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Engine vacuum test

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I have the vacuum fuel tank operating well on my 1929 White truck. While tinkering with the windshield wipers, I tested the vacuum at the manifold. A nice, steady, 16Hg, which seems reasonable. It's a 260 cubic inch 6 cylinder. Here's my question; When I disconnect the vacuum gauge and leave the vacuum line open, the engine idle speed increases, and it actually seems to be smoother. It seems to me that doing this would make the mixture leaner, since I'm adding air to the mixture.  Yet with everything reconnected, I can't adjust the idle mixture screw to get that same result. It's an old Zenith updraft, and backing the mixture needle out will only make it so lean and that's it. Could it be that a carburetor jet needs to be modified?  Or is what I am seeing normal. In short, I wish I could get it to idle as nice with the vacuum line closed as when it is open and disconnected. 

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Are you sure that backing the screw out is what makes that carb leaner? Most go the other way.

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Amongst other things you might want to check the carb float height, too high will give you over rich,  too low the opposite.

 

Have a look at the plugs after a lengthy idle period, if it`s really too rich there will be black sooty deposits on the plugs.

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