Jet Flow Test Results

modela-carb

modela-carb

Member
This may get a little long but I think it is interesting & may even be worthwhile.

I flow tested a group of 47 cap jets today and decided to record my findings. After all, how many Model A'ers can get their hands on this many jets at one time.

Disclaimer #1 - There are almost as many jet flow rate ranges as there are carburetor rebuilders. My range is closer to Chris Pelikan's, 170-190 for the cap jet.
Disclaimer #2 - I have labeled "original" jets as those with a number stamp and "Repro" jets as those with no stamp. I believe the "original" jets are a combination of original and OEM made between 1928 and some unknown later date. The unmarked jets are more modern. This is a purely arbitrary distinction and I have no evidence to support it.

The cap jet provides fuel at low speeds - between idle and about 25 mph. The range is larger than the other jets, but the critical factor is that it must have a higher flow than the compensating (comp) jet which meters fuel to the cap jet. The cap jet also receives fuel via the GAV.

Total jets Tested - 47 (25 orig, 22, repro)

Below Range 16 (4 orig - 8%, 12 repro - 26%)

In Range 21 (14 orig - 30%, 7 repro - 15%)

Over Range 10 (7 orig - 15%, 3 repro - 6%)

Total Low 16 - 34%
Total in Range 21 - 45%
Total Over Range 10 - 21%

Percent of Originals in range 56%
Percent of Repros in range 32%

These jets came from carbs & parts I have bought over the years and were randomly picked, so I'll go out on a limb and say they represent a fair picture of what is on the Model A's the Ford Barners are driving.
I can only conclude that:
1. There are a lot of cars with incorrect jets
2. It's not a good idea to buy & install any jets without testing them.

There are probably several themes for discussion in here and my only purpose is to share my findings...even if they don't add to the knowledge pool.
 
I have told several of the online vendors that there is a market for "Flow Tested" jets.
 
Jim,

Thanks for sharing. I found very similar information when testing jets a few years ago.

I have a Sears Marvel carb over 60 years old from my 1930 Coupe that produced 28 mpg at 45 mph all day for 12 continuous years.

It looks 60 years old. I never tried to clean it & I will never try to clean it.

Millions of vintage car jets were ruined by owners & jack leg mechanics improperly cleaning jets with steel wires.
 
Jim Parker Toronto said:
modela-carb, when you rebuild a carb, do you install it on an engine and road test it for any length of time? Jim
Click to expand...


The only real test is to install on an engine and run for at least a tank or 2 of gas, this way you can test of leaks after a while and test your fuel mixture The flow rate is essential to the carb for better gas mileage, We are testing some jets right now that are all original and have been able to improve gas mileage from 14.5 up to 17.5 right now and are shooting for 20 mph before we are done by just adjusting flow rates, jet sizes and working with the GAV...will report back again..
 
Mark, I have a carb on a B engine in my 4 door that I was getting 27 MPG, Canadian, but feel I could do better. I also have a couple of B carbs that I had rebuilt here locally, and am not at all pleased with their performance. I am just looking for someone to go over these carbs, that really knows what they are doing, and see if there is something wrong with them. Jim
 
Jim interesting however i know most of us are running a "A" engine with Zenith carb...That is what we have been working with, I dont think we have the time to look at others but tell me what are you not pleased with and how does the MPG compare to US
mark
 
Jim Parker Toronto said:
modela-carb, when you rebuild a carb, do you install it on an engine and road test it for any length of time? Jim
Click to expand...
Jim,
I do a static bench test for leaks, but no longer road test. All of my rebuilds have tested & correct jets, new float valves & tested floats and the work is guaranteed. As Mark pointed out, only an extensive road test can tell you how to set a carb up for your individual preference & performance. BUT if you start with correct jet & float settings & adjust the idle & throttle to suit your car when installing the carb, you're going to get very good performance.

If you want better gas mileage or higher performance than standard sized jets provide, then you can install & test different jets until you get it where you want it.

I will point out that millions of carburetors were installed on Model A's at the factories and the only road test they got was driving from the end of the assembly line to the parking lot....Same way it's done today.
 
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