Vacuum gauge test
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Trailering
Joined: Apr 2025
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Likes: 4
Vacuum gauge test
Hi, I bought a vacuum gauge to set my idle mixture screws (learning). What port on the carb do I hook it it up to? When standing in front of it. There is a small one to the left that is empty. The center large for the PCV and on the right another small that leads to the distributor
Laughing Gas
Joined: Aug 2000
Posts: 1,005
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From: Eastern, Idaho
Anything that reads vacuum at idle will work, preferably directly from the intake manifold. One port on the carb will not have a reading at idle. Depending on what you have access to you might need to make a tee and connect into an existing system.
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Trailering
Joined: Apr 2025
Posts: 21
Likes: 4
okay I found it. So I adjusted the mixture screws but still no reading change. It bounces between 10-15, no matter the change of the screws. What would cause that.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,154
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From: S/W Missouri, Polk county
https://www.classiccarrestorationclub.com/post/engine-vacuum-troubleshooting
Common Vacuum Test Results:
Normal Engine: On most engines, accelerate to around 2000 rpm and then quickly release the throttle. The engine should snap right back to a steady 17- 21″hg vacuum.Steady low between 5-10″hg vacuum: This indicates that the engine has a leak in the intake manifold or the intake gasket.
Steady low between 10-15″hg vacuum: This reading indicates late valve timing. There’s a chance the vehicle has jumped timing. Check the timing belt or chain depending on the application.
Steady low between 15-18″hg vacuum: This low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem, and recheck vacuum.
Fluctuating Needle: A fluctuating needle indicates there’s a problem with a valve or a there’s an engine misfire.
Needle drops during acceleration: If the needle drops steady during acceleration there’s a restriction in the exhaust or intake. This is typically due to a clogged muffler or exhaust system.
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