Yamaha Super Jet – Racing Mods

SINGLE CARB 633’s and 701’s – All of the Group K engine kits for the single carb 6M6 case-633cc and 61X case-701cc platforms are developed to retain the single carb intake system. Our testing showed us that these engine kits (with a well prepared single carb intake) can often generate as much speed as most riders would ever need, along with very easy tuning and maintenance.

The 633/701 Hammer 96 and Sleeper kits can both use your existing Mikuni “Super BN” 44mm carb, with internal circuit upgrades. These upgrades allow the carb to feed fuel at the increased rates demanded by the ported cylinder.

The only single carb arrangement that we found to be a substantial improvement over the upgraded 44 Mikuni was the SBN46 Mikuni. The 46 Mikuni, and a 3″ tall K&N flame arrestor, allowed the single carb Super Jet to accelerate, and top speed, with authority that easily matched many dual carb setups.

All our single carb kits (except the IJSBA stock) utilize a modified intake manifold plate. This modification offers noticeably improved throttle response and acceleration at a very reasonable price (note: this mod makes no change in peak rpms). This is an “all gain – no loose” modification that we recommend for all single carb arrangements.

Aftermarket dual carb kits are commonly used on 6M6 and 61X crankcase engines because they can offer slightly better mid range acceleration than a single carb arrangement. With plenty of time invested in tuning, some aftermarket dual carb sets can be made to operate better than a single in every way. However our testing showed many aftermarket “duals” to be questionably faster and unquestionably harder to maintain than the single carb. Based on these tests, we consider dual carb kits on 6M6 and 61X cases to be a questionably wise purchase for most recreational riders.

DUAL CARB 701cc PLATFORMS – The stock dual 38 Mikuni carbs (on ’96 – ’99 62T cased engines) are badly obstructed at the air intake mouths by the choke butterflies and other casting bosses. The Group K modification for these carbs not only removes these obstructions, but also slightly increases effective carburetor throat diameter. The Group K modified 38’s are likely the very best in quick overall acceleration, and impeccable throttle control…at a very affordable cost.

For the more serious recreational rider, or closed course racer, we recommend the Novi XR modified Mikuni carbs. These carbs, while a bit expensive, are the very best in the business for CFM ability and signal strength. We recommend the 44mm Novis for our Limited 2 kit, and the 46mm Novis for the 105 octane Hammer Big Bore kits. While these carbs can still utilize the stock reed cage assemblies, they do require different manifolds and flame arrestors.

For owners seeking a more affordable approach to dual 44’s (on 62T cases), we strongly recommend the stock arrangements found on the Blaster 2 and 760 WaveRaider. While these 44’s do not have quite the CFM abilities of the Novis, they are a very effective alternative that offers great power along with easy installation and tuning.

Reeds – The aftermarket is full of replacement reed petals and cages for all three of the different Super Jet lower ends. The main considerations for alternative reed petals are performance, longevity, and side effect damage from petal failure.

Yamaha employs stainless steel reed petals in all the 6M6 and 61X cased Super Jets. Some of the 62T cased Super Jets came with stainless steel petals, and some with non- metallic “fiber” reed petals. The stainless steel material is used because it has an outstanding flexibility to weight ratio, the metal material seals exceptionally well against the rubber coated reed cages. Regardless of which material Yamaha chose, all the stock Yamaha reed petals offer excellent overall power characteristics. From a sheer performance standpoint, there are no aftermarket reed petals that out perform the stock petals by any big margin. All the stock Yamaha reed petals also offer exceptionally good long-term reliability.

Unfortunately, after enough service time, the stainless steel metal reeds can eventually fracture and break apart. When these stainless steel petals break, and circulate through the engine, they have the ability to cause serious damage to virtually all the internal engine moving parts (pistons, bearings, etc). If a fiber reed petal breaks apart, the engine easily “digests” the material resulting in no engine damage at all. While this reed petal fracturing is not a big problem on stock engines, the added pressures of high performance modification can significantly shorten a metal reed petal’s life.

For most recreational riders, the stock stainless steel reeds (replaced every 2 seasons) are likely the best overall performance value. However for any owner preparing a Limited 2, or Big Bore racing engine, we would recommend the use of non-metallic reed petals. Owners of 62T lower ends can simply purchase the stock Yamaha fiber petals used in the later models. For 6M6 and 61X case owners that wish to convert to fiber petals, we recommend the reeds from Carbon Tech. In this application, the Carbon Tech reeds appear to offer the best aftermarket mix of good performance, good longevity, and good cage sealing. Carbon Tech offers different material tensions for their reed petals. The “LT” (low tension) petal should be used in Limited packages to maximize the low end improvement. The “HT” (high tension) petals should be used in all other modified applications. These petals should be replaced annually in recreational engines, and inspected every 15 operating hours in racing engines.

About Reed Stuffers – “Reed stuffers” is an industry term that describes a plastic insert that smoothly guides the incoming mixture from the round profile of the carb throat, to the rectangular profile of the reed cage. The net result of reed stuffers is usually a small improvement in overall acceleration and response. Stuffers generally have no effect on peak rpm. The ’96 – ’99 62T engines are the only Super Jet engines that employs a design that can utilize reed stuffers. Reed stuffers are not required on the 6M6 and 61X engines.

Upgrading to Larger Reeds – Many 633cc owners have considered upgrading from the 61X crankcases which hold slightly larger reed cages. While these larger cages can be an advantage on a higher rpm racing package, The improvement on a recreational (pump gas) arrangement would be narrow. We have built plenty of “very fast” 701cc engines that employed the 6M6 633cc style reed cages.

Much the same situation exists for 61X case owners wishing to upgrade the larger reed 62T cases. Going to the 62T cases is only worth the cost if you intend to construct a very high-output racing package

Flame Arrestors – Single Carbs – The stock flame arrestor assembly on all the single carb engines works surprisingly well, and offers good protection from heavy water exposure. For customers who don’t ride in “big water” (that exposes the engine compartment to water) an aftermarket arrestor can be an asset. We recommend the 3″ tall K&N non-fiber element with a short adapter.

Dual Carb Arrestors – For all the dual 38mm carb setups, we had the best performance results with the R&D Pro Lock flame arrestor. As well as offering good performance, this arrestor slips on and off easily to allow quick access for carb adjustments. Note that the Novi carburetors must be ordered with the “Pro Lock” adapters built on, in order for this arrestor to be used on them.

Ignition Systems – Of all the components of the Super Jets, few are as reliable as the stock ignition system. While different models have different rev limiter characteristics, they all share the same reliability and effective function. Even on our full out racing packages, we attempt to retain as many of the stock ignition components as possible. For this reason, our ignition mods for all of our kits consist only of rev limit changes made to the stock cdi box, and changes in ignition flywheel mass.

633 Engines – All the 633 (6M6) Super Jets have a stock cdi box with a 6500 rpm limiter. This stock limiter is okay for use with a Sleeper kit. However this limiter must be raised for effective use of any of the Hammer engine packages (where a higher revving aftermarket exhaust pipe is used). Pro Tec performs a modification to the circuitry of the stock cdi box that we consider to be the most effective and affordable rev limiter modification for the Super Jet. This Pro Tec modification comes with an adjustment for the limiter that allows for 7100 – infinity peak ability.

701cc Engines – All the 701cc Super Jets (61X and 62T crankcase) come standard with a rev limiter that falls between 7050 and 7150 rpm (there is a factory spec variation). Like the 633’s, Pro Tec also makes the best rev limiter modification for these models of ignitions. It bears noting that this rev limiter mod is not required for recreational level 92 octane modification sets. In truth, many of the 100 octane formats can easily operate within this rev limit as well…on smooth water. The main benefit of the rev limiter mod on the Super Jets is noticed when the machine is operated in rough water only. As a boat is run across rough water at high speed, the engine can easily “bump” the limiter each time the pump comes out of the water (that’s allot). When an engine, that is against the limiter, reconnects with the water, there is a momentary “landing with the brakes on” kind of feeling that makes it difficult to maintain steady high speed control. The rev limiter modification does not actually increase speed across rough water, but rather it eliminates this landing with the “brakes on” feeling that can cause the rider great difficulty during a race.

Ignition Flywheels – All Super Jets use the same stock steel ignition flywheel. As a percentage of the total rotating mass of the crankshaft, this flywheel is relatively small. Despite that, quicker throttle response can be had by reducing the flywheel’s weight. There are many shops that lighten the stock flywheels (Group K included). We have experimented with many different lightening modification levels on these flywheels. It is possible to lighten the stock flywheel by as much as 1.5 pounds. However our testing showed that when the stock flywheel is lightened by this margin, there is a very high risk of fracturing, and eventual flywheel disintegration (a very destructive failure). The lightening modification that we found to have no long-term reliability weaknesses reduces total weight by .4 pounds. While this is not a big reduction in total weight, we remove this weight near the outer perimeter of the flywheel (where it has the greatest effect on total inertia). The end result is a very noticeable increase in throttle response and acceleration, as well as no compromise in long term flywheel life (note: IJSBA only permits this mod in freestyle and Super Stock classes).

Lightweight aluminum “charging” flywheels (to replace the stock steel unit) have become common in the aftermarket. These lightweight flywheels (legal for use in freestyle and Superstock) can offer big reductions in rotating mass, that generate big increases in engine acceleration rate. While many racers have used these flywheels successfully, they can cause a “side effect” problem in a stand up application.

Most racers interested in a super-light aluminum ignition flywheel, for a Super Jet, are Super Stock class racers. Most of these racers are running big bore top ends in the 771cc – 781cc range. These large displacement top ends generate so much sheer horsepower that they can make a stand up machine very difficult to control, and even more difficult to keep hooked up. The increased engine acceleration from an aluminum flywheel (on a stand up) further complicates these problems. We have seen many instances where riders installed the lighter flywheel, and then cut slower laps as a result of increased difficulty keeping the pump hooked up. Some of this “hook up” problem can be resolved by installing a stainless steel Skat Trak pump assembly. The rest of the hook-up difficulties are left up to the rider.

Our basic recommendation for any non-pro racer (using a stock pump case) is to lighten the stock ignition flywheel. For pro racers that will be using the Skat Trak pump, get the aluminum flywheel…and eat your Wheaties (you’ll need ‘em).

Total Loss Ignitions – It has been popular for so top level pro racers to fit an aftermarket “total loss” ignition. The term total loss means that there is no charging system at all. The battery provides all the voltage needed to make sparks…and that energy is totally lost (instead of being replaced by a charging system). The lightweight flywheels of total loss ignitions can offer a huge increase in engine acceleration (for those that can handle it).

We have experienced “random” reliability with total loss ignitions to such an extent, that we will not fit them to any of our engine packages. If you choose to use a total loss ignition unit, be sure that you have a contact person at the purchase point that can help you with technical and/or diagnostic issues.

Cooling Systems – All of our kits include cooling system modifications of one sort or another. The following is our in-house terminology for the different types of cooling systems.

“Controlled” System – This refers to a cooling system whose total volume ability is controlled to a less than maximum flow capability. Our only Super Jet kit that uses a controlled system is the IJSBA stock kit.

By-Passed System – This refers to a system that maintains the stock cold-water inlet to the engine, yet has additional water outlets on the cylinder head for added cooling. This system is employed in all our pump gas Sleeper and Hammer kits.

Split System – This refers to a splitting of the water coming in from the single water input line. The cold incoming water is split to allow separate feeds to the top-end and the pipe. The intended advantage is to provide a smaller amount of cold water to the pipe, so that a larger amount of cold water can be processed through the top end (via bypass outlets overboard). This is most popular on recreational engine arrangements using an aftermarket pipe, where the owner wants to avoid the hassles of installing a second cold water input line.

Dual Waterline System – This refers to a system that has two cold water input lines from the pump to the exhaust manifold. There are a wide array of outlet/bypass options on this system. Dual Cooling can be employed on any racing engine platform (except IJSBA stock). The greatest advantage to this layout is that there is a backup water inlet system in the case of a blockage entering an input line.

Parallel System – This refers to a dual water line input, where the top-end and exhaust system are treated as two separate and parallel systems. This layout (used primarily by racers) allows the use of numerous bypass outlets on the cylinder head without affecting water pressure into the exhaust system. This setup offers the best in tunability, however a blockage in either input line will bring the machine to a stop (whether by a burnt pipe coupler or a burnt piston). This arrangement also requires separate flush fitting for each system.

Note that our instructions do not offer recommended plumbing schematics for the Split, or Parallel systems. We consider these to be custom arrangements that require individual tuning to suit the particular machine needs.

Crankshaft – Few moving engine parts are more durable, or trouble free, than the crankshafts in the Super Jet engines. Truing and welding is a popular racing preparation for the crankshafts in many high performance pwc’s and we strongly recommend it on a high-rpm racing platform. However truing and welding is questionable necessary for most Super Jet owners. Since the net boat/rider weight of the Super Jet is a fraction of the weight of a runabout, the crank experiences a lot less torsional loading. In addition to that, a stand-up is hard pressed to induce the same crankshaft “hook-up load” in comparison to most runabouts. We have constructed numerous high performance Super Jet packages with stock crankshafts…and never had a single crank failure. We only mandate truing and welding for race gas big bore setups.

Impellers – Impeller replacement is absolutely mandatory because all stock Super Jets come with an aluminum impeller that cavitates badly. We consider the Solas “I” pitch prop to be the most effective replacement for use with the stock pump case. This Solas prop offers excellent overall hookup, as well as the best peak speeds of any other impeller. When the Solas props for Super Jets were first released in 95-96, the steeper “J” pitch appeared to be best for the Super Jet. However since then, Solas has made a design change (without telling anyone) that made the full line of 144mm Yamaha impellers “act steeper”. That is, this newer design loads the engine much harder (we suspect as a result of a steeper “root pitch”). The end result is that the current “off the shelf” Solas “I” prop is much too steep for most applications (and it is the mildest pitch that Solas offers). Group K re-pitches the current Solas “I” prop to suit the particular engine sets we sell. We call those pitches Spec1 and Spec2. The correct pitch is denoted on the price list for each modification level. For customers who wish to have their existing Solas “I” re-pitched, we do so for $48. If you purchase your Solas “I” from us at the regular cost, the re-pitching is included free.

Pump Options – For most high performance recreational applications, the stock Yamaha pump assembly works fine. For owners that want to “fine-tune” the stock pump, we recommend pump blueprinting. This pump blueprinting does not increase smooth water peak speeds, however it can make a noticeable improvement in rough water hook-up ability.

For more serious racers, the stainless steel Skat Trak 12 vane “C-75” pump assembly is the last word in pump mods. This (somewhat expensive) assembly offers incredibly good rough water hook-up that cannot be matched with any other pump setup. It bears noting that the Skat pump uses a smaller diameter 140mm “swirl” type impeller (stock is 144mm). This means that any other prop you have for the stock pump cannot be used with the Skat pump. Since the IJSBA rules mandate that a stock pump “housing must be retained, Skat Trak installs their stainless steel insert into your pump case, hence your pump case “core” is required for this modification.

Handling Components – There are many different ride plates, scoop grates, etc, made by various aftermarket shops for the Super Jet. Since all riders have different handling preferences, it’s impossible to name any one part as “best”. However it is possible to name the parts that appear to have gained the widest acceptance as “good in the most applications”. With respect to ride plates, the Pro Tec plate seems to get the nod from more racing and recreational riders than any other. Many plates allow for much better tracking in the turns, but let go suddenly with little notice. The ProTec plate offers a very predictable “sliding” in high speed turns that allows the rider to feel when the plate is getting ready to “let go”. In addition, the Pro Tec plate offers very good high speed tracking (for a plate with a predictable slide characteristic).

With respect to scoop grates, the Worx grate seems to be the best combination of hook up and peak speed ability. To be sure, there are more aggressive top loader grates available. But most of those grates pay a big price in peak water speed ability that comes along with a very questionable improvement in hook-up (compared to the Worx).

One popular handling item among closed course racers is a lever controlled trim nozzle (aka “dump” nozzle). This nozzle kit helps the rider to control the porpoising of the hull under hard acceleration. Such a kit is manufactured by ProTec. While a bit expensive, this trim nozzle kit is a big asset for controlling the machine during high speed operation in rough water. It took our test riders a while to get used to the operation, but once acclimated, they all preferred it. The only down side to the nozzle set was that the siphon bilge did not work as well as with the stock nozzles. Some fiddling with the siphon tube did not resolve the problem on our test unit (a longer tube in the thrust stream may do it). Since most racers have an aftermarket bilge pump fitted, this is not considered a big problem compared to the handling benefit.

Complete Engines – While many owners prefer to do their own mechanical work, there are plenty of folks that don’t consider engine assembly as “fun”. For those customers, Group K can construct a complete ready to install short-block, modified to your specifications. The 150 lbs. UPS weight limit easily accommodates the Super Jet engine (about 115 lbs.) for customer outside of the Southwestern USA region.. All short blocks are assembled with all new gaskets and seals, and then pressure tested to assure no “rouge” air leaks.

Note that complete short blocks carry a $35.00 fee to cover handling and container costs.

Complete Boats – Group K can construct a complete turn key race boat to your specifications. After assembly, all complete race boats are broken in and water tested to confirm tuning and performance. Specific quotes to your specification are available for $50 billed to your Visa/MasterCard. This quote charge is applied to the cost of the machine upon ordering.