Fuel Consumption @ Peek Speed
Fuel Consumption @ 50 mph
Background – Sea Doo built the carbureted GSX 785 (actual 782cc) during the sales boom years of 1996 & 1997. The “rock-reliable” 785 engines are the smallest displacement PWC engines ever built that have an integral counter-balancer (that makes for exceptionally smooth engine operation). In addition, these GSXs were the smallest of all the Sea Doo hulls that got the large15 gallon gas tank. Given that, the stock GSX785 had a very uncommon combination of excellent turning and handling abilities, along with silk smooth operation, and a very long fuel range. These qualities made the GSXs one of the most popular and successful endurance racing platforms of the late 1990s, and one of the most versatile pwcs of the late 1990s. The 785 GSX is something of a “forgotten” model because it often sat in the shadow of it’s wildly successful little brother (the smaller 96XP 785). Unlike the smaller XP, the GSX was seldom used for closed course racing because it couldn’t quite match the horsepower to weight ratio of the little XP. However the little XP was no match for the larger GSX in big rough water…. And a strong rider on a GSX could easily match any high speed turning maneuver of the little XP hull.
In 1998, Sea Doo released the GSX 951cc model, which was 45 pounds heavier than the 785, but carried the same quantity of fuel. Endurance racers quickly learned that the 951 hull was far worse handling in rough water than the lighter 785. As a result, the newer GSX951 series of machines was less popular and less desirable than the older GSX785. Eventually all GSX versions were discontinued in favor of the longer (and much heavier) RX model.
Even today, a good GSX 785 offers a measure of speed, handling and fuel range that can’t be matched by most new model 4stroke machines. All these qualities make the GSX785 an ideal choice for a Resto-Mod project that delivers the best in lightweight sport handling, and long distance abilities.
Our Resto-Mod Goals – Like all our Resto-Mod build-ups, our project GSX785 needed to be reliable, 91-octane safe, and have the strong horsepower that the GSX hull utilizes so well… and get the best possible fuel economy. To accomplish this, we would start out by performing one of our tried and proved “Sleeper” Engine modifications. This kit consists of modifications to several crucial engine parts, as well as the addition of a custom pitched Solas Concord impeller.
The Finished Project Boat – Our “Resto-Mod” Sport-Cruiser 785 offered strong acceleration all the way to the 60-61mph peak speed… even with a full 15-gallon gas tank. The extra dose of power allowed for full speed turns with no engine bogging at all, and out handled all the big heavy 4-strokes boats in our riding group. Out on the open water, we finished an 85-mile loop at 45-50 mph, with fuel to spare. The only 4-stroke pwc that could match our speed and fuel range was the Kawasaki 1500F ….. it costs about triple what it cost to buy and build our GSX…and (unlike our GSX) it has no trim system to improve comfort and handling under differing water conditions. From our wallet’s view…. This resto-mod GSX is untouchable.
Going to the smaller 38mm carbs made for even better fuel economy, better bottom end, and a 1mph loss of top speed… still the ultimate “bang for the buck” sport-cruising PWC.
Tech Info About The Build-Up
Exhaust System Modifications – One of the natural first choices for many high performance minded PWC owners is the installation of a high performance exhaust system. Since the 785-engined models were so often used for racing, there was a large abundance of different high-rpm racing exhausts made for them. Of all those exhausts, the ones made by Factory Pipe Products offer the biggest performance increases. However these exhausts were developed mostly for racing applications that required very high rpms. These high rpm pipes could often operate on the 92-octane fuel blends of 1996. However, today’s premium fuel blends have far less detonation resistance than 1996 fuels had. In short, the big power increases of the aftermarket pipes generate a temperature increase that cannot be supported by today’s 91-octane pump gas. In addition, the very high rpms of these racing pipes causes a big increase in fuel consumption… which is exactly what we did not want for this project.
As a result, the best option, for this build-up, would be to optimize the stock exhaust system. The best way to accomplish this is by modifying the plumbing system that injects water into the pipe at the head-pipe and waterbox. In short, we removed the regulator valve on the waterbox, and created a simple plumbing system that meters (through changeable jets) water to the head-pipe and waterbox. While the plumbing kit appears very simple, it is the result of many hours of on-water testing to determine the ideal orifice sizes for the best performance. This exhaust jetting kit nets a very noticeable increase in response and performance on a 785 motor with the “Sleeper” modifications already in place, however the gains on the “Limited” setup are more modest.
Cylinder Porting & Compression – Cylinder porting has often gotten a bad reputation as a modification that hurts low end power as well as reliability…. And that may be true for poorly executed porting. The porting modification used in our 785 Sleeper kits was developed for use in endurance racing engines that put a premium on long term reliability, and strong power at all rpms. Our Sleeper porting involves changing port heights and widths, while at the same time addressing numerous port shaping and mismatch problem areas. The result is a significant improvement in performance at all rpm ranges, along with improved high rpm ability.
Increasing compression ratio is also a very popular mod for 785 owners seeking to get an overall power increase. Like every other engine specification, there are limits to how much is “too much”. With compression, the limiting factor is the detonation resistance of today’s 91-octane fuel. Surprisingly, our tests showed that the whenever we used excessive compression, we experienced very heavy detonation at 6600rpm, and only trace detonation at wide-open throttle. This happens because the ignition curve has greater advance at 6600 than it has at 6900+ rpms, hence a higher detonation risk at 6600. It bears noting that our head modification specification is the same for both the Limited Kit and the Sleeper Kit, so the head mod does not need to be done a second time for customers upgrading from a Limited to a Sleeper.
Squish Clearance – One important part of getting the 785 SD engine to make it’s best power is properly setting up the squish clearance (this is the clearance between the piston crown and head dome at TDC). The base gasket surface deck height on the 785 cases varies so much that Sea Doo offers a range of different thickness base gaskets that allow technicians to setup the correct squish clearance for each particular motor. For this same reason, we prepare our Sleeper ported cylinders and milled heads as a matched pairs to assure that you have the correct compression ratio when the squish clearance is properly set. Our kits come with detailed step-by-step instructions on how to perform this squish clearance setup.
The net result of our Sleeper top end modifications is an engine that makes “a lot” more torque at all rpms. This added torque allows for the use of a much steeper impeller, so you get your speed gains from puling a steeper prop instead of turning sky-high rpms.
About Cylinder Boring – There are many PWC models that net significant gains in power from big-boring to a significantly larger piston size. Sadly, repeated tests have shown that the 785 Sea Doo engine does not react very well to this mod. However the 785 does respond well to having a very close clearanced fit between the piston and cylinder wall. Out project GSX had very worn bores, so we bored it to 1st oversize Sea Doo pistons. The oem Sea Doo pistons are a bit more expensive than the aftermarket versions, but we have found the performance and long-term wear of the oem pistons to be far better than any others.
It’s also important to note that anytime a 785 engine is bored, the Rave-valves “must” be clearanced to avoid the valves making contact with the new larger pistons. Valve clearancing is included with all Group K 785 cylinder boring.
Air Inlet Modifications – There are many different “free-er breathing” aftermarket flame arrestors available for the 785 Sea Doo models. While they all have good performance abilities, very few of them include a “carb to head” brace. Without this import brace, there is a very high risk that the unsupported weight of the carbs and filters will cause the inlet manifold to fracture.
Since our GSX was not intended as a high rpm race machine, we opted to modify the stock flame arrestor case (which does have the much needed carb-to-head brace). We simply removed the fine screens from the arrestor element, and added a few air access holes on our flame arrestor lid. This modified part matched the peak rpms of a set of aftermarket “pods”, and offers considerably better water protection.
Ignition System – The ignition system is the one area where the 1996 & 1997 models vary. The 96 model has a rev limiter at 7050 rpm, and the 97 model limiter is set at 7150. Our GSX was a 1997, and so we felt no need to further raise the rev limiter. If we had done a 1996 model, we would have installed a Micro-Touch rev module that offers an adjustable limiter. Beyond that, the ignition system of the GSX is an excellent setup that requires no modification.
Carburetor Modifications – Another modification included in our 785 Sea Doo Sleeper Kit is the “True-Boring” modification to the stock 40mm Mikuni carbs. While the stock carbs are denoted as 40mm, the actual minor diameter at the throat is about 37mm. With this mod, we make the stock carbs a full 40mm along the entire throat. These modified carbs offer easy staring, steady idling, and a good improvement in overall acceleration characteristic. The True-Bored carbs are a very popular modification because they net a nice overall improvement in power, with only a small increase in fuel consumption over stock carbs. The carb True-Boring is included in both the Limited and Sleeper Kits.
Increasing Fuel Economy – For our GSX project, we decided to test an option that could significantly increase fuel range while having a minimal impact on overall performance. The “option” would be the installation of a set of re-jetted dual 38mm Mikuni carbs that came stock on all the 720 models. Externally, the 720 carbs look cosmetically identical to the 785 carbs, and have all the identical cable connections. However internally the 720 carbs use a 38mm butterfly instead of the 40mm butterflies in the 785s. To be sure, we suspected that the overall performance of our project GSX might be harmed slightly by the smaller carbs (but we didn’t know by what margin). Since we were not constructing a high-rpm racing engine, a minor performance loss from the smaller carbs could likely be worth the fuel economy gains that might come along with them.
We fitted our GSX785 test boat with a digital fuel flow meter so we could measure the exact differences in fuel consumption while we were simultaneously evaluating performance differences of the two carb sets. The following chart shows the data:
Liter = .2642 Gallons
Stock GSX 785
Sleeper GSX 785 – 40 mm Carbs
Sleeper GSX 785 – 38 mm Carbs
Fuel in Liters/hr
44 Iph @ 6780
48.0 @ 7120 (12.94 gph)
42.5 @ 7050 (11.22 gph)
37.0 (9.77 gph)
31.5 (8.32 gph)
Surprisingly, there was very little difference in the “feeling” of riding the GSX785 with the two different carb sets. It’s possible that the difference in overall performance might have been bigger if we conducted a series of high-speed turning tests with a heavy overlap racing impeller. But again, this GSX is not supposed to be a race boat…. Just a very responsive destination cruiser.
Our overall impression is that the 38mm carbs were an excellent choice for a 785 owner wishing to get the best possible fuel range (a roughly 15% increase).
Fuel Supply – The large fuel tank, and excellent turning ability of the GSX are two of this boat’s most desirable features…. However these qualities also contribute to a hidden mechanical problem. When the GSX is put into a hard turn, with less than 1/3 tank of fuel, there is enough space in the large 15gallon tank for all the fuel in the tank to be thrown to one side…. Thus allowing a large number of air bubbles to enter the fuel system. The only way that these air bubbles can escape, is to pass through the jets of the carbs…. causing a very serious lean condition that results in mild detonation, and/or a big “bog” in throttle response. If the engine is repeatedly exposed to enough of these lean conditions, they can result in stronger detonation, and possible piston scoring.
To eliminate this problem, we installed a Group K Fuel/Air Separator kit. This kit consists of a remote fuel pump, and an air-separator canister that plumbs into the fuel system between the carbs and the fuel petcock. As the fuel level gets low, this kit maintains a solid supply of air-free fuel to the carbs, and returns all the air bubbles back to the gas tank. Besides the obvious benefits of better reliability, this kit also helps to allow the boat to pull every last drop from the fuel tank, hence further increasing the useful fuel range. The “on” and “reserve” features of the petcock are not affected in any way…other than each one will offer about 10% more range before running out of gas.
Pump & Impeller – The stock GSX stainless steel prop is a good part, but much too mild in pitch for the extra power and torque of our project GSX. We opted for a Solas 16-24 Solas Concord impeller with custom pitching. The Solas concord design offers the best combination of rough water hook-up, and smooth water peak speed abilities. With a little additional pitching, this Concord impeller offered us excellent overall acceleration, and still held the engine just a few rpms away from the 7150 rev-limiter.
Owners that ride their GSX in extremely rough water condition may experience a considerable amount of increased cavitation as a result of the random contact with the water surface. Installing an aftermarket scoop grate can greatly increase the water being delivered to the prop, hence reducing this cavitation. Sadly, all aftermarket grates will cause some loss of peak speed. During our testing, we found the Worx grate to offer the best combination improved hook-up, along with a nominal loss in peak speed (about .5mph).
The stock GSX pump is a very good part that requires no modification. However it is “very” important to assure that the changeable “impeller wear-ring” in the pump is in excellent condition. If the surface of the wear ring is badly damaged, it makes for a poor sealing surface around the impeller’s outside diameter. A damaged wear ring will cause cavitation, and a significant loss in overall performance.
Another optional addition for the GSX is the use of an R&D 85mm exit nozzle. Besides having an exit diameter that is slightly smaller than stock, the R&D nozzle has a much more precisely bored diameter. The end result is a slight reduction in peak engine rpm, along with a slight gain in speed. The most important part of the nozzle, for our project GSX, is that the nozzle also helps fuel range because the mid-range speeds are attained with measurably fewer rpms. For our project GSX the R&D nozzle was an all-gain, no-loss proposition.
Package / Upgrade / Parts
Group K Price
Limited Kit –
Cylinder Head Mill and Re-Cut Squish Bands
40mm Carburetor “True-Boring”
Re-Pitched Solas 16/24 Concord Impeller
Includes: Recreational Finish Cylinder Porting & Decking, Cylinder-Head Modification, Carburetor “True-Boring”, and Exhaust Plumbing Kit
Group K Fuel/Air Separator Kit
Cylinder Boring and Valve Clearancing
Optional Competition Porting Finish (on Sleeper Kits only)
38mm OEM 720 Carb Set
Micro-Touch Rev Limiter Module (1996 models)
Re-Pitched Solas 16/24 Concord Impeller
R&D 85mm Exit Nozzle
Optional Worx Scoop Grate
*prices subject to change based on manufactures pricing
ORDER INFORMATION: SEND ALL PARTS REQUIRED FOR MODIFICATION VIA UPS TO:
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 !!