FlowTesting Jets --- HELP

[170bdlx]

I have tested over 30 jets and the flow rates seem high. My flow tester is made with 3/4 PVC, 36 inch column, vented at top, fed with a fountain pump in a bucket. Retesting jets give the same result.

16 Cap Jets 160/220. 6 over 180

7 Idle Jets 55/190. 3 new ones 55,60,65.

Soldered an Idle Jet and drilled with #76 and got a 75 flow rate.

Any Ideas on what I am doing wrong?

[SAJ]

Are you testing flow in the same direction as the fuel flows? It is tempting to shove the threaded end into a bit of poly tubing. But the flow is from the slotted end. The flow direction makes quite a bit of difference. I can't recall exactly without looking up my results which are at my workshop. But I think I got higher flows testing in the wrong direction before I discovered my error.
SAJ. in NZ

[MikeK]

Make sure you are actually delivering 36.0" water column by attaching a clear tube as a manometer at the test fitting connection. The water in the clear tube should stop EXACTLY 36" above the point where your jet orifice normally would be. It is easy to get too much pressure if you have a pump the forcefully expels water from any type of overflow at the top of the 36" column.

[Gary WA]

Originally Posted by MikeK

Make sure you are actually delivering 36.0" water column by attaching a clear tube as a manometer at the test fitting connection. The water in the clear tube should stop EXACTLY 36" above the point where your jet orifice normally would be. It is easy to get too much pressure if you have a pump the forcefully expels water from any type of overflow at the top of the 36" column.

no pump should be needed!? Correct? that may be the problem.

[Pete]

If you are trying to get a certain flow from a jet, it will be hit and miss with drills.
Hand drilled, the hole will be bell mouthed and out of round.
Machine drilled, it will be out of round.
A 2 flute drill will not drill a round hole.
For accurate jet holes, they need to be machine reamed.
If you are talking super accuracy, soldered up and resized jets will not be the same size as brass reamed with the same reamer.

For most backyard model A work even a dull drill will get by.

[Ron/IA]

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I have a document from a workshop on flow testing and adjusting Zenith carb jets. It is too big to attached to this reply; I will PM you my email address, and you can reply. To your reply I can attach said document.

You can compare notes between what you are doing, your flow tester, and how to drill the jets; see if there is anything different.

I hope it helps.

[170bdlx]

Thanks for all the ideas.

I will start with smaller drills.

I am testing in the right flow direction, thanks to an article I found.

I will check the water column. I have jets attached with a small short hose and may be pointing it down making the water column longer.

Gary W - Not sure about the pump thing. It fills the column from the bottom where some use a water hose.

Back to the drawing board

[160B]

Here is a photo of my flow test equipment.

It has a submersible pump and a throttle valve so you can regulate the flow so you get a 36 inch column from C/L of the overflow tee to the C/L of the outlet tee.

Make sure that your outlet connection and jet being tested in not lower or higher than the C/L of the outlet tee; otherwise you are changing the water column height which will result in a higher or lower pressure.

Pump pressure has nothing to do with the column pressure as long as you a good overflow vent and you are not creating a vacuum at the overflow and the water level at the overflow is at the centerline of the overflow tee.

[MikeK]

Originally Posted by Gary WA

no pump should be needed!? Correct? that may be the problem.

That would depend on the design of your test setup. Unless you want to be often bothered by refilling some elevated reservoir, a pressurized supply is nice. The setup I have uses feed from the city water main, essentially the same as using a pump and a bucket. Design flaws creating a head pressure on the overflow will cause errors.

Irrespective of design, it is essential the pressure remains as needed at the connection point while the jet is flowing. It is also essential the jet orifices are always at the same vertical point, level with the point at which the pressure is verified by manommeter. It does no good to have a 36" column then attach a 6" convenience hose at the bottom to attach jets. If the jet orifice is moved up or down to fill a container, the actual pressure will not be 36" WC.

[RonC]

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