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EFV-8 Club Forum / General Ford Discussion / what size main jets?

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Discussion Topic: what size main jets?

buford8

09-30-2012 @ 5:18 PM

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I want to know what the standard size is for main jets in a 1940 Ford 91-99 carburetor is. Any help will be appreciated...thanks, Karl.

You mess with me, you mess with the whole trailer park!

TomO

10-01-2012 @ 7:23 AM

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.050

Tom

supereal

10-01-2012 @ 10:04 AM

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Below 5,000 ft altitude: .050. 5,000-10,000 ft: .048 Over 15,000 ft (rare): .046. Unless you live in a place like Denver, the .050 is the "standard" size. It is not advisable to drill jets to size, as the passage surface is critical to proper operation.

Stroker

10-01-2012 @ 4:19 PM

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Karl: I noticed that your beautiful Ford has dual exhausts. Changing the exhaust system can affect the fuel mixture, as reduced back pressure tends to make engines run leaner. Rather than
simply adhering to an average jet size, consider doing a simple diagnostic test. Drive the car hard in high gear, push in the clutch, and then coast to a stop and look at the plugs. There are many plug reading charts available on the net. If the plugs are "sooty", you are running an overly rich mixture. If they are grey/brown you are spot-on. White plugs would indicate an overly lean condition. Super is right regarding drilling main jets, but you should be able to source intermediate sized jets for a 94 from Edelbrock, and jets for a 48-97 from www.stromberg-97.com. If you do decide to drill jets, just remember that the "probable hole size" will be slightly larger than the number size drill.

buford8

10-01-2012 @ 5:28 PM

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Thanks for all the input...I have noticed that .048 jets are actually closer to .043 measured with a wire gauge...it seems that most jets I have measured come a lot smaller than the number stamped on the jet. I have no intention of drilling the jets because I no that the factory holes are very precise and I can't be sure of my own ability.
I live at 4600' elevation and have put .048 jets in my carb...it runs lean and stumbles when holding a steady speed. I have installed .050 jets today and it seems a lot better.
Will keep trying to find the right jets to get this thing running perfect...

Also I forgot to mention that while the engine appears stock it is bored and stroked to 276" (3 5/16" bore X 4" crank)
Again thanks for the input...Buford8

You mess with me, you mess with the whole trailer park!

TomO

10-03-2012 @ 9:27 AM

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Before changing to larger jets, try a fuel delivery test. You may have a partially clogged fuel line.

To check fuel delivery, you need a graduated 16 ounce or larger container to determine the amount of fuel delivered in a measured time. Disconnect the fuel pump line from the carburetor and direct it into a graduated container. Crank the engine long enough to get 2 ounces of gas in the container and then continue cranking for 15 more strokes of the pump. The pump should have delivered 6 more ounces of gas.

Tom

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