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We at JETWORKS have been modifying carburetors for watercraft since 1983. Some of our early carburetor modifications were for the Mikuni 38mm round body. It consisted of a taper bored top with a true 38mm venturi utilizing a thin tube atomizer. The carburetors worked very well and the response was exceptionally crisp considering we eliminated the outer venturi and the inner venturi. They were very popular among 440 super stocks because the IJSBA rules allowed any internal modifications as long as you maintained a 38mm throttle bore. In 1986 the 44mm Mikuni BN round body was introduced on the JS 550. We attempted to do the same mods as we had done earlier to the 38mm. We found out we could not remove the outer venturi without losing considerable response when we utilized a thin tube atomizer. The carbs worked great at top end but were not consistent enough to satisfy our high level of standards.

We came to the conclusion that the air speed and signal strength through the modified carburetor were much too low in the larger 44mm carburetor even when the outer venturi diameter was not increased. In 1990 we tried the same mods on the newer Super BN 44. It wasn't until Mikuni made available different size needle and seats and springs along with different size jets to choose from that allowed the Super BN to be modified in the same similar way. After many hours of testing, we decided we could get them to work fairly good. The bottom end response was good but again it was not up to our high level of standards. We had to run them at lower pop off pressures than we liked to. What we had to do was one of two things, increase the signal strength and decrease signal rise time or fit an accelerator pump to perfectly tune the transition using a higher pop of pressure. This is when we decided to come up with a different setup, which eventually evolved into the BlackJack Carburetor of today.

By us choosing to use the annular discharge on the BlackJack instead of a thin tube or modified booster venturi doesn't mean these other methods of high speed fuel discharge are not good, it's just that we found the best overall results with the annular discharge outer venturi. This also eliminated the need to fix or install an accelerator pump to the system.

A lot of late model watercraft are using the Mikuni I Series large bore carburetors. These are 44mm or 46mm. They feature an accelerator pump circuit and were designed primarily as an emission reducing carburetor. By using the accelerator pump the engineers were able to lean out the lower speeds and reduce emissions, yet prevent any hesitations by using the accelerator pump as an additional tuning aid.

Some of the models that use these types of carburetors are: Sea Doo 951, Yamaha GP800, GPR800, XLL1200 and GPR1200.

These carburetors were designed to function with the OEM air filter assembly. This means the low speed circuitry, jetting, pop off pressure are all tailored or designed to work with the OEM air filter system.

WE HAVE BEEN DOING extensive testing on different types of carburetor modifications for these specific carburetors. Since these carburetors were designed from the beginning as an accelerator pump carburetor, the number and placement of the off idle transition fuel ports, the angle and degree of the throttle plate in relation to the throttle bore and how it affects the off idle transition fuel ports, plus the fact that they were all designed to work with the added fuel discharge from the accelerator pump means modifying these late model big bore I series carburetors is quite different than modifying the traditional Super BN carburetors.

If we effectively modify the existing carburetors that come with these late model watercraft, we would be saving the cost of a new carburetor. The cost of a new manifold and any linkage, extra fuel lines and the pulse line problems.

We have received calls from customers wanting to modify these types of carburetors. Some have called extremely concerned if not down right scared to modify their carburetors. This is based on what they have been told or have read somewhere. Their major concern is a well talked about lean spot at 3/4 to 7/8 throttle opening. From our testing, this lean out condition is usually apparent when the carburetors are not properly jetted and the craft has a little more than simple basic modifications. Air filter changes, increased compression, a taller impeller, dry pipe kits or aftermarket exhaust kits, cylinder porting etc. All of these require the engine to run with a much narrower margin for error. Octane requirements, jetting and temperature become much more critical.

This is what really happens at 3/4 to 7/8 throttle opening when you are feeding a single cylinder per carburetor with a large bore carburetor such as the I series that uses an accelerator pump. A few things happen when you back off the throttle from full throttle. You drastically slow down the air velocity through the carburetor and since it is of a larger bore size and the required amount of air needed to sustain these somewhat higher speeds is easily passed around the partially closed throttle plate. What really happens is you block the engines suction vacuum from reaching the booster venturi which relies mostly on air velocity to increase signal. Since the booster venturi is of a blunt design.

The slower air velocity caused by the partially closed throttle plate, which is also forcing the incoming air towards the sides of the carburetor, decreases the vacuum signal through the booster venturi which in turn reduces the amount of fuel and in some instances basically closes the high speed check valve. What you end up with is a relatively small slow speed jet unable to supply enough fuel for the amount of air being flowed at these throttle settings. We must remember that because of the accelerator pump discharge, we must run slow speed jets much smaller than would be needed to supply the needed fuel at these throttle settings. The reason this is not a problem on stock engines fitted with the OEM spark arresters is the increased resistance imposed by the stock unit creates a slight vacuum which helps pull in more fuel from all the circuits. Increasing the slow jet helps to a certain degree, but you can't go too big before you run into a rich condition when accelerating. If you remove the accelerator pump, you must run lower pop pressures and much larger slow jets to not hesitate under acceleration. This might work for stock carburetors, but if you modify the carburetor for more air flow you must somehow improve or, or at least not decrease, the signal strength or lessen the ability to help draw the fuel In through the high speed circuit especially at these 3/4 to 7/8 throttle openings. The law of physics says that the sir speeds will be slower, and since the booster venturi is more speed dependent than the slow speed circuit, it will suffer most. As we have stated elsewhere, at part throttle cruise, the majority of the air is flowing near the outside walls of the carburetor bore. This is due to the fact that the throttle plate is blocking the air from going through the center of the carburetor, also by the fact that most air filters don't flow from the top but rather from the sides. Now you can cover part of the booster venturi but this doesn't make it any less air speed dependent, just more restrictive to air flow. Covering part of the booster shows improved signal when tested on a flow bench at high test pressures, mainly because the throttle is wide open and there is no air filter to deflect the air flow. Air enters straight into the booster. Now, on this same carburetor, if you put on a spark arrester and close the throttle slightly it is obvious the signal strength is going to be effected much more than on a carburetor which discharges it's fuel from the sides where the air flow is still taking place, even with the partially closed throttle plate.

At Jetworks, we feel that the R&D time we spend on these types of carburetors was well spent. We don't look at the large bore I series carburetors as the carburetor with problems, or the one that can't be modified because of it's design. Instead of scaring people from modifying their craft or carburetors because of a problem, we find solutions to the problem. 15 years of carburetor modifications flow bench experience with velocity probes, vacuum probe and signal strength graph plotting data have all taught us a lot throughout the years. We are proud to offer our expertise in this field along with our other products and services. We offer knowledgeable technical assistance to our customers. Feel free to contact us with any questions before and after your purchases.

As always, we explain to you why our carburetors are superior instead of telling you we are the best, and that's the way it is and, don't ask me why, you will hear words like 'revolutionary breakthrough', 'we are the center of technology' and patent still pending. Along with 'explosive', 'extreme', 'best' and 'awesome'. One thing you can be certain is that every racer running our carburetors has bought them instead of giving some carburetors away to a few key racers and then giving little or no support to most pros, experts and novices. We understand that it's the novices and experts of today that will be the pros of tomorrow. This is why we help and sponsor in a more even and unbiased manner.

The BlackJack SFI (Super Flow I series) and BlackJack SFI Max were designed to be a high performance carburetor from the start. Emphasis was put on air entry geometry and how it would help instead of delaying the high speed circuits' fuel flow by using a larger diameter top with a radius at the largest and highest point of the billet top then achieving a taper angle over a longer area which gradually reduces to a true actual venturi. We are able to achieve a substantial increase in air velocity by literally compressing and accelerating the air through this tapered area. The fuel for the high speed circuit is discharged at this actual venturi area. This area has the highest air velocity of the carburetor. Right after the fuel is discharged the area opens up again to the actual bore of the throttle plate. This air expansion is what helps pull more air in from the top. By removing the accelerator pump circuitry, modifying the low speed circuit and relocating the high speed discharge point lower we are able to make better use of the much stronger engine suction (vacuum) to help initiate the high speeds circuit earlier and greatly reduce the signal rise time. This creates a clean, crisp, instant throttle response along with a very linear and precise fuel metering.

The SFI retains the same throttle bore size as the original. The SFI Max is bored to a throttle bore of 48mm. The SFI Max also uses a much larger outer venturi. The SFI Max will require enlarging, or matching, of the intake manifold, spacer, and possibly matching of the reed cage to spacer or manifold. The SFI and SFI Max do not require extra or additional fuel pumps on the Yamaha GPR, even on heavily modified race boats using triple pipes.

Modification Prices
44mm & 46mm SFI Mod	$149.95 each (includes billet top)
See T-Bar Adjuster Screws	Price does not include T-Bar adjuster screws