* Engine condition: A well-tuned, healthy engine will generally have higher vacuum than one with worn components (rings, valves, etc.).
* Cam shaft: A performance cam with more aggressive valve timing will produce lower vacuum than a stock cam.
* Intake manifold: Different manifolds can affect vacuum readings.
* Engine speed (RPM): Vacuum will be highest at idle and decrease as RPM increases. The measurement is typically taken at idle.
* Carburetor settings: The carburetor's adjustments can influence the vacuum reading.
Instead of aiming for a specific number, focus on these guidelines:
* Idle vacuum: A generally acceptable range for a *street-driven* 305 Chevy with a relatively mild cam at idle is 17-22 inches of mercury (Hg). However, readings outside this range aren't necessarily a problem if the engine runs well.
* Smooth running: The most important factor is that the engine runs smoothly and consistently at idle. If the vacuum is too low, it could indicate a problem. If it's too high, it might also indicate a problem, like a too-lean fuel mixture.
* Consistent readings: Fluctuations in the vacuum reading, even at a relatively high number, can indicate problems like vacuum leaks.
Troubleshooting low vacuum:
If your vacuum is consistently low, investigate these potential causes:
* Vacuum leaks: Check all vacuum lines, the intake manifold gasket, and the carburetor base gasket for leaks.
* Worn engine components: Worn piston rings, valve guides, or a worn camshaft can all contribute to low vacuum.
* Incorrect carburetor adjustment: A lean fuel mixture can cause low vacuum.
Recommendation: Instead of focusing on a specific vacuum number, observe your engine's performance. If it runs smoothly and efficiently, the vacuum reading is likely acceptable for your particular setup. If you suspect a problem, a qualified mechanic can diagnose the issue. They'll use a vacuum gauge to assess the situation alongside a comprehensive inspection of your engine's components and operation.
