Flow testing a carb

Russell Reay

Russell Reay

Well-known member
My car came with a Tillotson, and it ran fine. A friend gave me a Zenith 2 that had sat on a shelf for at least 20 yrs, so I rebuilt it with a kit, including new jets, and it seems to run fine. Many threads about carb performance recommend flow testing the jets--how is that done?
 
You are basically setting it up to see how many units flow over time... just like when building an airplane. You are wanting to see if it is flowing enough for the motor.
I saw a Youtube video about it once(so you might try to find that) but however you rig it up, you run fuel through the full throttle carb and collect the results over a certain time. Not too technical. This will not measure possible resistance of the shutoff and gascolator unless included in the test. Do remember that you should set the fuel supply at a level over the carb that is the same as the tank outlet is over the carb while in the car. Better yet, do it in the car.
D
 
You can buy a device for measuring the A/F ratio in your exhaust pipe. This is probably the best way to make sure your carburetor is set up for different driving conditions. They are expensive, so it might be something that a club would buy and share with members.

Here is one company that sells the A/F meter: https://www.innovatemotorsports.com/shop-all.html?cat=25
 
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I would suggest you get a copy of Steve Pargetar's "Restoring the Zenith Carburetor." He has a few paragraphs on how to build the tester out of PVC pipe. Basically you are making a precise 36 in. column of water that has a carburetor jet mounted in it for testing. Then you record how many mini-liters of water pass through the jet in one minute using a stop watch. Ed
 
here is my flow tester made from the instructions in Steve Paraters Carb booklet.
36" tall water column made from PCV pipe. Diameter of pipe is not criticle.
Water is fed to the top with a small fish tank pump an a piece of flex plastic hose.
Carburetor jets are connected at the bottom next to the on/off valve.
The fish tank pump MUST pump more water than the carb jet will flow to maintain the constant 36 " column. Overflow water drains back int the white plastic bowl.
Sciene lab or medical beaker measure in Milliters.
Timing device is a cheep analog clock from Walmart.
Turn the valve on and at same time watch the clock for 60 seconds.
Water in the beaker will tell you the flow within 1 milliliter.


Easy as pie. Slow as Christmas and very tedious, BUT very accurate.
Until your dog comes in, jumps in your lap and you loose count or spill everything.
 

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My flow tester is very simple. Just a 1 gallon milk container connected to a rubber tube with a paper clip at the bottom. There is no need for a pump. The water level will drop less than 1/2" during the test. No need for overflow container etc. KISS Keep It Simple Stupid.

Bob

flowtest01.jpg
 
Swapping out jets to get a well running car might just be easier and take less time than flow testing them.
 
Gene F said:
Don't you only need to swap out the longest jet?
Click to expand...


Basically, yes. The main jet. That's the one that makes the most difference.

However, If any of them are plugged with crud, then any of them plugged will make a difference.
 
What I've learned about flow testing jets for the Zenith.
If the engine starts easy,runs smooth then leave it alone.
Use original jets that are marked and use Plug readings to determine if your running Lean or Rich.
Remember it's a Model a running on today's gas.....keep it simple.
 
I think flow testing a carburetor is different than flow testing jets that go into the carburetor. When a carburetor is flow tested it is put on a bench that measure air flow and fuel flow (or water if the difference in viscosity is taken into account). There is a vacuum system to suck the air through the carburetor. The air/fuel ratio is measured at different air flow rates and throttle settings and the pressure drop across the carburetor is also measured.

The jets in my Weber are stamped with the diameter of the hole. I measured the diameters with a set of small drills and a micrometer to measure the drill shanks. I found the holes to not match the size that they were stamped with. I drilled out the jets to match the numbers. My flow testing was to insert different jets and go for a ride. People have asked me what jet sizes I use but each carburetor/manifold/fuel/location/engine is different. So I would recommend doing your own testing.
 
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Thanks for all the feedback. With all the posts recommending flow-tested carbs or jets, I didn't want to foolishly ignore something critical. As a big advocate of "if it ain't broke, don't fix it" I think I know how to proceed.
 
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