Yamaha 701 – Dual Carb Mods

About High Performance PWC and the New Reformulated Fuels – For the last 7 years, Group K has offered two lines of pump gas engine modification kits known as “Sleeper Kits” and “92 octane Hammer Kits”. The Sleeper kits are an affordable group of modifications to stock components only. The 92 Hammer kits were designed to be the highest horsepower combination, of modifications and bolt on parts, that could run reliably on 92 octane pump gas. All our Sleeper kits are still safe to run on reformulated 92 octane. However the high performance “unfriendliness” of the new reformulated fuels is so profound that we have had to increase the octane mandate for the Hammer kits to a minimum of 96 octane.

Part of the reason for the increased octane need is the nature of the modern personal watercraft themselves. During the days of stand up pwc’s, it was rare for anyone to ride “full throttle-full rpm” for extended periods of time. However, the later model (sit down) machines handle so well that extended “full rpm” operation has become commonplace. We test our kits with this very heavy use in mind. Initially we were able to avoid the 96-octane mandate by using stock rev limiters and milder compression. However, the progressively increasing output of new machines and the decreased quality of 92 octane pump gas has brought about this unavoidable situation.

There have certainly been many isolated situations where owners have “gotten away” with running pump gas in machines that we might consider as 96 or 110 octane formats…good for them. However these exceptional situations will become more and more rare. We have no idea how the other aftermarket high performance shops will be dealing with this issue, but we do know they cannot ignore it.

Octane Options – Not many folks want to drive a long distance to spend $5.00 a gallon on race fuel for their recreational pwc. However anyone that has paid an expensive bill for the repair of a seized piston has considered it. The repair cost of a seized piston is the financial basis we will use to look at the cost effectiveness of various octane options. The average repair bill for a seized piston is about $350 for a twin cylinder pwc.

By far the most cost effective octane option is a 50/50 mix of avgas. A gallon of 100 octane avgas costs about 50 cents more per gallon than 92 octane unleaded. Based on a 10 gallon tank of gas, you could buy 140 tanks full of 50/50 100 octane avgas/ 92 pump gas for the same money it costs to repair one seized piston.

Another option is a 25% mix of 110 octane race gas with 92 pump. With race gas at $5 per gallon, you could buy 28 tanks of this mix to equal the repair of a seized piston.

Lots of owners ride in areas where they need to buy fuel at outlets on the water. These folks are more interested in octane “boosters” that they can carry along. Unfortunately no octane booster can turn 92 octane into 96. At best, 3oz of booster per gallon can normally make 87 into 92, or 92 into 93.5-94 (the increase offered by booster becomes less as base octane gets higher). Using more than 3oz per gallon offers no further increases. In addition, this small increase comes at an expensive $1 per gallon (the 25% mix of race gas costs $1.25 per gallon).

Design Weaknesses – The Twin Carb 701 engines have a few inherent design weaknesses. Modifying your boat with a keen awareness of these weaknesses permits the construction of an effective yet reliable modification package. If these weaknesses are ignored, engine life and overall reliability can begin to suffer immediately.

Head Gasket – All Group K kits are intended to be used with the stock head gasket. There are many aftermarket alternatives to this gasket, i.e. .020″ copper gaskets, and “O” ringed heads, etc. It has been our experience that the primary cause of most head gasket leaks is low spots on the cylinder deck surface. To address this problem, all Group K kits include cylinder decking to assure a totally flat deck. While all of the after market gasket replacements may be capable of maintaining a seal, none of them can resolve a leak caused by an irregular cylinder deck surface.

Drive Coupler – The stock drive couplers and rubber cushion mounted at the back of the motor can work well on a stock machine. However these parts may have difficulty standing up to the added rpm’s and torque loads of the modified engines (particularly in the Wave Raider hulls). As a safety measure, we recommend a set of “billet” drive couplers for modified machines that will frequently be run at full throttle.

Air Leaks – The lower end of your engine should be totally air tight up to 8 psi. New engines pass this test with no problems. However engines with older deteriorated crank seals or brittle intake gaskets can allow outside air to leak into the lower end during operation. This “air leak” can make the fuel mixture lean enough, at high rpms, to cause a piston seizure. Air leaks, not engine modifications, are by far the number one cause of piston seizures.

TOP END MODIFICATION – All Group K kits include cylinder porting. In the past, cylinder porting has received an undeserved reputation as a modification that causes a loss of reliability and engine life. If the porting is performed by someone with very little Yamaha experience, that may be true. The porting mods used in all of our kits are the result of many months of performance and wear evaluation testing. We confidently claim that Group K cylinder porting will yield a wider power band and longer piston life than any other porting modification available anywhere. All Group K modified cylinders and heads are typically prepared as matched pairs to assure that you have the proper squish clearance and compression ratio for the quality of gasoline that you intend to use. These cylinder and head specifications play the key role in determining the amount of overall power that the engine can make as well as the quality of fuel that you must use.

Piston Clearance – It’s common for new 701 cylinders to have as much as .005″ of piston to cylinder wall clearance. (set up clearance is .003″ – .0035″ ; wear limit is .007″). On any racing or high output recreational packages, we recommend set up clearances of no more than .004″. Engines intended for moderate recreational use can easily live with .004″ – .005″ set-up. Piston clearance inspection is available on request, and cylinder boring (using pistons furnished by you or us) is available in all oversizes. Since Yamaha only offers only two oversize pistons, boring beyond second over will mandate the use of Wiseco pistons.

Exhaust Systems – We recommend that a stock waterbox silencer is used on all the 701 Sleeper and Hammer kits. Contrary to common belief, the stock Yamaha waterbox yields excellent overall power levels in all high output applications. Unlike most after market waterboxes, the stock waterboxes can offer quiet sound levels along with strong power when water is diverted away (water diverters are a part of every Group K engine kit). Even the Hammer 96 runs best with the stock waterbox. We have tested numerous aftermarket waterbox replacements and found none that significantly increased overall power.

For all the dual carb Yamaha engine formats, we have had the best overall results with the Factory Pipe Products exhaust systems. Owners looking for an exceptional increase in acceleration should also get the optional ECWI (electronically controlled water injection) system from Factory Pipe. While this system greatly improves power from 3500 – 6800 rpm, it makes no change in peak rpm or peak water speed abilities.

Intake Systems – The stock dual 38 Mikuni carbs are badly obstructed at the air intake mouths by the choke butterflies and other casting bosses. During all our tests, the stock 38 Mikuni carbs (as modified by Group K) consistently offered the better overall acceleration and peak speed of any arrangement tested. Dual 44 Mikuni conversions are heavily promoted and being made widely available. In our testing on the 701cc Raider style engine, dual 44’s worked well in every way. However in back to back tests done on a modified Raider, the modified 38’s offered exactly the same authority of acceleration as the 44’s, along with 100 more peak rpm. Along with the slightly better rpm, the modified 38’s also offered significantly less fuel consumption. During our 701cc tests of aftermarket exhaust pipes, the modified 38’s had no problem delivering strong acceleration to speeds slightly over 60 mph (Raider hull).

Flame Arrestors – The stock flame arrestor assembly works well and offers good protection from heavy water exposure. For customers who don’t ride in “big water” conditions that expose the engine compartment to a lot of water, aftermarket arrestors can be an asset. We had the best performance results with the R & D arrestor.

Reeds – The stock Yamaha reeds and cages have consistently outperformed any aftermarket reeds we have tested. Some aftermarket reeds can offer better power for 20 – 30 operating minutes. However after that amount of time the material “looses its memory” or begins to chip. As this happens, the performance of these weakened aftermarket reeds becomes worse than stock.

Aftermarket reed stuffers offer a noticeable improvement in overall acceleration. We consider them to be a good horsepower value and recommend the aluminum Boyesen stuffers for these engine packages.

Ignition – These stock ignitions have a built-in rev limiter that comes in at about 7100 rpm. Modifying this limiter is recommended “only” for the 96 octane Hammer kits. For these machines, the limiter cannot be effectively disabled by simply disconnecting a wire in the electrical box. Pro-Tec offers an effective and reasonably priced brain box modification that eliminates the rev limiter. The Pro Tec mod features a manually adjustable upper rpm limit that may appeal to some customers. (NOTE: Aftermarket billet drive couplers are mandatory for any Raider hull machine that has the limiter eliminated.)

Lightening the stock ignition flywheel offers slightly quicker throttle response at a very reasonable price. The aluminum “charging flywheels” available from various aftermarket shops are a little lighter (and a whole lot more expensive) than a lightened stocker. We only recommend these flywheels to racers who have an “open” budget.

Cooling System – All of our Raider kits can operate reliably with the stock single input cooling system along with our cooling system upgrades. All the Sleeper and Hammer kits come with 2 additional water bypass outlets. All fittings and hoses are included. (note: for customers who have aftermarket exhausts, our cooling upgrade kits require that you send your exhaust manifold along with your top end parts.

Group K 753-780 Big Bore Kits – Group K will be offering two different Big Bore modifications for the 701 engines, based on the cylinder that you have. The first, and most affordable, is the Raider Cylinder Bore Only. The 701 Raider cylinder (which bolts onto SJ/Blaster cases with no case boring) can be bored to 753cc using the existing stock sleeves. While the base sleeve areas do become somewhat thinner, the are easily strong enough for high output long term use. Boring this cylinder leaves the ports badly undersized for a 753. That’s why cylinder porting will be a mandatory part of all the Group K big bore kits.

We will fit the original 84mm bore to Wiseco pistons because they typically run .002″ – .003″ under the size of the stock Yamaha pistons. In the event that this original bore becomes slightly worn or scored, the Group K big bore cylinder can be honed slightly to a perfect clearance fit on the slightly larger Yamaha piston. After that, the .010″ Yamaha piston can still be used as another overbore size. After that, the sleeves would need to be replaced. This amount of over bore allowance would easily last most owners a couple of seasons.

The second type of big bore is the Sleeve and Bore, which must be done on the SJ/Blaster 701 cylinders. These cylinders do not have enough base sleeve thickness to safely accommodate an 84mm piston (753cc). For these cylinders, Group K installs a thicker wall sleeve (made to our specifications) that can safely accommodate pistons up to 85.5mm (780.44cc). Top case boring is required to accommodate these larger sleeves. Thick wall sleeve installation will also be available for customers who desire to bore their Raider cylinders to the 85.5mm diameter (boring the Raider cases will be necessary).

Case Porting – Case porting by itself doesn’t offer a giant overall power increase on the 701 engines. However, this is a modification that can make a noticeable improvement in overall power that comes without any loss of reliability at all. The case porting is an “all gain – no lose” modification that should be done on any Big Bore Hammer (note: cylinder is required with top crankcase half to do case porting)

Pump Modifications – Pump blueprinting offers much better “hook up” in rough water conditions and is an important asset to machines that will be run hard in rough water conditions (as race boats are). If your machine will be operated primarily on smooth water, the blueprinting will offer little improvement for the money spent.

The aluminum props on all modified Super Jets must be replaced with a Solas “I” pitch or a Skat Trak 10/18°. The Stock (stainless steel) Blaster prop works well, but the Solas “I” offers a little better acceleration with no loss of speed. The stock Raider impeller is too steep for any application. The Sleeper kit Raider should run the Solas “I”, while both Hammer kits will run the steeper Solas “J” pitch.

Handling – For recreational boats that will be operated primarily on smooth water, cutting out the center bars of the stock scoop grate will offer all the additional water access that’s needed. Aftermarket scoop grates are recommended for all machines that will be run at high speed in very rough water conditions. For riders who run their machine very aggressively in rough water conditions we recommend the “top loader” type scoop grates. Be aware that while these top loader grates offer excellent rough water “hook up”, they also scrub 1 – 2 mph off peak water speed (4 – 5 on the Raider), and often cause “eye opening” deceleration when you release the throttle.

Of all the ride plates and scoop grates we tested on the S/J and Blaster models, the “Jet Dynamics” and “Worx” components consistently out performed every thing else. These plates and grates are more expensive than the rest, however in our judgment…well worth the price. On virtually all the Raiders we tested, cutting the stock ride plate flush with the back of the hull offered the best water speed and handling characteristics.

Assembly Information – All Group K kits are accompanied by a step by step instruction pamphlet that outlines the assembly, break-in, fine tuning, and maintenance of your kit. For any further questions you may have regarding your Sleeper or Hammer kit, you’re welcome to contact us directly for assistance.