buy one from Riva

    - add a thru hull fitting to be used for your water source to the rear of the exhaust pipe

    - use the siphon tube thru hull fitting for the water that needs to be bypassed from the exhaust

    - add a manual adjuster valve in-line beside the Jetworks valve (this allows you to tune the water to the waterbox) Riva sells this valve for $25 - fine tuning the water flow routinely gives me an extra 30 rpm

    Riva Restrictor Valve

    Bill Cosca has also provided an alternative method for installing the Jetworks valve. I really like this method because of it simplicity.

    Riva CDI

    I have both an Advent and Riva CDI. If using the Advent I would suggest you try the 94 or 84 curves. If Riva, use the stinger curve.

    Air to Motor

    In stock form the GPR motor is starved for air. You can raise you RPMs by allowing more air into the motor area. To fully satisfy the motor air demands it doesn’t take much.

    Air Opening

    Create an air opening on the right hand front cowling (to match the left hand cowling). Use a sharp exacto knife and cut away the black plastic as shown above.


    Storage Bucket

    I have also cut holes as shown above in the inside layer of the windscreen and in the top of the storage bucket. If you don’t want to make the holes in the storage bucket you can simply remove the storage bucket and leave it at home when you do speed runs. I have 2 storage buckets, one with holes and one without. I use the storage bucket without holes on rough water days when I am concerned with water intrusion.

    Do not cut holes in the windscreen and storage bucket if you ride in the surf.

    Adding additional air flow beyond what I have shown here does nothing - I have tried using a bilge blower forcing air into the engine compartment - rpms stayed the same.

    Steering Nozzle
    Protec nozzle - 9 mm longer than OEM - same diameter as OEM - oh yeah it is metal so it won't break like the cheap plastic OEM nozzle

    Protec Nozzle Test

    The cheap crappy stock one works fine - it just breaks and that was the reason I replaced it - I saw no speed gain from the Protec nozzle.

    Delta VForce III Reeds and Valve

    I currently use a set of Delta VForce III reeds and custom spacer plate.

    Novi Carbs

    As someone I truly respect (Carl) once said "They Work". They are being used with 3 OEM fuel pumps.

    Coop and I have been all over the place trying different jetting specs. If anyone needs our current specs please send me an e-mail.

    Fernando thread on tuning Novi Carbs - triple piper

    It should be noted to build a high performance ski aftermarket carbs are not mandatory. There are numerous ski's on the forum using the stock OEM carbs or fuel injection system that have been able to achieve very good results.

    With the stock carbs it is recommended that you cut a large hole in the exhaust mounting bracket to provide visibility to the T handle on carb # 2.

    Hole in exhaust mount

    Intake Grate and Shoe
    I am using an OEM pump shoe properly sealed with a GP800R double bar intake grate.

    When I sealed the pump shoe in I used a straight edge and made sure the shoe was not sitting lower in the water than the hull line.

    By trying to raise the pump shoe even higher in the hull you can get a small gain in speed. This comes at a cost, if you raise the shoe too much you reduce hook-up and create instability (porpoising). I do not think this speed gain is worth the loss in handling and I do not recommend raising the transom on the hull in order to raise the pump shoe higher in the hull.

    Impeller and Pump
    I am using a 2004 FS HO pump (same as the 2005/2006 GPR pump). With the new design pump I use a Dynafly impeller. For most BB skis you will need to run a pitch higher than 14/20. When you pitch up a Dynafly impeller the blades will rub on the sides of the pump liner. This means when you raise the pitch you also need to have the impeller machined so that it doesn't rub against the side of the impeller liner.

    Impeller pitches I am using - Dynafly 15/25 SB

    GP1300R Pump Spacer

    I use the 1300 pump spacer set-up on my 1200 ski. To add the 2 inch pump spacer to a 2000 or 2001 GP1200R you need to swap out the intermediate shaft and the impeller side coupler. To add the spacer to a 2002 GP1200R you need to swap out the driveshaft. Coop is not using the pump spacer. (this is one of the few areas are skis are different)

    Pump Tunnel

    I have reinforced the pump tunnel. To do this I used layers of fiberglass cloth and Epoxy resin. With the SMC hulls any glue or resin used must be epoxy based in order to stick. One alternative for this is this product made by MarineTex.


    The following links are for various test results I have posted over the years.

    Test Results - Misc

    Test Results - Rough water set-up

    Test Results - Shimmed Trim Tabs

    Hydrotoys - all sorts of stuff!

    CajanDude - GP1200R Manual & Info

    GP1300R Manual
    Last edited by philip_gpr; 05-28-2009 at 08:14 AM.

  2. #2
    Moderator RX951's Avatar
    Join Date
    Apr 2005
    League City, Texas

    How to build a fast ski !

    Thanks for posting that Philip !

    Very good information !

  3. #3
    There is no VooDoo like BadVooDoo NeVerMind's Avatar
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    Sep 2005
    Hammond, IN
    Great Info...

  4. #4
    Hydrotoys's Avatar
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    Jul 2005
    Mesa, AZ
    Bout time, ya slacker!!! You plan on transposing that over here by chance?

    Thanks Phil!

  5. #5
    Moderator RX951's Avatar
    Join Date
    Apr 2005
    League City, Texas
    done !

    Phil, you may have to check some of the hyperlinks as they did not carry over from the copy/paste

  6. #6
    Happily Self-Employed WFO's Avatar
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    Nov 2005
    Houston, Texas

  7. #7
    Moderator RX951's Avatar
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    Apr 2005
    League City, Texas
    Thso bullet sponsons look weird

  8. #8
    Old Ski Insanity w/rpm500's Avatar
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    Aug 2005
    Crystal River Fl
    Really Great Post !!!
    Thanks Phillip for putting this all down in one post ...
    I dont own a GPR ....yet,
    This posting ,with some great pictures ,helps to understand what has been done and what has worked in your setup.

  9. #9
    Moderator RX951's Avatar
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    Apr 2005
    League City, Texas

  10. #10
    Crash Test Dummy Smitty's Avatar
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    Jun 2006
    Cullman, AL
    Bump for all the new guys trying to go faster.

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