Carbflow

About The BlackJack Carburetor

bjcarb.gif (53999 bytes)

Now enter the BlackJack era.

We designed the BlackJack Carburetor so that it could be used as a recreational model yet perform like a race only carburetor on the track. The requirements are quite different between a racer and a recreational rider.

The racer requires the maximum available air flow for the best top end speed possible. Most race craft leave the starting line between 2,500 to 4,000 rpm, usually with 2 holders holding the boat back. They have plenty of compression, lightweight flywheels, and relatively low pitched impellers to allow for the high rpm's needed to maximize the cylinder porting and pipe tuning requirements. This type of watercraft seldom needs to accelerate from a real idle speed.

The recreational - weekend rider requires instant response right from idle speed with frequent accelerations from idle to top speeds, and consistent response through turns. These type of watercraft usually run lower octane pump fuel and thus, lower compression. They also have heavier flywheels. This causes a longer dwell time relying on the low speed - transition circuit. This is the critical time because the air velocity through the carburetor is too low to start acting upon the high speed circuit. This is especially true when dealing with larger-than-stock carburetor bore diameters, let alone modified larger than stock-bore-diameter carburetors.

When we set out to design the BlackJack Carburetor, over 3 years ago, the design started at the flow bench. A superflow 600 flow bench was used instead of the traditional 110 model. This allows higher test pressures. We tested at 28" H2O D. P. This high differential pressure is needed if you want to detect small changes in air flow. Ten different billet air velocity stacks were used, each with a different diameter, transition taper and venturi design. Emphasis was put not only in cubic feet per minute of flow, but also on one detail seriously overlooked in the industry - static pressure decrease through the venturi at part throttle openings. This is the key to fuel curve management. In other words, the vacuum measured at the main jet port at part throttle openings. The higher the vacuum measured at this point, the quicker the high speed circuit comes into effect and the less dwelling time you have relying on the low speed circuit. If you have a modified carburetor, and it has to be run at very low pop-off pressures otherwise it will hesitate upon sudden acceleration. This is usually due to a weak signal to the main jet or high speed circuit. Things that effect the signal strength are outer venturi diameter, air entry geometry and pressurization, and high speed fuel delivery placement and design used. all of these are air velocity dependent. The weaker the signal strength to the main high speed circuit, the lower the pop pressure and richer your lower circuit must be in order to not hesitate until higher air speeds are obtained to pull in the high speed circuit.

Ironically, during our initial testing, we found that increasing the venturi diameter past a given point did not yield any significant increase in cubic feet per minute (cfm) but did decrease the vacuum to the main jet port. We chose a particular venturi size and varied the shape to get the highest cfm flow possible. We chose a billet air velocity stack for one primary reason - it allows a larger diameter top opening. This allows a smoother, better designed transition taper that could not be obtained by machining the carburetor alone, or placing a top over the existing carburetor.

The BlackJack Carburetors have the highest vacuum at the main jet port of all the high performance carburetors we have tested. This is evident by the fact that the BlackJack Carburetor can consistently run higher pop-off pressures than our competitors, and still retain instant and crisp throttle response. Stock and limited reed engines frequently run 24 to 30 psi. Some of our competitors recommend pop-off pressures as low as 8 psi. This low pop pressure makes for inconsistency in low speed tuning and frequent loading up and rich conditions. This is especially noticeable at idle or just off idle speeds.

fig4.gif (16553 bytes) When we flow tested our competitors carburetors, we found that they all had larger outer venturi's than ours, yet they flowed less cfm. Using velocity probes and a smoke stream to visually check how the air flowed, we found that the obstructions caused by the high speed fuel delivery system, whether it be a tube, half tube, bombsight, booster venturi or inner venturi, they all seemed to cause the air to split or deflect in multiple paths around the obstruction.

Just like a wing or a car in a wind tunnel deflects the air around it's shape. This deflection causes high pressure areas which further deflect the incoming air to it's surrounding areas. This hereby fig5.gif (10118 bytes) reduces your total net usable flow space. In other words - what the eye sees is not what the air sees as it passes through the carburetor. I'm sure everyone has noticed what happens when you're driving and encounter or pass by a truck at a high speed on a two lane highway. It is this high pressure area that forces your car to move to the side. You don't see it, but you know it's there.

fig6.gif (14981 bytes) One of the carburetors we tested uses a tube running at 90 degrees to the throttle shaft. What we noticed was an air flow "dead spot" caused by the air going around the intersection point of the throttle plate and the tube. One must remember that the air must split at this point into multiple paths. The faster the air moves, the larger the high pressure air deflecting zone becomes.

On this type of carburetor, the air rushing through the carburetor does little to pull the fuel out of the high speed circuit. This is especially true at lower speeds or part throttle openings. The fuel is discharged on this type of carburetor primarily through the pressure difference between the atmospheric pressure and the pressure in the carburetor bore only, and not by the air acting upon the high speed circuit.

The above three pictures depict how high speed air flow causes any obstruction
to decrease the net usable air flow space. The yellow is the high pressure area,
air deflecting zones. The orange is the net usable air flow space.

On BlackJack Carburetors, there is no performance robbing restrictions or obstructions to decrease the flow area at higher speeds (see diagrams below). We can optimize our air entry geometry and venturi size to create a stronger signal to the main jet. We can create an air accelerating ramp to speed up the air just prior to reaching the point of high speed fuel delivery. We can incorporate an expansion area after the venturi to help pull through more cfm at higher air speeds. We can utilize the highest air speed flow area of the carburetor, at part throttle, to help pull in the high speed circuit earlier for better response around turns.

fig1-x.gif (8163 bytes) fig2-x.gif (8390 bytes) fig3-x.gif (7171 bytes)
The three pictures above show air speed at 1/4 throttle opening. Here the yellow area shows
air flow at lower speeds. At these throttle openings, the throttle plate is covering most of the
carburetor bore. Air is flowing from the sides, down the sides. The orange area shows airflow
at higher engine speeds. Note how the air filter top contributes to the air flow pattern.

What we came up with was a carburetor that more precisely matches the air-fuel curve requirements of a high performance engine. The BlackJack Carburetor will offer you more peak power with a broader power curve, while maintaining excellent throttle response and the best race course ride-ability and throttle control in competition. What this means is you have instant throttle and power control at any time, anywhere around the race track. No more feathering of the throttle to reduce hesitations or bogging around turns.

Peak horsepower is very important if you are consistently running at high speed - wide open at or near your peak horsepower rpm. Here your engine needs the maximum amount of air and fuel that it can use. If you are closed course racing, peak horsepower is not quite as important. The figures just before and after peak horsepower and peak torque, are more important. It is these numbers that get you from point "A" to point "B" in the least amount of time. This type of broader power band is also helpful in choppy water conditions. It allows the engine to be closer to it's peak torque rpm. When a boat re-enters the water in rough water conditions, it can re-accelerate quicker and this maintains higher speeds in choppy water conditions.

Numerous racers, national champions and even factory backed teams have taken notice of the BlackJack Carburetors ability to perform. One particular factory team manager was quoted as saying that he could not believe how well the boat responded around the turns and how responsive it was at instant 1/4 throttle openings.. We must note that this team had every conceivable aftermarket carburetor available in it's inventory, including the pretty billet ones, and had tried them all. This particular engine was a triple pipe engine of only 261 cc per cylinder, yet was performing excellent with the 46mm BlackJack Carburetors.

Some of our best testimonials for the BlackJack Carburetors are from racers and tuners who have actually compared them, side-by-side, to the so-called best carburetors in the industry. After testing, they have consistently chosen BlackJack Carburetors over the others.

Since it's early prototype in 1994 to the current production model, including our 46 and 48mm H.S.S. models, we have stayed ahead of our competition. One way we have done this is by our customers sending us our competitors carburetors to test. To date, we have tested 8 different types of carburetors, even some from companies in foreign countries. Some resemble ours. To the untrained eye, they may appear almost identical. Flow benches, vacuum gauges, velocity probes and smoke streams don't lie, only people lie. We believe that testing shows we have the best carburetors in the business. The more time goes by, more and more people agree. Distributors such as Western Power Sports, Parts Unlimited, Tucker Rocky and White Brothers have all bought and sold the BlackJack Carburetor.

Email