Testing carb jet flow rates

700rpm

700rpm

Well-known member
I spent the better part of today testing all the new and used carb jets in my carburetor drawer and in my cars. The results were eye-opening to say the least. Measurements were wildly divergent, especially for the main jets, and most were not even close to specs. I selected main, cap, comp and idle jet sets that were the closest to being what they should be, and put those in each of my cars.

Of the new jets only the cap jets came close to being within tolerance. Most jets, even originals, were larger than they should be.

I'll be watching mileage and smoothness over the summer to see if there is significant improvement and will report my experience. The lesson learned is, check the flow if you can, and be wary of new jets.
 
One thing I have noted when I was reading up on flow testing, not all charts match up. There are some differences in the different charts. Some numbers are larger and smaller then maybe the next chart. I guess one would need to pick one and stay with it.
 
Ray,

I found exact same results a few years ago.

Had a gang of both old used, tarnished, & formerly tinkered with jets & also bright new jets.

Trying to guess how each would perform because of exterior appearance was about as accurate as one million individuals choosing "winning" lottery numbers.

Flow numbers was so far off they resembled my Lottery numbers after the winning numbers are published.

Some wonder why their Model A's cannot fire up on 1 or two engine revolutions, or why they get poor gas mileage -- checking & finding "good" jets is not only fun, but so beneficial.
 
Mark, I was using Pargeter's book, and you're right, it's different than what Gordon Fischer says. I was told today about an article written about 1984 that said the Zeniths were originally built for 66 octane gas, so what worked well out of the factory in 1929 is hardly relevant today.

Henry, that's funny! My coupe was starting on the first turn when warm before I put in the "correct" jets. Now it has to crank a couple times. But it was burning real rich before, and getting poor mileage. Everything's a trade-off, I guess.
 
700rpm said:
. . . I was told today about an article written about 1984 that said the Zeniths were originally built for 66 octane gas, so what worked well out of the factory in 1929 is hardly relevant today. . .
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The octane rating has no effect on the ideal stoichiometric air/fuel mass ratio. It was about 14.7:1 then, and was that when you could get 100+ octane gas in 1984.

The relevant factor is the energy content of the fuel and the mixture flow characteristics of the fuel itself, which is not the same as water flow. E10 lowers stoic slightly, and very light HC's, like butane additions to modern gas, flow at a different rate through the same orifice compared to 1929 gas. For those reasons the 'old' flow rates lose relevance, not any change in flame travel rate or detonation point (the octane rating).

To get a real picture of how carb jets perform you need to run them in a car with an air/fuel ratio meter. When you find the jet orifices that give you ideal performance you can go back and test their water flow rates to use on other carbs. Keep in mind fuel formulas vary both by geographical area and seasonally. Jet test bench water flow rates that work well in carbs used in Dallas in the summer would probably be way too rich in Maine with winter 35% butane gas. In 1929 gas was pretty much the same everywhere and any season, so factory jets worked everywhere with minor GAV adjustments. Today vehicles with primitive carbs suffer. In modern vehicles sensors and computers constantly adjust and give the illusion that all fuels are similar.
 
Ray,

(Wish just a few Model A owners would please try, (& not just read about), the experiment in the last sentence for a "real" experience on tiny air leaks one will never forget).

Anyway, so often mentioned problems & so often overlooked problems for obtaining very "quick" instant Model A starts & smooth idling are very minor air leaks such as at:

1. The intake manifold; and/or,

2. At the carburetor top & bottom mating surfaces & carburetor mating surface at the manifold, especially on slightly warped pot metal carbs; and/or,

3. At worn throttle shaft joints & worn choke shaft joints; &,

4. Vacuum wiper leaks and/or vacuum hoses/tubes leaks.

5. If not thoroughly convinced, please try getting (2) plastic straws, a glass of water & a needle or safety pin; & in one straw, pierce a very fine hole midway in this one straw; & next please witness the difference in drinking water through both straws -- now think of a thin gasoline/air mixture, or better yet borrow two cigarettes, insert a tiny pin hole midway in one cigarette & try drawing smoke through both to experience & "see" the difference with a very tiny air leak.

Hope this helps.
 
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H. L. Chauvin said:
Ray,

(Wish just a few Model A owners would please try, (& not just read about), the experiment in the last sentence for a "real" experience on tiny air leaks one will never forget).

Anyway, so often mentioned problems & so often overlooked problems for obtaining very "quick" instant Model A starts & smooth idling are very minor air leaks such as at:

1. The intake manifold; and/or,

2. At the carburetor top & bottom mating surfaces & carburetor mating surface at the manifold, especially on slightly warped pot metal carbs; and/or,

3. At worn throttle shaft joints & worn choke shaft joints; &,

4. Vacuum wiper leaks and/or vacuum hoses/tubes leaks.

5. If not thoroughly convinced, please try getting (2) plastic straws, a glass of water & a needle or safety pin; & in one straw, pierce a very fine hole midway in this one straw; & next please witness the difference in drinking water through both straws -- now think of a thin gasoline/air mixture, or better yet borrow two cigarettes, insert a tiny pin hole midway in one cigarette & try drawing smoke through both to experience & "see" the difference with a very tiny air leak.

Hope this helps.
Click to expand...

I agree with you 200%, also I have seen some "professional" rebuilt carbs that there is no way that they should work, a total mess. I agree that in a perfect world flow testing if done right for the area you live in and the gas you use will improve performance, there are many other far more important things to be done right first. If EVERY passage way is not 100% clean and has not been enlarged by someone using a drill to "clean" the passage, all gaskets in good shape....jet threads properly cleaned and restored..... etc..... you are wasting your time flow testing the jets. A good carb rebuild takes time, and a lot more than a can of carb cleaner, a bag of new gaskets and jet parts, and a can of spray paint.
 
good morning all, have read this thread with interest, as a newbie I am naturally interested in getting all I can out of my A, QUESTION how do you preform checks , what equipment do you use thanks for any reply
 
Running drill bits or welding tip cleaners through the orifices will change the flow rates greatly.
 
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