951/800 951 ROTAX SEA-DOO We have found huge gains in a very inexpensive way.  Head milled together with a redesigned squish band (an alteration of the lead in angle).  Both squish bands are brought back to bore size which is 88mm.  Typically, we find the OEM stock bands off by 1 to 1.5 mm and the squish band diameters not symmetrical.  In addition, we find high and low spots all over the domes.  An Engine-Tech 951 "head mod" corrects all these very critical, but almost always overlooked defects which causes detonation and detonation head pitting.  For those owners with slightly larger bore sizes, all you need to do let us know what you require and consider it done. FLY flame arresters, a jetting kit and "outerwears" for the FLYs (Prok style 4 inch by 2 inch) are a must for this very "water intolerant" motor.  It is our experience that any water that migrates into the engine compartment from any source, such as the tray on the XPL/XPs for an example, will get sucked into the stock air intakes as it drips down.  As a consequence, the visual evidence always appears to be scoring of the rear cylinder (pto side) and piston.  Simply put, the outerwears repel moisture.  If an owner were to run the Flys without outerwears, the result would be the same as running the stock airbox and perhaps worse.  Full bodied outerwears can be ordered in addition to the Fly outerwears and are the same as those found at www.outerwearsracing.com  PRICES: Head mod $120.  re-jet carbs and jet kit: $95.  (includes cleaning of the internal carb fuel filters)  FLY flame arresters: $150.  (all "Flys" come with Fly outerwears) (jet kit includes 90 pilot & 170 main jets & (2) 2.3 needle & seat) (lows 1 & 1/4 turn out on both) (mag high 1/4 turn out / pto high 1/2 turn out / add 1/4 turn on both for 175 psi) (all settings tuned for outside temp of 72 F / warmer and humid air requires less fuel so turn adjusters in slightly) (2 new aftermarket (thicker than OEM) carb gaskets are provided)   Total kit price: $365.    Tuning Note: Your 951 comes stock with Mikuni BN 46-42 "I" series carbs, (unfortunately), which are known as the "emission" series of Mikuni PWC carburetors.  Consequently, there exists an issue when adding less restrictive flame arrestors to this motor in as much that the "venturi signal" is reduced at a certain rpm by the lessening of the air intake restriction created by the "Flys" or any aftermarket flame arrestor for that matter.   Therefore, one will not be able to cruise or "sit" at or very near the 5800 rpm range.  No amount of jetting or tuning will change this unavoidable situation.  When one does hit this area of rpm, one will find that the motor will start to run very smooth and very much quieter.  This is the initial sign that your motor is running lean and heading eventually into a lean seizure.  Naturally, you can move back and forth through this particular rpm range without any ill effect.  Also, it must be noted that the mechanical accelerator pump action must be "leaned" out by approximately 50%.  That is to say, that the stock setting of the accelerator pump arm must be reduced so that just a little "squirt" of gas is injected into the carbs.  This is a most simple procedure when the carbs are off the ski which must occur for the re-jet.    Compression, Gas, Oil & Plugs: The stock cranking compression of a 951 @ sea-level is 150 psi.  An Engine-Tech head mod will raise that to about 173-175 psi which requires a minimum of good, fresh 92 octane.  However, for those of you that are interested in using 87 octane, your 951's head can be cut to produce 160 psi.  That combined with the cooling mod will allow for 87 octane safely.  Even if an owner does not require an increase of compression, it is highly recommended that the zero squish lip (if present) be removed to eliminate squish band detonation/pitting.    Stock 951s have an oil ratio of 50:1 @WOT.  If you can name any high performance "big bore" 2-strokes that operate at this ratio then by all means contact the webmaster and tell him.  It is highly recommended that the gas/oil ratio be increased to 40:1.  This can be accomplished simply by adding 3/4 ounce to every gallon of gas or 50ml to every litre.    Also available is the dual cooling kit which includes; 2 drilled and symmetrically taped fittings on the high sides of the domes, dual cooling lines and all installation fittings. Special care is taken when taping the domes not to install the fittings below the ceiling of the water jackets. Doing so could trap small pockets of air at these locations leading to problems of unreliable heat transfer which plague the OEM head. Not only do the dual cooling lines look cool, they ensure even and maintainable head temperatures. This service is only available with the head modification.   Dual Cooling Kit:  $95.      A few additional 951 tips courtesy of the Webmaster:   The number one reason to own a jet ski is to be able to ride it when you want to, not when it decides to let you.  All jet skis have their issues.  IMO, Sea-Doos are excellent all around jet skis.  Some of these issues are actually quite minor in relation to the entire machine.  But it doesn't take much to keep these machines from, quite frankly, screwing up!  In addition to the above-mentioned items for sale and modification already listed, the following are an additional few items that can interfere with your 951 ride.   1.  The Manifold/Head Pipe Exhaust Water Leak:  There is an excellent article about this at www.sea-doo.net in the technical section.  Essentially, this exhaust leak is a function of how much pounding the ski takes.  The pipe is heavy and it gets loose and the leak happens.  Sea-doo has a repair kit called a "Y2K" something and it contains stronger head bolts, stronger pipe support bushings, a "recovery envelope" and a thicker improved gasket.  It's inexpensive and I have it on my 99xpl, although I never did have the leak.  I experienced the water intrusion through the STOCK air intakes which, even though Sea-doo has tried with a few plastic and rubber pieces to remedy, still allows water in.   2.  The 99 951s are the first year to be equipped with a throttle postion sensor (TPS).  It was first developed for Ski-Doos.  Essentially, it assists in engine timing.  However, water will fast destroy these TPS units as they contain a printed circut board which will short when wet.  On a 99xpl, the TPS is located under the oil tank fitted into a pad.  All that is needed to protect this device is to smear it with grease (electrical type grease would be best).  Personally, I have just used the same Castrol syntheic grease that I pump into the PTO fitting. If water does penetrate the TPS, the ski will not run correctly.  Typical symptoms are having to hold the start button on to run and no more than 4000 rpm.  Please note, that for the 2000 951s and up, Sea-Doo re-engineered the TPS and they seem to be operating without incident.   3.  Bad bendix gears in 98 and 99 951s.  Occasional problem with the starter bendix on these years is that teeth can shear off or chip off the bendix gear and  pieces of metal will fly about the flywheel and on occasion get stuck in the stator and short it out.  If your ski stops and won't run and the fuses blow and re-blow every time you replace them, then unplug the wire connector to the front mag cover and  then try another fuse and if it doesn't blow then you need to open up your mag cover and very closely inspect the stator and other parts for metal fragments.  The 2000 and up 951s have a different part number for the bendix gear so it appears Sea-doo figured it out.   4.  An additional website for good technical information regarding all skis in picture format can be found at www.sbtontheweb.com       800 ROTAX SEA-DOO The 785cc engine is one of the best, but it has a lot of performance room left. The head is in need of milling, together with re-cutting the squish band to optimize compression. Flame arresters finish the kit off. Result is a tons of low end, a hard hitting mid and very fast top end. A good aftermarket pipe also boosts performance, but requires a rev limiter.   Performance Note: The 800 is under aspirated.  In the past, engine-tech manufactured 42.5 mm butterflies to augment over-boring of the stock Mikunis, but no longer do.  We do not recommend boring the stock carbs without enlarging the butterfly.  Aftermarket carbs, although expensive, make a difference but usually require an external fuel pump.     Kit includes: Head mod: $100. Carb re-jetting and jet kit: $96. (includes cleaning carb filters) "Fly" Flame Arresters and adapters: $195. (all "Flys" come with Fly outerwears) (jet kit for just F/As are 80 pilot jets/stock 142.5 main jets/ 2.0 needle & seat with 3/4 turn out on both lows & 1/2 turn out on the MAG highs & 3/4 turn out on the PTO highs)  (warm/humid climates above 72F could require 75 pilots with an additioanl 1 half turn out on all adjusters) Total kit price: $395.