Tuning & Reading With A Vacuum Gauge

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.


Vacuum gauge needle readings explaination


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


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


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)


Improper carb setting or minor vacuum leak


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


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


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


Worn valve guides

cant understand la bro.. can giv example for efi engine at simple explanation?

efi n carb same reading la
juz install a vacuum meter then c how the meter react n u wil no wat is the prob

must hav TT la bro.. then can discuss n understand more lah

u guys come to penang ltt a
i treat u all

hahha.. penang ar.. so far ma...

not really war la
around 3++ hours wil arrive d

c la if can kautim wit satria975.. car pool 2 penang..

u blanja makan + hotel + leng lui ar?? hehe

leng lui hard to find la
aunty wan or not?

auntie below 35 can la.. but must c face 1st... hahahha...

walao...

e191 wrote:

c la if can kautim wit satria975.. car pool 2 penang..

u blanja makan + hotel + leng lui ar?? hehe

y wan 9dim me?

must he belanja all!!!

gsrturbo9166 wrote:

walao...

i think he more hungry aledi!!!
e191...how old r u???

not included "panggil ayam"

gsrturbo9166 wrote:

not included "panggil ayam"

"nasi ayam" can gua!!!
(makan nasi lagi panggil ayam)!!

satria975 wrote:

e191 wrote:

c la if can kautim wit satria975.. car pool 2 penang..

u blanja makan + hotel + leng lui ar?? hehe

y wan 9dim me?

must he belanja all!!!

ya la.. we go there we claim fr him lor...

me how old ar... still kiddo oni lor.. 25

25 icic
should cal u "go go" liao

how old r u ..??? n satria??

cannot la
anything that included ayam cant d

e191 wrote:

how old r u ..??? n satria??

me 21 wan 22 aledi!!!
still poor kia!!

e191 wrote:

how old r u ..??? n satria??

i 22 liao

same same only nia... sume skolah menengah sekali wan
)

e191 wrote:

same same only nia... sume skolah menengah sekali wan
)

no lah!!!that kind duno come from where de!!!\
somtime ipoh sometime penang somtime kl!!!

satria975 wrote:

e191 wrote:

same same only nia... sume skolah menengah sekali wan
)

no lah!!!that kind duno come from where de!!!\
somtime ipoh sometime penang somtime kl!!!

wei wei wei
don talk bad thing bout me k

ipoh where ar?? my wife fr jelapang