Next, turn one idle-mixture screw in -turn and note the change on the vacuum gauge.
The vacuum gauge hose as close to the intake manifold as possible and start engine.
Where to hook up vacuum gauge on holley
A quick opening and closing of the throttle should cause the vacuum to drop below 5 in.
Inch of the idle transfer slot (you can use a spark-plug-gap wire gauge to measure this).
Where to hook up a vacuum gauge
The goal of adjusting the idle-mixture screws is to achieve the highest possible idle vacuum at a set idle speed.
Note the location and action of the vacuum gauge needle, and use that information to determine the engine problem.
If the vacuum reading increasessay, from 14 inches to 14- inchesmove around to the other side of the carburetor and turn that mixture screw in -turn as well.
For instance,If the reading on your gauge peaks at 18 inches of vacuum, that is the.
Head gasket leakagewith the engine at idle, the vacuum gauge pointer will drop sharply every time the leak occurs.
Have your vacuum gauge hooked up, it is a good time to set the idle.
Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.
If vacuum decreases at higher rpm, an excessive amount of back pressure is probably present due to a restriction in the exhaust system.
Elevation the vacuum reading will be about one (1) inch lower per each 1,000 feet rise in elevation.
Testing with a Vacuum Gauge - Technical Articles at Greg's Engine & MachineTry timing your engine using this vacuum method.