Carb Jet Flow Rates

[AL in NY]

Just finished building a carburetor jet flow tester. Water column is 36 inches. I had an unrestored "B" carburetor which had unmolested original jets in it. I flow tested those 4 jets and the flow rates aren't even close to what the Rex Reheis book says they should be.

My results: main jet = #18 and flows 134 ml/min, compensator jet = #19 and flows 176 ml/min, power jet = #18 flow 140 ml/min and the idle jet = #12 flows 91 ml/min.

Book values: main jet = #19 = 136-142 ml/min, compensator jet = #18 = 110-118 ml/min, power jet = #18 = 110-118 ml/min and the idle jet = #12 = 43-47 ml/min.

My question is: Are the flow rates the same if you flow liquid the reverse direction through the jet? If not, I don't understand the large differences in the expected values and the actual test results on original jets that have NEVER been modified. Do jets get larger through use?

[160B]

Originally Posted by AL in NY

My question is: Are the flow rates the same if you flow liquid the reverse direction through the jet? If not, I don't understand the large differences in the expected values and the actual test results on original jets that have NEVER been modified. Do jets get larger through use?

First, on two of the jets you are comparing jets with a different numbers ie Main Jet 18 to a Main jet 19, and a Comp jet 19 to a Comp jet 18. The number identifies the opening size on a jet.


The internal design of the nozzle is such the flow rates will not be the same if you reverse flow test the jet. See the below photos of some Model Carburetor jets showing the internal design. Soldering and drilling changes the entrance flow characteristics of a jet.

[AL in NY]

Just did a second flow test of the same jets, but this time, in the direction of gas flow. Only big change from the first reverse flow test was for the idle jet #12, was 91 ml/min now is 59 ml/min. The other three jets were within a few percent of the first reverse flow test numbers. Main jet (#18) was 134 ml/min now is 128ml/min. Compensator jet (#19) was 176 ml/min now is 172 ml/min. Power jet (#18) was 140 ml/min now is 148 ml/min.

Two things I don't understand here: First, the Reheis book states the main jet is #19 and the compensator and power jets are #18. In my carburetor, The compensator jet is #19 and the main and power jets are #18. Could there be a misprint in the Reheis book?

Second, if the jets don't show any signs of being tampered with when examined under a microscope, why don't they flow as stated in the Reheis book?(See first post for values) Can jets become enlarged just by normal usage?

[Bob Johnson]

There were different jets available depending on the application. Your main jet is a high altitude jet. You compensator jet #19 is not a standard size. They came as either #17 or #18. The flow rates for your compensator jet are way too high for a #19 it is likely that the hole has been opened up. See the link below for more info.

Bob

Jet Link

[AL in NY]

Bob, under microscope inspection, there are NO signs that these jets have been tampered with.

[Bob Johnson]

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Al,

Alright, they have not been altered. I do not know why the rate is incorrect. But the #19 comp jet should be replaced or modified to achieve the correct flow rate.

Bob

[Richard Lorenz]

When flow testing, the 36 inch distance must be between the top of the water level and where the water exits the jet. Some people connect the jet using a rubber tube that allows the jet to hang down which increases the height of the water column.

I use an electronic scale to weigh the water. One gram equals 1 ml or 1 cc. With the scale you can zero out the weight of the collection cup. The scale is much more accurate than reading levels in a graduated cylinder or in a measuring cup.

[Mike V. Florida]

Originally Posted by AL in NY

Do jets get larger through use?

Yes, If they did not, they would last forever, never needing to be changed.

[MikeK]

Originally Posted by 1930 coupe

...
NOS jets are too large because smaller jets are needed for modern higher octane gas. Gas in the 30's was much lower octane than current gas...

Octane rating is unrelated to needed jet size. Fuel fluidity and energy density determine the flow needed through an orifice to achieve a given air:fuel mass ratio. In the 'old days' 12.5:1 was common. Today people tend to run closer to the stoichiometric point, approximately 14.7:1 for gasoline. Octane rating merely defines the compression detonation point of the mix in a standardized test engine.

[Richard Lorenz]

Getting back to using correct measuring technique, a graduated cylinder should be dry when used. Left-over water droplets from a previous test would add to the quantity measured and give a small false high reading. Also, the height measured in a graduated cylinder should be the meniscus, which is the low area at the center. Reading the high area at the outside would again contribute to a high incorrect reading.
There is no way of telling what degree of accuracy was obtained by many of those who have reported jet flow rates in the past.
Thanks to 160B for the excellent cross-section photos. I and others who have compared measured flow rates with measured drill sizes have obtained generally very good results. Altered jets might not fit this pattern.

[AL in NY]

Richard, I too have an electronic scale which I used to calculate the volumn of a plastic 1 cup measuring cup. The vessel was slightly smaller then one cup (236.58ml) at 230ml. I just time the flow until the vessel is full then divide 230ml by the number of minutes.

MikeV, thanks. That explains why the unmolested jets flow more then they should.

[mrtexas]

Originally Posted by MikeK

Octane rating is unrelated to needed jet size. Fuel fluidity and energy density determine the flow needed through an orifice to achieve a given air:fuel mass ratio. In the 'old days' 12.5:1 was common. Today people tend to run closer to the stoichiometric point, approximately 14.7:1 for gasoline. Octane rating merely defines the compression detonation point of the mix in a standardized test engine.

Octane is simply a measure of the resistance of gasoline to ignition by compression. The only advantage to higher octane is spark can be advanced vs lower octane fuels.

[Bob Johnson]

Al,

I do not see how MikeVs post explains your jets. I think he is saying that with today's fuel you want a higher air/fuel ratio. That would suggest using a smaller jet. To do that you would use say a #15 jet instead of a #19. The flow rates for the jets should match the published rates. Correct me if I am wrong but I think you are thinking that you have an old unmolested #19 jet and since it is old it flows more fuel than a modern #19 jet which flows less because it is made for modern fuel. The size number on the jet, in this case #19, determines the size of the hole and therefore the flow rate. All #19 jets should have the same size hole regardless of when they were made.

Does this make sense Al ?

Bob

[MikeK]

Originally Posted by mrtexas

Octane is simply a measure of the resistance of gasoline to ignition by compression. The only advantage to higher octane is spark can be advanced vs lower octane fuels.

I'm confused here. You quoted me and your first sentence seems to be a simplified version of my last. Did I miss something?

[RonC]

http://www.zenithfuelsystems.com/doc...istory2009.pdf

[AL in NY]

Bob Johnson, I think you looked at MikeK's reply, not Mike V 's. Mike V answered my question as to why my unmolested jets flow more then they should................they get larger over time from use.