4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Group K Air-Fuel Separator

– by the Technicians of Group K

The Short Story

High performance pwcs (and jet-boats that use pwc engines) all have a shared problem. As fuel level gets low, air bubbles routinely make their way into the fuel pickups and then into the fuel lines. Since there is no way for this air to be purged out (once it has entered) the air is passed through the carburetor jets where it causes a momentary, but serious, lean condition. As more and more bubbles are admitted, these moments of lean condition becomes so prevalent that the operator experiences a noticeable “bog” or hesitation.

On PWCs being run at low speeds, these “lean moments” are of little consequence. However on high performance pwcs being run at high rpms (particularly 3 cylinder engines), these lean moments often result in momentary detonation, and “more than occasionally” result in piston scoring or piston seizure. Countless times, owners of high performance pwcs and jet-boats have experienced a sudden unexplainable engine “shut down”. Examination afterwards shows the shut-down was the result of a scored piston … buy why.??

Our testing showed that in many cases these mysterious scored-piston shut-downs are a result of a “more than momentary” lean conditions being caused by air entering the fuel pickups. To resolve this problem, we created a fuel/air separator kit that assures an “air free” supply of fuel to the carbs at all times. This kit can be easily fitted to most pwcs and jet-boats, and brings with it improved starting abilities and reduced carburetor maintenance.

The Technical Story

Like all engine builders, we had experienced random failures of high performance engines that we “suspected” were attributed to air being introduced into the fuel system. However we recently conducted some tests that allowed us to pin point this fault.

The Jet-boat Tests
We obtained a single motor Yamaha Exciter (135) jet-boat for performance increase tests. After spending a few weeks of testing with various props, compression ratios, porting formats, etc, we got down to doing the carburetor fine-tuning. Like all our fine-tuning tests, the boat was equipped with a digital tachometer, and a detonation sensor that had a lead to each cylinder. We were in the second day of fine-tuning, and the boat was operating perfectly, even under the heaviest high-speed loads we could inflict upon it.

Mid way through our testing, another boat we had at the water needed fuel. We pulled up the jet boat (with it’s 35 gallon tank about half full) and siphoned out 6 gallons to put in our other boat. We figured we still had at least 10 gallons of fuel on board, which would easily be enough to complete our testing. With the siphoning done, we headed our Exciter jet-boat back out on the water to finish our fine tuning. Immediately, we started seeing mild detonation on all three cylinders as the jet-boat ran across the bumps of some small boat wakes. We then ran the boat to peak speed for our standard “full speed left turn” test (this test loads the engine harder than any other kind of operation). As soon as we snapped the wheel to full left, all three cylinders showed heavy detonation that would have easily scored a piston if we maintained the turn for a couple of more seconds. We were stunned that this engine (that had been performing flawlessly for days) was suddenly on the verge of burning down a piston. We checked our entire engine compartment to make sure that some random component failure or fuel restriction wasn’t causing this new and very lethal detonation. After exhausting all possibilities, we went back to the launch ramp and added back the 6 gallons of fuel we had just siphoned off. Back out on the water, our jet-boat once again performed flawlessly with no visible detonation even under the heaviest high rpm operation and hardest turns.

For us, the message was clear. The fuel in the long and narrow fuel tank of our jet-boat could easily be ”sloshed” away from the fuel pickup tubes. Once the pickup ends are exposed to air, that air enters the fuel lines. Our jet-boat (like all pwcs) has no way to purge out this air in the fuel lines, other than passing it through the jets in the carburetors. As soon as these air bubbles reach the jet circuits, they create a very brief, but very serious, lean condition. These brief lean conditions were causing the mild detonation our jet-boat exhibited as we drove across the bumpy boat wakes. However, our full-speed left turn sloshed virtually all the fuel to one side of the tank, causing an instant and very lengthy lean condition. A lean condition of this order can easily score a piston in less than 3-4 operating seconds (under load).

The Solution
We created a fuel/air separator for our project jet-boat to purge all air bubbles from the fuel lines “before” they reached the carbs. We tested with both electric and “pulse” type fuel pumps, and various separator canister designs. Through the whole process we maintained a “keep it as simple as possible” mindset.

We finalized on two separate designs. (A) The crankcase “pulse” fuel pump kit, and (B) the electric fuel pump kit. The electric fuel pump kit is intended primarily for the twin-engine jet-boats (Yamaha and Sea Doo), however it could be used on any pwc with a strong charging system as well. The “Pulse” fuel pump Kit is by far the more simple and reliable, however it does mandate a primer pump (or carbs with an accelerator pump feature to prime the carbs). Since our pulse pump delivers 18 gallons per hour, it can easily feed any race engine (including triple pipe 1200s).

After installing our finished separator on our test jet-boat, we re-ran our jet-boat tests with only 5 gallons of fuel in the tank. This is a level that would have certainly delivered heavy detonation during high-speed rough-water operation, and piston seizing detonation during our “full speed left turn” test. No matter how hard we ran our test jet-boat with this low fuel level, we never experience even the slightest detonation. The fuel/air separator was obviously doing a perfect job of purging all the air from the fuel input lines to the carbs.

Other Applications
While we originally developed these kits on a pwc driven jet-boat, we believe that the most effective uses for the fuel/air separator is in PWC applications. The following are the applications where PWC owners would stand to have the greatest benefit. In short, any PWC application that entails a machine being operated at high rpms with a low fuel level.

Yamaha GRP / XLL / XLT Owners
For several reasons, the Yamaha 1200cc “exhaust valved” models have an exquisite need for fuel/air separators. They are as follows:

  • With their large 16-18.5 gallon fuel tanks, and hulls that encourage high speed operation in rough water, these Yamaha models have the greatest risk of splashing fuel away from the fuel pickups and taking air into the fuel system lines.
  • These Yamaha models are equipped (stock) with three 46mm Mikuni “I” body carbs that each have their own “pulse” fuel pump. These individual fuel pumps each have two plastic check-valves that maintain fuel pressure to the carbs. Unfortunately, the check valves in the fuel pumps of these carbs are prone to “crease” damage. When this creasing takes place, fuel pressure to the carbs is seriously comprimised. PWC technicians disagree on the reasons for the high rate of check valve “creasing” on the valved 1200 Yamahas, however they all agree on the negative effects that this “creasing” can have on fuel delivery, fuel pressure, and overall machine performance.

About 50% of all GPR/XLL/XLT carbs we inspect have creased fuel pump diaphragms like these. As the engine is modified, the rate of creasing is substantially higher.

  • C) “IF” an owner is wise enough to recognize the poor performance caused by “creased” fuel pump check valves, he (or she) can go to the local Yamaha dealer to have the problem attended to. The set of check valves themselves cost about $12. However replacing them requires the full removal and dismantling of the carb rack…..which can only be reached by removing the entire exhaust system. Generally, the labor bill for this process is $500-$600. When the separator kit is installed, all these pump check valves are removed, thus eliminating the need to service them ever again. The “pulse” pump in the separator kit also has check valves, however this pump can be placed in a way that allows easy check valve inspection/servicing that can be done without removing the carb rack or exhaust pipe.

Our Testing on Stand-ups
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 arrangement that would be on a competitive par with the stock class 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 Super Jet prototype, we decided to restrict the 91 octane machine to a single 44 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 51.5 with a 200lb rider.

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, I encountered a few ripples at peak speed that set the nose bouncing a bit…. But I 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 I 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 in 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 F800SJ 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 that is better than a scored piston. We made a simple aluminum strap to mount the remote pump & chamber on top of the battery (worked like a charm). For all you guys out there running modified stand-ups and sport machines …. It’s something worth thinking about.

Closed Course, Endurance, and Freestyle Competitors
As stated above, the greatest risk of piston damage (from air in the fuel lines) is when a PWC is being run at high rpm … especially when fuel levels are low and the fuel is being agitated by rough water.

Closed Course – Each time a closed course race boat is put into a turn, the fuel is pushed to one side of the tank. On open-class machines with 15+ gallon fuel tanks the tank must be almost full to avert getting air into the fuel lines. If air does make it’s way into the lines, the brief lean condition can often cause a serious “bog” in throttle response driving off the turn. With a separator in place, there is never any fuel starvation “bog”, and the rider gets consistent throttle response no matter how low the fuel level gets.

This problem of aerated fuel is so pronounced the Factory Pipe Products urges their triple pipe STXR owners to NEVER operate their machine with less that a half tank of fuel. On a 7700 rpm 1200, a momentary lean condition from aerated fuel results in instant piston scoring.

The new SXR800 has become the new staple for stand-up closed course racing. The Limited & Superstock SXRs run exceptionally fast, but do so by consuming “lots” of fuel. On larger courses, an average SXR Ltd/SS race boat will be very low on fuel in the last laps of a race, and runs the same risks of “corner bogging” or piston scoring from aerated fuel. The separator is compact enough to be fitted to any stand-up, and it assures an air-free fuel supply to the bottom of the tank.

Endurance – Among all pwc racers, endurance racers run the highest risk of damaging a piston from aerated fuel because they are constantly trying to run as far as possible on each tank of fuel. In addition, these owners are running their engines constantly at high rpms, where the risk of a lean condition has the highest consequences. With an air free supply of fuel, the risk of scoring a piston is dramatically reduced.

Free-Style Riders
While freestyle riders are at lower risk for experiencing piston scoring from aerated fuel, they have their own special issues. Any time a freestyle boat get in a sideways or upside down attitude, air bubbles can easily enter the fuel lines. It sometimes happens that, after a big maneuver, a freestyle rider experiences serious bogging from a large amount of air bubbles being admitted to the fuel lines. The separator cannot stop the air from getting to the carbs while a boat is upside down. However once a boat is right side up again, the separator processes all the air out of the lines in moments so the rider can proceed to the next trick immediately without having to “wait” for the fuel lines to completely refill with fuel.

Drag Racers and “Grudge” Racers
While this type of PWC racing is not often sanctioned, it is widely participated in by owners across the country. In pursuit of the highest possible peak waterspeed, these owners often run with the least possible amount of fuel to reduce nose weight. Among racers, these owners have the greatest risk of scoring a piston from aerated fuel because the load of running at peak rpm on smooth water is the heaviest of all engine loads. A boat being operated in this way needs only to get aerated fuel for a few moments in order to experience detonation and piston scoring. With a separator in place, that risk is virtually eliminated.

Package / Upgrade / Parts

Group K Price


PWC Pulse Pump Separator Kit
Includes separator chamber, 70 lph pump, all fittings, hoses, and installation instructions


Separator Chamber Kit – (no fuel pump included)



Jetboat Electric Pump Separator Kit – (for twin engine jet-boats)
Includes separator chamber, 30 gph pump and all fittings, hoses, and installation instructions


Separator Chamber Kit – (no fuel pump included)



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 !!