[Tom Vaught]

Today going to post a bit about Holley WET FLOW TESTING, Holley's Wet Flow Bench, and A bit more on why the Main Jets could be removed on the Main Circuit and still not affect the Idle Fuel Mixture much, in some cases. Opening the Power Valve (blocked signal that would normally keep it closed might have more of an Idle effect).

What does "100% wet tested" mean?

Wet tested means the carburetor is tested with air and fuel flowing to simulate actual engine operation. The test stand is comprised of a large vacuum pump and fuel supply. A fuel line is connected to the carburetor using mineral spirits at 6 psi (typical). The test stand is instrumented to measure fuel flow and air flow. The vacuum pump is turned on, and air and fuel will flow through the carburetor in the same way as on the engine. From this the A/F ratio can be calculated. The idle airflow and fuel flow is adjusted to spec, then the A/F ratios at different throttle positions representing off-idle part throttle and full throttle are measured. If the flows do not meet specification, the carburetor will be inspected and parts replaced as necessary to make sure it is correct.

Holley has a Wet Test Stand that has a 6000 cfm flow capability. (originally used for WW-II LARGE Aircraft Engine Carburetor Testing).

The Stand has special Radiused Inlet Orifices, only one used at a time, that controls the 100% air flow capability for that Flow Test.
The air goes thru the "Orifice", then thru large piping to a Special "Test Box" that can be sealed up so there is no air leakage. It can also be raised up to allow mounting a carb inside the bench on the flow fixture, hooking up the fuel line to the carb, and the throttle linkage.
The "wet stand" uses Stoddard Solvent, a much safer flow liquid vs gasoline to simulate the gasoline in a normal engine. It has a similar Specific Gravity. So air goes Thru the orifice, into the pressure box, into the carb, out of the carb, and after fuel is removed back to the atmosphere. The throttle changes the air flow read by the calibrated flow orifice.
(From the carburetor Flow Box the air/fuel mixture goes to a fuel separator where the Stoddard Solvent is reclaimed for use in another test. The air is cleaned and passed back into the atmosphere). So no air pollution.

The carb is flowed at multiple test points. Graphs are created for each carb circuit and test points recorded with the measured data.

Topic #2

The Idle Circuit and the Main Circuit.

Because the IFR (Idle Feed Restriction) is connected to the Main Well, fuel flow through the IFR will be affected by the fuel level in the Main Well. Think of it as the Idle and Main systems playing tug of war.

At idle, the Idle system draws fuel from the Main Well. Since the Main Jet offers some restriction, the fuel level in the Main Well drops.

This results in a lower head of fuel (height of the fuel above the IFR). If the Power Valve is open, it’s just like opening up the Main Jet. This lower restriction raises Main Well fuel level and the Idle air/fuel ratio becomes richer.

It is by design that the Idle Circuit feeds from the Main Well. With the Idle System pulling the Main Well level down, a higher airflow is required to get the Main System started. This helps prevent the Main Nozzle (boost venturi) from dripping or starting prematurely. (If you have the fuel level too high in the Fuel Bowl you can get Nozzle Drip too). As the throttle is opened, the Venturi Signal increases, and the Idle Signal eventually DECREASES, (but does not stop) so there is a smooth transition to the Main System. When the throttle is opened suddenly, the Idle System drops out (stops) because it is sensitive to Vacuum, not Airflow. This raises the fuel level in the Main Well and the Main System will now start faster to compensate for the WEAKER Idle Signal.

You can observe this effect - with the vehicle in neutral, open the throttle with the idle speed screw slowly until you see fuel flow just beginning to discharge from the Booster. Holding the Throttle at this position, open the Mixture Screw and the Nozzle should stop.

This is common Carburetor Theory that is taught to every Carb Engineer at Holley so you could hear the same exact explanation from different Engineers but the Holley Carb principals will not change.

Tom V.