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What are the Different Power Valves

Carburetor technical help: difference in the power valves for Motorcraft/Autolite carburetors

Power ValvesPower valves are tightened at 40-50 inch lbs.

2 Stage Valve Used on Motorcraft
Power Valves

Two stage power valves are not adjustable. The screws in the ends are there for the factory to set them. You tune a power valve by replacing one with other hg rated valves.

How to Determine Correct Two Stage Power Valve Rating

Here is an explanation I found on the internet from Web Wheeler about measuring for a power valve. I cannot attest to the accuracy.

Selecting a power valve means you need to take a vacuum reading.
The vacuum reading is best take from the BASE PLATE of the carb, where it bolts to the intake manifold, since this is the same vacuum signal the ‘Power Valve’ “Sees”…

While driving long at ‘Part Throttle Cruise’ (PTC) you get a vacuum reading.
Throttle steady, you tooling along with a STEADY THROTTLE PEDAL at about 55 MPH.

The vacuum reading should be STEADY.

If that vacuum reading is about 9 In.Hg. on a vacuum gauge,
Your power valve size should be about 2 In.Hg. BELOW that PTC signal reading.

9 In.Hg. at PTC,
The power valve should start to open around 7 In.Hg.

If your vacuum reading is 8.5 In.Hg.
Then the power valve should start to open at around 6.5 In.Hg.

With TWO STAGE power valves, you get two different fuel flow rates,
The primary should start about 1.5 In.Hg. below the PTC signal reading,
While the second stage (fully open) will come in about 1 In.Hg. below the first opening point.

The power valve is there to let you run LEANER jets at regular PTC, while the power valve adds extra fuel when you load the engine.
It’s a fuel saving device, since you don’t have to run larger jets to compensate for small loads…


Older engines will have less manifold vacuum, so power valves will need to be adjusted once in a while as your engine wears, or the ‘Perfect’ tune will be too rich as vacuum drops with wear and the power valve stays open much longer than it should.

You are probably working on a MC 2100 or 2150 carb since MC 2100 series was the only common ‘Jeep’ carb to use two stage power valves.

Updated on 05/26/2021

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