Vacuum Gauge

Screw Driver

Vacuum "tee"

Carb Spray

Optional Tools:

Smoke Machine

(leak detection)


1) Normal: (stock engine)

vac_g2.jpg (2855 bytes)

2) Sticking Valves:

vac_g8.jpg (2882 bytes)

3) Incorrect Valve Timing:

vac_g5.jpg (2818 bytes)

4) Carb Adjustment Needed:

vac_g7.jpg (2841 bytes)




5) Blown Head Gasket:

vac_g4.jpg (2960 bytes)



6) Burnt or Leaking Valves:

vac_g9.jpg (3007 bytes)

7) Clogged Exhaust:

vac_g3.jpg (3067 bytes)

8) Worn Valve Guides:

vac_g6.jpg (2830 bytes)


Tuning With A Vacuum Gauge ... And What It Tells You

vac_g1.jpg (5792 bytes)Reading the VACUUM Gauge:

Many racers and engine tuners (I get caught myself sometimes) have opted for state-of-the-art diagnostic equipment but have forgotten one of the simplest, as well as most accurate tuning tools  the vacuum gauge.

Of course if your engine is barely able to sustain 2"Hg vacuum at idle, it will be hard to tune using this method, but it is still useful. In most cases if you use a quality large faced Vacuum gauge, with some experience you can effectively tune your race car's fuel and timing systems. This is a starting point, we all know that dozens of passes and laps will more than likely be required to verify tuning adjustments and compensate for variations.

First, A Few Basics:

  • Connection of the gauge is to a simple "manifold" vacuum source. "This must NOT be "ported" vacuum that rises as RPM increases". In most cases this will be a direct manifold source or in many cases the PCV port (larger port on the carb) on a typical carb.
    • NOTE: Connection to EFI cars is best done "Tee'd" into the source line. If you are not careful, disconnecting a particular line can affect the idle speed as well as information that the ECM needs to see. For this reason it is best to tee into your vacuum source for the gauge.
  • You must have the timing "Pre-Set", or at least do all ignition settings "Before" any carb adjustments.
  • After each adjustment is made, you MUST RESET your idle speed setting. You do this to have a standard or baseline to compare against.
  • Small adjustments are best, and in fact "optimum" carb settings on the vacuum gauge (highest reading) is usually richer than it needs to be. In other words, after the highest reading is reached, the best setting (depending on engine) is to set mixture screws back "lean" approximately 1/16 to 1/4 turn.

Proper Carb Adjusting Procedure:

With the above items taken into consideration, the first thing you do with the gauge connected is to lean one of the mixture screws until the gauge as well as the engine begins to shudder.

Note: With a properly jetted carburetor, turning either of the mixture screws all the way lean, should kill the engine. If not, you're too rich! This may require re-jetting, or drilling the primary butterflies to add more idle air. Many of the newer "race" carbs allow you to change Idle air bleeds to fix this.

You now bring the screw back towards rich, watching the gauge. (I'm not being specific as to "in or out" movement of the mixture adjustment for there are a few carbs that are actually "In" for rich instead of the standard "Race style" Holley that is "Out" for richening the mixture) As the gauge climbs, you will stop adjustment when the gauge reaches it's highest reading.

 Now do the same process for the other mixture screw. You may have to repeat this process a few times to get optimum results, plus it's worth the time an effort.

 For carbs with the 4-corner mixture screws, you have to take a bit more time setting these. You can also run the engine at a "steady-state" RPM of say, 2500 RPM to double-check your secondary mixture screw settings. Do this with temperament! It takes time to get used to what you are seeing as well as if it is actually helping. each engine will behave differently.

What Does The Above Tell You?    It can help you find errors in:

         Incorrect Jetting

         Vacuum Leak

         Leaking Power Valve


         Leaking Component Diaphragms (distributor vacuum canister, EGR Valve,

         Internal engine problems (valve adjustment, burnt valves, head gasket condition)

         Clogged exhaust system (catalytic converters, failing mufflers)

 When I was younger, I was taught to effectively tune an engine with just the vacuum gauge and track time. Not even a timing light (which can be scary)! Once the education and practice is there, it is a very effective technique. It takes some patience, but the knowledge of how your engine behaves to tuning changes is worth it. The typical screw up will be that at first you will set the engine with too much timing (more timing increases vacuum).


Detailed Vacuum gauge needle readings explained below (images at left):






Steady Needle

Normal reading (usually 17-22" Hg. in stock engines) Race engines vary "a lot" and in most cases will be considerably less.


Intermittent Fluctuation at Idle

Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft


Low, though Steady Reading


Late timing, low compression, sticking throttle valve, carb or manifold vacuum leak (remember most race engines with a big cam and a tight centerline and high overlap will be naturally low ... you must decide your baseline vacuum reading)


Drifting Needle

Improper carb setting or minor vacuum leak


Fluctuating Needle as RPM Increases

Ignition miss, blown head gasket, leaking valve or weak or broken valve spring


Steady, but Needle Drops Regularly

Burnt valve or incorrect valve setting (too tight), "needle will fall when bad valve operates"


Gradual Drop at Idle

Clogged exhaust, excessive backpressure (extreme cases engine will die at idle)


Excessive Vibration that Steadies as RPM Increases

Worn valve guides


In Closing:

 Now, of course all this info sounds really good, but don't throw out your timing light, multi-meter and other required tools. What the information above offers you is one more way to check for proper tune of your engine, and also a few tricks to do some preliminary testing of other components that typically require special tools and equipment. The vacuum gauge does not replace the timing light, multi-meter, exhaust system back pressure tester, leak down tester and other tools, it only enhances them. No one likes to tear down an engine to fix something broken, or spend excess hard-earned money on a trained mechanic or tuner to find a problem.


Always remember to use the right tool for the job!