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Vacuum Guage
#1
I hate asking questions like this but I have never used a vacuum gauge and it is time to learn. My buddy just gave me one. Where do I tap in to get a reading? I have an Edelbrock carb with the stock manifold on a 351 Q Code. The car is idling rough but otherwise runs well at speed. I was thinking the carb might be an issue (and it might) but it also sounds like checking vacuum is one of the basic things I should start with. Any instruction you all can give would be greatly appreciated.
Thanks.
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#2
here is a quick intro to vaccum gauges.
http://www.secondchancegarage.com/public/186.cfm

to hook one up you want to tap into full manifold vacuum on the front or rear vacuum tree off the intake manifold.
do not use ported vacuum on the carb which usually connects to the vacuum advance on the distributor.


reading a vacuum gauge can be confusing and lead you to incorrect diagnoses without more information. the gauge is another tool in your arsenal but not a 100% end of argument based on a reading you need to put it together with other information to get a detailed idea of what is causing a problem.

you would want a vacuum gauge, a compression test, dwell meter if you have points, rpm meter if you don't have a tach or to confirm if your factory tach is reading correct.
additionally you may want a fuel pressure gauge for diagnostics.

additional information comes from the spark plugs, how they look. a Timing light is used to check timing and can also find electrical misfire.

how the exhaust smells also gives you information, sweet is rich, sour or metallic taste lean.

seat of your pants tells you a lot as well.
if the car Gets up and goes then falls flat, you are too rich.
if you stomp on it and falls flat, misfires, then goes, you are too lean

so do not get all your information just from a vacuum gauge

carb problems usually come from aged internal rubber parts that have failed or ethanol gas has eaten something, you could have rust in the tank that has gotten through the filter and clogged up the carb internally.

if you find a carb is clogged with junk internally, then cleaning it is only half the problem, because it is picking up junk from the fuel tank and you must inspect the inside of the tank or it will just keep clogging the carb up.
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#3
72HCODE;196561 Wrote:here is a quick intro to vaccum gauges.
http://www.secondchancegarage.com/public/186.cfm

to hook one up you want to tap into full manifold vacuum on the front or rear vacuum tree off the intake manifold.
do not use ported vacuum on the carb which usually connects to the vacuum advance on the distributor.


reading a vacuum gauge can be confusing and lead you to incorrect diagnoses without more information. the gauge is another tool in your arsenal but not a 100% end of argument based on a reading you need to put it together with other information to get a detailed idea of what is causing a problem.

you would want a vacuum gauge, a compression test, dwell meter if you have points, rpm meter if you don't have a tach or to confirm if your factory tach is reading correct.
additionally you may want a fuel pressure gauge for diagnostics.

additional information comes from the spark plugs, how they look. a Timing light is used to check timing and can also find electrical misfire.

how the exhaust smells also gives you information, sweet is rich, sour or metallic taste lean.

seat of your pants tells you a lot as well.
if the car Gets up and goes then falls flat, you are too rich.
if you stomp on it and falls flat, misfires, then goes, you are too lean

so do not get all your information just from a vacuum gauge

carb problems usually come from aged internal rubber parts that have failed or ethanol gas has eaten something, you could have rust in the tank that has gotten through the filter and clogged up the carb internally.

if you find a carb is clogged with junk internally, then cleaning it is only half the problem, because it is picking up junk from the fuel tank and you must inspect the inside of the tank or it will just keep clogging the carb up.

Thanks. That site was a huge help in that it goes over the possible issues based on the readings. I'm not prepared to tear down an engine myself but I want to start learning how to read the diagnostic tools we have for these cars and go from there.
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