Hot Rod Forum banner

Thermo-Quad Idle Circuit Function/Duration

120 2
Hello. Could someone with Thermo-Quad knowledge please advise if;

1. Thermo-Quad Idle Circuits supply fuel only during idle.
or
2. Thermo-Quad Idle Circuits supply fuel throughout idle and early part throttle operation.
and
3. Does it cease to supply fuel once the High-Speed initiates?

In other words....are all the circuits cumulative? Yes or No?


Thank you.
1 - 3 of 3 Posts

· Registered
Joined
·
16,303 Posts
Assuming the cam is mild thus the curb idle is achieved without more throttle position that doesn’t expose the transition slot more than .040 inch.Beyond that both the idle circuit and some of the transition circuit will supply fuel as they are ends of the same circuit. This circuit mostly is dependent on manifold vacuum. As the throttle opens manifold vacuum decreases this shuts off the idle flow due to insufficient force at the outlet later transition flow as well.

There is a transition period established by how much the transition slot is exposed to manifold vacuum as throttle blades pass the slot. Then as the throttle blades move to a more open position the vacuum on the slot decreases till reduced to where the slot stops feeding fuel. In concert with this as the throttle blades continue to open the flow velocity in the boost venturi picks up. Here a venturi functions to the reverse of the manifold in that as flow velocity demanded by the opening throttle and resultant increase in engine speed demanding more air flow the pressure decreases inside the venturi essentially forming a local vacuum source. This first brings on the boost venturi and finally as the throttle blades further open the main venturi flow deepens the local vacuum feeding more fuel.

Initial fuel flow on the venturis is controlled by the main jets. Flow is proportional to the vacuum applied to the feed passages and the size of the jets. Jetting covers an air flow range that has an upper and lower limit so one size cannot service all conditions from off idle and through transition to Wide Open Throttle (WOT). Approaching WOT more fuel than main jetting can provide if reasonable cruise mileage is expected, then as manifold vacuum further falls with throttle opening a power enrichment circuit is switch on. This is essentially a spring loaded valve held closed by manifold vacuum.

In the case of the Thermoquad as with the Carter now Edelbrock AFB and AVS, also the Rochester/Edelbrock QJet, this function is a metering rod that has stepped diameters passing through the main primary jets or secondary jets as well for the QJet. With the Holley and it’s many clones, the Ford/Summit these use a diaphragm valve held closed by manifold vacuum against a spring. In all of these the switch on point is controlled by spring strength selection. As I said with the Carter/Edelbrock/QJet these control fuel flow with main jet sizing and metering rod size selection. For the Holley/Ford and their clones this is controlled by various diaphragm valves with different strength springs and specific internal jets of the carb that augment main jet flow.

This a rough over view, there are also trimming circuits that are essentially bubblers that add or not air to the fuel flows to further refine fuel volume to airflow through the throttle blades. Air correction is found on every circuit from idle to main metering.

Big cams get to be problematic especially at idle because they need to idle fast and their manifold vacuum is low. This typically pushes the tuner to open the curb idle position of the throttle blades to where too much if not all the transition slots are exposed. This results in an overly rich idle where the idle fuel screws have little to no authority over the idle mixture and the engine has a lean stumble between idle and main metering. The answer here is an alternate air source other than jacking the throttle blades too open. solutions here range from four corner idle circuits to drilling small holes in the primary throttle blades to using aquarium air control valves to leak air into the intake at a vacuum port. Some older Carter’s had a screw controlled idle air port some speciality Holley’s include this feature under the air cleaner stud.

Bogie
 
1 - 3 of 3 Posts
Top