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EFV-8 Club Forum / General Ford Discussion / Engine RPM

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Discussion Topic: Engine RPM

silverchief

07-07-2026 @ 3:16 PM

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How are twin screws at base of 94 carb on my 47 Ford sedan used to adjust idle?

carcrazy

07-07-2026 @ 3:27 PM

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Those two screws are the idle mixture adjustment screws. They control the Fuel/Air ratio of the air flowing into the engine. To richen the F/A ratio turn the screws counter clockwise and to lean the mixture, turn them clockwise.

If you want to increase the idle RPM speed, you want to change the position of the idle set screw which is located on the side of the carburetor where it contacts a lever connected to the throttle shaft. Turn this screw clockwise to increase idle speed and counter clockwise to reduce it.

ford38v8

07-07-2026 @ 3:35 PM

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In conjunction with the throttle screw adjustment screw, keep idle at 500 RPM throughout entire operation. Engine fully warmed to operating temp, both screws at base of carb scr*w*d out 1 ½” turns. Working with one screw at a time, screw in slowly till engine begins to lope. Screw out slowly, just enough to smooth out again. Do the same with the other screw. Repeat both once more. That should do it.
If I’m remembering wrong, someone please correct me!

Alan

marko39

07-08-2026 @ 5:32 AM

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you can also use a vacuum gauge to set these, adjust till highest reading is obtained.

silverchief

07-08-2026 @ 10:30 AM

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Thanks very much goys. I owe you.

Drbrown

07-15-2026 @ 8:01 AM

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I also recommend getting/having a vacumm gage as it can indicate other concerns such as a sticky valve, manifold leak, etc. The one I have can also test fuel pump pressure.

carcrazy

07-15-2026 @ 9:50 AM

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A vacuum gauge is probably the most important diagnostic tool you can have. You can easily use it to determine the health of the engine.

It is easy to use, simply connect it to the port on the intake manifold that feeds vacuum to the windshield wipers. If the engine shows a steady intake manifold vacuum reading of about 19 inches at idle speed, the engine is operating properly.

The next item you want to have to diagnose problems with an engine is a compression gauge.

To use the compression gauge you will have to remove all of the sparkplugs. Insert the hose from this gauge into one of the spark plug holes and with the throttle plates and choke plate held wide open turn the engine over for 6 complete revolutions. The reading you obtain should be between 100 and 125 PSI. For our stock Flathead this would indicate a healthy cylinder.

Repeat this procedure for each of the remaining cylinders. If all of the cylinders show pressures that are withing 10% of each other and are within this range, your engine is mechanically sound.

51woodie

07-17-2026 @ 3:09 AM

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Here is a chart that interprets the engine vacuum gauge readings, that aid in troubleshooting engine performance issues.

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