Resto-Mod Build Up – Yamaha Super Jet

OVERVIEW: In the same thought line as our Formula 650 project, race promoters asked us if it was possible to develop an affordable pump gas Super Jet package that would be on a competitive par with the stock class racing SXR 800s. The idea being to expand participation in the 800 Stock class, and bring back many Super Jet owners to a class they can run in… and be competitive.

With this Formula 800 SuperJet prototype, we decided to restrict the 91-octane machine to a single carb, wet pipe, and 84.25mm bores (barely doable on a stock 61x cylinder with no sleeving). All our test riders agreed that the finished prototype was easily the quickest and fastest single-carb stand-up that they had ever ridden. The F800SJ easily accelerates as well as any SXR, and radars 52.5 with a 200lb rider.

After all our testing, We realized that the F800 SJ was not only a reliable and practical race machine….it was also a very economical and reliable recreational machine … at half the price of a new SXR. Since constructing our prototype we have built many “F800” SuperJets for our high-performance recreational customers. Their simplicity and reliability makes them a boat that can easily last for several seasons of hard use with little maintenance. On top of all this, our F800 SuperJet has almost twice the fuel range of an SXR (an important feature given recent fuel prices).

The Objectives – The key to this build-up is that we did not want to make the machine “as fast as possible”…. We only wanted to make it a small margin faster than a stock-class Kawasaki SXR800. That easily attainable goal allowed us to make tuning choices that were lower cost, lower maintenance, and yielded better bottom end power (which every stand-up rider wants). The following text will outline those tuning choices, and how they contributed to the goals of the build-up. At the bottom, you’ll find the complete price list.

Cylinder – Most full-out racing SuperJet cylinders are fitted with larger cylinder sleeves that can accommodate 85.5mm pistons (781cc). The 781 setups works great, but they required engine complete teardown to bore the top-case, as well as the extra cost of sleeving. For our F800, we opted to bore the stock sleeves as far as was safe, thus avoiding the costs of sleeving and case boring. The maximum diameter limit of the stock sleeve is 84.5mm. Given this, we opted to bore to 84.0mm to allow us room for later over-boring (84.25 & 84.50). While the base sleeve (that sticks out of the bottom of the cylinder casting) gets very thin, it is still stable enough to endure the operation of a 7200-7300 rpm engine. One nice side effect of boring the stock sleeves to 84mm is improved cooling (a result of the shorter “steel” heat path from the piston to the aluminum casting).

The porting mismatching of the stock 701 cylinder is some of the worst in the industry. In addition to that, the port dimensions are not up to the job of properly feeding the new 84mm bores. We chose a porting layout that maximized low range acceleration characteristics, and put high rpms as a second priority (because we already knew we didn’t want to rev the engine sky high). In addition to this, we employed the use of piston fed “boost” ports. These ports have a two-fold purpose. First, they improve low range power. Secondly, they pass cool gases across the underneath of the piston crown, and actually help reduce piston crown temperatures. This is an important feature when trying to keep temperatures low enough to allow 91octane operation.

Head – The stock 1994-1995 701cc “61X” cylinder head casting works great for 81-82mm engine set-ups. However the 61X head casting is not beefy enough to maintain strong head gasket sealing pressure on 84+mm bore diameters. The 96-2005 SuperJets (& Blasters) come with a much beefier 64U cylinder-head casting. These casting are easily recognizable by the two carburetor brace holes on the intake side of the casting. 701 Raider engines and 760 engines use this same style of casting (and can be used for F800 kits). Cutting higher compression 84mm domes in the beefier 64U casting is an easy and safe modification. In addition to the dome cutting, we added a second water outlet on the cylinder head to more evenly pass water across both combustion chamber domes, and net more even cooling of the top end.

Last but not least, we added a Pro-Tec “Half Girdle” in order to provide added support for the inlet side of the cylinder casting. Failing to use this half girdle greatly increases the risk of fracturing the cylinder casting around the inlet side base bolts. The cylinder-head does require a bit of machining clearancing to properly accommodate the Pro-Tec girdle….but this small bit of work is well worth the long-term protection offered by the girdle. We intentionally avoided using an aftermarket “O” ring type girdle head because we felt the long-term sealing properties of the 64U-head/blaster2-gasket were far better than the long term sealing properties of any “O” ring head we have used.

Head Gasket Sealing – The stock sleeves of the Yamaha cylinders very often will drop .002”-.003” in the aluminum casting. This makes perfect head gasket sealing very difficult (if not impossible). To resolve this problem, our Cylinder Porting Modification includes cylinder top decking and lapping. This decking allows us to set up a more desirable squish clearance between the head domes and piston crowns. After this machine work, each cylinder top surface is “lapped” to assure the perfectly flat surface to maintain a lasting head gasket seal.

The stock Blaster 2 has 84mm bores, and employs a very tough 3 layer metal head gasket that is second to none when it comes to maintaining a lasting seal. We opted for this Blaster2 head gasket for it’s great sealing properties, and it’s ability to be re-used a few times (with the help of some sealer).

Exhaust System – Most full-out racing SuperJets employ the Factory Pipe “Dry-Pipe”. The term dry pipe refers to the absence of any full time water being injected anywhere inside the body (water is only admitted at the stinger tip). The dry pipe is actually an expansion chamber with a full-length water jacket (literally a pipe inside a pipe). The dry pipe allows much higher rpms than the more conventional “wet pipe”, and is much more expensive to build. For our F800 build-up, the extra rpms of the dry pipe were simply not needed, and the (much more affordable) wet pipe could generate all the rpms we would need. In addition, the wet pipe had a very desirable “adjustable” feature. The wet pipe comes with 3 water-inlet points on the head-pipe, and “tuning” adjusters for each of those inputs. Allowing different amounts of water in at different parts of the head-pipe makes for big changes in power delivery. For our normal “closed course” setup, we opened the bottom adjuster on the pipe ½ turn out from bottomed, and the other two adjusters closed. This allowed for great overall power, and 7250-7300 rpms. However if we simply opened the top adjuster screw ½ turn, we got a huge boost in bottom end power (as would be needed for freestyle). Opening this top adjuster took about 100rpm from our peak…. But that was a small price to pay for converting our top-end racer into a “gobs of bottom end” freestyle boat.

Cooling System – It’s common for full-out racing SuperJets to have “dual waterline” cooling systems. The owners would actually drill another water-feed hole into the pump case, and run a second cooling line from the pump to the exhaust manifold. Dual cooling systems are a great idea, but they do involve a lot of extra hoses, fittings, outlets, etc). For the sake of simplicity, we opted to retain the stock “single input” cooling system on our F800 build-up. The higher rpms and pump pressures of the F800 actually allow the single cooling system to pass more water than a stock boat passes, and we felt we were turning rpms that could easily be contended with by a well laid out single input system….And it works fine.

Crankcases & Reeds – All full-out racing SuperJets use the 96-05 “62T” crankcases for the larger 8 petal red cages that they use. These large reed cages are needed to pass the fuel mixture needed to turn the very high racing rpms. For our F800 build-up, we used the 1994-1995 “61X” crankcases and reeds. The 61X lower end utilizes a very well designed single carb manifold, and two 6-petal reed cages. The 61X case/inlet arrangement is not very popular among racers because it does not convert very well to a dual carb configuration. However the single inlet 61X case design (and the 6 petal reed cages) are ideal for the kind of power we were seeking from our F800. It bears noting that the 1990-1993 650 SuperJets employed a similar crankcase/inlet system (called 6M6 cases) that had 6 petal reed cages only slightly smaller than the later 61X cages. The 6M6 cases and reeds are much too small for modern racing SuperJets, however they are ideally suited for an F800 buildup, offering performance that is virtually identical to the 61X case/reed setup. Note that the 6M6 cases would need to be bored slightly to accommodate the base sleeves of the 61X cylinder employed in this build-up.

All year models of the 6M6 and 61X reed cages came stock with stainless steel reed petals mounted to the six petal reed cages. These stainless steel reeds offered great sealing and reliability in the low rpm stock configurations. However the increased rpms and crankcase pressures of the F800 greatly increases the risk of breaking one of these steel petals. If that happen, the loose steel petal can do considerable damage to the internal moving parts. To avoid this, we fit “fibre” reed petals that offer the same performance as the steel reeds, but cause no internal damage at al, in the event that one breaks.

Inlet System – Most full-out racing SuperJets use dual carb arrangements to feed the air and fuel needed to turn the very high rpms of top level racing. Since our F800 was not turning those high rpms, a single carb would easily meet our engine’s needs, not to mention making carb tuning “a lot” easier. We did lengthy testing between a 46mm and 44mm Mikuni carbs. In the end, the smaller 44 allowed exactly the same peak rpms, along with considerably better low range acceleration and throttle control….who knew that the re-jetted stock 44mm carb would still be the top performer on our F800.

For many years, Group K has included an “inlet manifold modification” on our Sleeper kits. In this modification, we reduce the actual volume of the inlet manifold interior, thus increasing inlet “signal”. The end result is quicker acceleration, and better throttle response. This manifold modification (available on both 61X & 6M6 manifolds) made a very noticeable improvement on our F800 boat.

Fuel System – While our F800 consumes far less fuel than a full out racing SuperJet, it does consume considerably more than a stock SuperJet. Given that, the fuel system needed some modification.

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

While testing props and nozzles one morning, we were making repeated, long, smooth-water passes turning consistently 7250 rpm. About 2 hours into that testing, the test rider encountered a few ripples at peak speed that set the nose bouncing a bit…. But he still maintained full rpm. Suddenly, the boat surged a couple of times (obviously from air bubbles getting in the fuel pickup of a 1/3 full tank)… but he still maintained full throttle. After about 2-3 seconds of the intermittent surging, the engine shut down. Back at the shop we found a scored rear piston…. obviously from air bubbles entering the fuel lines while running the engine at full rpm.

We realized that we needed to re-define the term “pump gas safe”. As long as our F800 had a full tank of fuel, it was totally “pump gas safe”. However as soon as the tank got under half full, the occasional air in the fuel line, from rough water riding, could easily create piston-killing detonation in mere moments. For any closed course race boat, it is impossible to keep air from entering the fuel pickup tube…the only thing you can do is eliminate those air bubbles before they reach the carb….so we did.

The solution was to install a pulse-pump fuel-air separator. With the separator mounted, we were able to run the tank to less than a half gallon at peak rpm with no detonation or surging at all. The only down side of the separator is that when you run out of gas…. you are “out of gas”… there is no notice. Just the same, we figured this inconvenience is better than a scored piston. We made a simple aluminum-strap bracket to mount the remote pump & chamber on top of the battery (worked like a charm). For any modified closed course or freestyle “pump-gas” boats (like our F800), we strongly recommend a fuel/air separator to avoid damaging a piston when fuel levels get low.

Ignition – Most full-out racing SuperJets employ the use of a “total-loss” ignition system. These systems use ignition flywheels with no magnets (thus no charging) in an effort to reduce the rotating mass of the crankshaft. Since these ignitions have no charging abilities, they require frequent battery changes (and/or charging) during a day of riding. There is nothing cost effective or easy about having a total loss ignition…and we didn’t chose one for this build-up. Instead of the high maintenance of the total loss ignition, we opted for the stock ignition with some minor modifications.

The stock SuperJet rev-limiters are 6500rpm (’90-’93 650s), and 7050rpm (1994+ 701s). The most reliable (and affordable) means of raising the rev limit is to get the Pro-Tec CDI modification. This modification includes a small adjuster on you cdi box that allows for settings from 7300-8000+ (we ran 8000+).

Pump & Impeller– Most full-out racing Super-Jets are equipped with an aftermarket stainless-steel pump that offers greatly increased hook-up (compared to the stock pump). The added hook-up of these pumps is mandatory to deliver the considerable horsepower of a high-rpm racing engine. Sadly however, these pumps also hook-up so well that they dramatically increase the loads on the entire driveline…including the crankshaft ands all it’s bearing components. These extra loads are not a problem for full-out racing boats, because they get routine crankshaft rebuilding/replacement to avoid the expensive destruction of a failed or broken connecting rod.

For the lower rpms and lower horsepower numbers of the F800 SuperJet, the added hook up of a stainless steel pump is not necessarily required. The hook-up of the stock pump can be greatly improved with the use of an aftermarket impeller, top-loader scoop-grate, and pump blueprinting. These improvements do not match the hook-up of a stainless steel pump case, however the “less than perfect” hook-up actually helps to extend drivetrain (ie. crankshaft) life.

The impeller we had the best results with is a re-pitched Solas 13/17 Concord. “Out of the box”, this prop is much too steep for the 800cc SuperJet. However with the repitching, it offers excellent hook-up at all speeds, as well as great peak speed abilities.

For owners that want to “fine-tune” the stock pump, we recommend pump blueprinting. Blueprinting essentially removes all the surface interruptions and casting drafts from the pump case interior to allow for improved hook-up at all speeds. This pump blueprinting does not increase smooth water peak speeds, however it can make a noticeable improvement in rough water hook-up ability.

Handling Parts – 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 two most popular choices are the Jet Dynamics plate and the Worx “Dominator” plate. The Worx plate is 100mm longer than the stocker and is biased for closed course riding. The Worx plate is designed in a way that offers a strong “nose-down” ride attitude. This plate is great at reducing porpoising in rough water and under hard acceleration. Unfortunately, when you run at high speed on smooth water, this plate drives the nose down so hard that there is a very real loss in smooth water peak speed, not to mention difficult control caused by the hull “steering on the nose”.

The Jet Dynamics plate has a unique concave design, and is not as long as the Worx plate. The JD plate offers much easier control during high-speed smooth-water runs, as well as very good cornering characteristics. We consider the JD plate an excellent choice for “non-closed course racing” type riding.

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 scoop-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).