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willysmjeeps.com :: View topic - vacuum gage hook up
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vacuum gage hook up

 
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gunnerb17
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Joined: May 09, 2005
Posts: 24

PostPosted: Sat Nov 03, 2018 7:29 pm    Post subject: vacuum gage hook up Reply with quote

When using the vacuum gage on a M38.Should the suction(vacuum) line from the vacuum pump be disconnected ? If so would this be at the brass tee on the intake manifold nearest the firewall? How much difference--if any would there be if it was not disconnected?THANKS IN ADVANCE
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mdainsd
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Joined: Oct 20, 2017
Posts: 199

PostPosted: Sat Nov 03, 2018 9:20 pm    Post subject: Reply with quote

as long as the input side of the vacuum section of the pump is not open to air it will be fine to leave it in place.
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2004 M1025, 1952 Dodge M37, 1968 Dodge M615 export ambulance, 1942 MB script, 1948 FrankenJeep CJ2A/M38, 1951 M38, !942 WC-57 Command Car, 1944 WC-51 Weapons Carrier, (2) M1941 Sperry 60" Anti-Aircraft Searchlight, John Deere M-gators.
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Jeff_Lee
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Joined: Jan 27, 2014
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Location: West Palm Beach FL area

PostPosted: Tue Nov 06, 2018 11:10 am    Post subject: Reply with quote

I have pondered this same question. Does the air/fuel mixture change when you turn on the vacuum wipers?
Just thinking out loud... isn't the "input side" always open.... even if the wiper suction needle valve is closed?
Consider the path from the wiper T to the distributor and the dizzy to the air cleaner.

That said, wouldn't you want to measure vacuum with the engine in the state in which it is "normally" operating?

This should smoke out some of our vacuum diagnostic experts on the forum into this thread.
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mdainsd
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PostPosted: Tue Nov 06, 2018 12:22 pm    Post subject: Reply with quote

Yes the fuel air ratio changes, but insignificantly. The displacement of the wiper motors is about 2 cubic inches each per stoke and slow. But the engine is using thousands of cubic inches or air per minute, so the difference is so far down in the noise that it matters not.

The "input" side of the vacuum pump is closed effectively when the wipers are off.

Wiper Tee to the distributor? the Tee on the air cleaner supplies clean air to the distributor which is drawn through into the cross over pipe. the other side of that tee goes to the wipers so that when they are operating they are getting clean dry air to replace the air that is being vacuumed out the other side to the vacuum pump during operation.
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2004 M1025, 1952 Dodge M37, 1968 Dodge M615 export ambulance, 1942 MB script, 1948 FrankenJeep CJ2A/M38, 1951 M38, !942 WC-57 Command Car, 1944 WC-51 Weapons Carrier, (2) M1941 Sperry 60" Anti-Aircraft Searchlight, John Deere M-gators.
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