Holley Avenger 870 orafice sizes

[1969Camaro]

I get how the choke functions, what does the intake vacuum accomplish?

[57 210]

1969camaro
I would say the vacuum which comes from a small bleed hole, stabilizes the heat buildup inside the cover.

[GTO Geoff]

The vacuum has nothing to do with heat inside the choke hsg. The vac is applied to the piston. If the driver suddenly opens the throttle, with a cold engine & choke blade nearly closed, the engine will stall from lack of air. The sudden loss of vac to the piston allows the attached linkage to open the choke blade, preventing the engine from stalling.

[tbarb]

On a typical hot air choke set up there is a small vacuum feed hole in the side of the carburetor that applies a metered vacuum signal to the choke piston. This vacuum pulls on the piston and opens choke blade approx 1/4" on cold start and that metered vacuum applied to the piston balances against the bimetallic spring to open the choke blade gradually as heat expands the spring.

The heat source on our old cars is from a tube inside the exhaust manifold that's piped to the hot air choke housing vs the electric heat source used with this hot air choke. The small vacuum hole pulls hot air into the choke housing where the bimetallic spring is located and then through the piston slots and into the engine.

[57 210]

tbarb and GTO both make sence.

[rgalajda]

The vacuum source is to actuate the choke piston pull off . This opens the choke blade slightly on start up, to lean the mixture slightly and prevent stalling. more air.

Geoff if you look at the picture , the piston is attached with a floating pin . If you rev the engine, the air flow across the choke blade will cause the choke blade to open farther. The floating pin in the piston allows this. When you let off the throttle, the choke blade will return.
Vacuum to the choke on this particular unit ( external vacuum ) being discussed is not metered. It is connected to the base of the carb below the front throttle plates and receives full vacuum being what ever the particular engine produces on high idle.
These 850 holley carbs with the large 1.56 venturi use an external vacuum source electric choke for a reason. The wall between the venturi is very thin . There is very little room to drill the passage without damage. The smaller venturi carbs have ample thickness for drilling.
The 80531 850 holley I have on my bench uses a choke blade that is notched ( cutout ) at the rear lower section of the blade.

Some of new holley carbs do not have a choke pull off piston in the electric choke. These carbs will have a choke blade with a very large cutout at the rear lower section of the blade. This is what allows sufficient air to enter on startup with nearly closed choke. On startup with fast idle the air flow across the choke blade will cause the blade to open slightly farther. This system without a choke pull off works with mixed reviews. The bean counters at holley like it.

The electric choke thermostat receives a full 12v constant. Of course there is a resistance in the thermostat. It is unusuall for a holley thermostat to burn out even with a constant feed ( key on running ).
These thermostats have a rather quick opening as compared to the ford Motorcraft thermostat, which was used on late factory installed OEM Holley carbs. Usually the Motorcraft thermostat will fit the after market Holley carb.

[rgalajda]

[57 210]

Rgalajda
Thanks for the update, good work.