Tuning carb with vacuum guage?

[Peregrine]

I remember seeing a thread on thsi awhile ago. I tried to seach for it but got around 75 pages of threads. We talk about carbs a lot Anywho, I recall someone saying that you can use the guage to tune for better milleage on the highway, or something to that effect? Can someone give me the rundown again, tahnks.

[71-longbed]

im only aware of being able to tune your idle mixture with a vacuum gauge .... once you press the pedal the idle circuit is no longer in use .... .. are you running a rochester ??

[john]

This is from GMC PAULS site. He has a lot of other good info on there.
Vacuum Gauge Use
Connect vacuum gauge to base of carburetor to full vacuum source at idle. Then monitor your readings and compare readings to chart below.
1-Steady gauge reading from 17-22 indicates normal engine in good condition.

2-A intermittent fluctuation at idle indicates a ignition miss or sticking valves.

3-Needle fluctuates as you increase engine speed indicates ignition miss, blown head gasket, leaking valve or weak valve springs.

4- Steady needle that drops at regular intervals indicates burnt valves or improper valve clearance, (the needle falls when bad valve is in operation).

5-A drifting needle goes high then drops goes high then drops in a steady movement indicates minor intake leak or improper carb adjustment.

6- A low reading but steady from 10-16 indicates late ignition or valve timing, low compression, stuck throttle valves, leaking carburetor or intake gaskets or a big cam.

7- Needle vibrates heavy at idle but becomes steadier as engine speed increases indicates worn valve guides.

8-A gradual drop in reading at idle indicates blocked exhaust system or excessive back pressure in exhaust system.
That's troubleshooting with one. Now for fuel mixture adjustment with one.
Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.
Ok now for monitoring fuel consumption with a vacuum gauge.
Connect the vacuum gauge to full unported vacuum and run vacuum line long enough for gauge to be in cab. Go for a drive monitoring the vacuum gauge. When you have high readings you are getting the most fuel economy possible, when you have low readings fuel consumption to efficient combustion ratio is lowered. By easing into your gas pedal rather than moving it rapidly you will notice vacuum readings stay higher which increases MPG


GMCPauls Truck Parts
Phone-(765)344-2906 / Fax-(507)242-0952
R.R.#1 Box 665 / Rockville, IN / 47872
Email- robbinspd1199@yahoo.com

[gonebad2]

I thought a vacuum gauge was used more as a diagnostic tool than a tuning tool for a carb. I use a really touchy Snap On digital tachometer for idle mixture adjustment. It works great! Just adjust the idle mixture screws until you get the highest rpm reading.

[RON WOODGEARD]

Originally Posted by gonebad2

I thought a vacuum gauge was used more as a diagnostic tool than a tuning tool for a carb. I use a really touchy Snap On digital tachometer for idle mixture adjustment. It works great! Just adjust the idle mixture screws until you get the highest rpm reading.

You can use a Vacuum gauge for Timing, carb. setting, and engine diagnostics.. It was very common in the OLD DAYS :p It is widely overlooked anymore.

[70c10]

You can also use it to check your fuel pressure but I have never done it.