Sea Doo Exhaust Upgrade Kit 951 Q and A

Frequently asked questions - answered
for.............
Black Jack Carburetors

At JETWORKS we've been listening. Many of you have called us with questions and we have been tracking them. Here we have posted the questions and answers most commonly asked.

Q Why don't you publish flow bench figures and signal figures so we can do comparisons?
A 1 - We have decided to not publicly publish our figures because some of our competitors could use these figures and then claim a higher figure or manipulate their test figure to appear better without doing a test at all. Signal readings taken on a flow bench at high test pressures like 20 or 25 inches of water without a spark arrester can be misleading and not a reliable comparison for all aftermarket carburetor manufacturers. Obviously you can't do a signal test with an air filter installed, but an open air top at these high flow test pressures will benefit a carburetor utilizing a booster venturi. As the air comes straight down there is no filter top to deflect or redirect the flow of air into the carb so the air speeds up through the booster venturi increasing the signal figure. Since a good portion of the signal figure is based on air velocity, it would appear to be good, especially at these high flow test pressures. Now lets do a test with a simulated air filter top 4 inches in diameter and 3 inches above the carburetor. You are now asking the air to flow around the sides of the filter top and into the carburetor such that the air is not directly fed into the booster venturi. Not until enough air flows around the sides of the carburetor and it forces more air to find it's way through the booster venturi. Now install the same plate on the BLACK JACK carburetor. The simulated air filter top actually increases it's main signal strength by directing the air flow down the sides to where the fuel discharge is taking place at the outer venturi. Most of us don't have a choice about running a air filter or spark arrester, it's the law. One very important and almost always overlooked aspect of signal strength is signal rise time. How long does it take after sudden wide-open throttle acceleration to build up signal. Since the BLACK JACK carburetor flows the fuel from its' outer venturi even at off idle speeds, there is air flowing past its' high speed discharge point, so any sudden increase in air flow, even at the air filter, is instantly passes on to the outer venturi where the fuel is instantly discharged.

2- A second reason why we don't publish these figures is because we have seen airflow bench test and signal data published by other companies (some refer to themselves as being "unbiased"). Their figures don't resemble what we see when we do our tests. One test showed the CFM reading from 3 different aftermarket carburetors of the same size as flowing all within one CFM of each other. This particular test was done at 20 inches of water on a superflow 600. This information might be absorbed by most people and maybe even considered to be the final word. If we then publish something different from the pattern seen on this particular test, we might be confusing some people, upsetting some people, and maybe even risk being called lairs. Anyone who has done any flow bench testing, even on a smaller superflow 110 knows that these carb flow capacities differ by a lot more than one CFM, even at 5 inches of water let alone 20 inches of water. This particular test gives out information on main circuit and slow speed circuit signal in inches of water. Most consumers don't know or have any idea how much an inch of water is when it is used as a unit of measure for a high vacuum reading.

Q How important is signal strength to the operation of a good responding carburetor?
A Signal strength was one of the key factors in developing the BLACK JACK carburetors. This was long before all the recent talk about signal strength and carburetor comparisons. Most tests are done at wide open throttle and without air filters. You need a big flow bench and high differential pressure, like 20 to 25 inches of water, to flow large masses of air down the carburetor. If you check signal strengths at wide open throttle you will see the numbers range from the 30's to the 70's at 25 inches of water pressure. Less at 20 inches of water and a lot less at 5 or 6 inches of water, this being measured at the main circuit and using inches of water as a unit of measure. The low speed signals will range the mid 20's to the mid 30's at the same 25 inches of water. most of the variation of the pilot circuit is due to the different types of high speed fuel discharge methods being used and how the air flowing around them acts upon the low speed passages. One must remember that there is a large volume of air passing by them. One very important issue to consider is that in the actual engine operation the low speed circuitry is more influenced by the engine vacuum around the throttle plate (known as manifold pressure) and pop off pressure than is by the amount of air passing by the passages at part throttle opening. This is why more emphasis should be placed on the main circuit signal and not on the low speeds circuit.

If we check out the figures we will find out that even at the high test pressures, like 25 inches of water, we will yield over 350 cubic feet of air per minute on large bore 48mm carburetors. The signal strength measured at 53.1 inches of water, tested with an open carb top and no deflector.

How much is 53.1 inches of water? Most people don't have gauges that read inches of water so you can't compare it to something you know. Most of us have vacuum gauges that read in inches of mercury or PSI. I'm sure everyone has sucked on the end of a vacuum gauge to see how sensitive it is or to get a perception of what vacuum is. If we multiply In. H2O by .0735 we get In. Hg. or inches of mercury - something we all know. We come up with less than 4 inches of mercury, not bad for such a big carburetor and no spark arrester, but not what you expected to see. That's under two pounds per square inch. What this tells us is that these figures, however small they may be, are still very important for comparisons and research and development. This only assures us that one of the main reasons carburetors flow fuel through their circuits is because of the difference in pressure between the outside atmospheric pressure acting on the fuel and the much lower pressure inside the carburetor bore when the engine is running. This lower pressure is a combination of engine suction signal, airflow through the carburetor and spark arrester resistance, if so fitted.

Q Is it true that carburetors that use a booster venturi require a larger outer venturi?
A Yes. If you introduce an obstruction in the path of air you must increase your outer diameter to allow sufficient air flow at high speed. This is why some carburetors appear bigger than others yet don't flow more CFM. Basically it's like borrowing to pay a debt, a catch 22. The larger you make the outer venturi, the more dependent you are on air velocity through the carburetor to build up signal strength. Additionally, the higher the air flow, the more of a restriction the obstruction becomes.

If you make the outer venturi large enough you might have to cover part of your booster to increase signal or design a booster that is highly restrictive to get the job done.

A carburetor with a large outer venturi has a longer signal rise time and will rely more on a richer low speed circuit with a lower pop off, like 15, 12 or even 8 PSI, even when this same carburetor shows a strong signal under unrestricted high flow testing on a flow bench.

Go to the tech page BLACK JACK carburetor.

Email