Vacuum gauge readings help

[Zonaman]

A little help from you more experience fellows would be apprecaited. I had a vacuum guage on the PCV port of my carb, The needle was jumping rapdily between 19 & 21 inches and doing so consistantly. Adjusting the idle screws has no effect on this. Is this indicating some sort of valve problem?

[69TowRig]

Copied from GMC Paul's Website:

http://www.gmcpauls.com/Tech%20Tips1.htm

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

[pmpski_1]

Thanks, this was helpful!

[john]

Another one with pics.

http://www.classictruckshop.com/club...ts/vac/uum.htm

[Zonaman]

Thank you all very much! The closest description seems to be "Wavers irregularly in one range despite engine speed. Unbalanced carburetion; improper spark plug gap; ignition timing; poor valve seating" from John's chart.

For some reason, on my truck, the balancer TDC mark doesn't seem to be properly lined up with the timing tab. How close can I get the timing with the vacuum guage? Also, since the idle screws don't seem to effect that wavering, what else might I be looking at on that carb (Edelbrock 1406)?

[WorkinLonghorn]

I always do a compression check when attempting a diagnosis.The more info the better and it's a real easy test,provided your spark plugs are easily accessed.

[BLACK AND BLUE 67-72]

Check your valve springs you may have one broken.

[Zonaman]

Originally Posted by BLACK AND BLUE 67-72

Check your valve springs you may have one broken.

Yikes! What other symptoms would go along with that?

Thanks.

[BLACK AND BLUE 67-72]

slight loss of power and a ticking sound is what mine did.

[Zonaman]

Originally Posted by WorkinLonghorn

I always do a compression check when attempting a diagnosis.The more info the better and it's a real easy test,provided your spark plugs are easily accessed.

Here's the results (about 28,000 miles on a '96 crate 350):

8: 165, 7: 160
6: 160, 5: 159
4: 150, 3: 150
2: 165, 1: 155

Could it have readings like this and still be a valve spring? Haynes says they needed to be atleast 150 and within 20 psi so this looks OK. Plugs all looked good. There's a slight stumble at idle. Does it raise any questions with any of you?

Meanwhile, I'm going to try working on the timing. Once again I'll be using a vacuum guage for that. Can I get it right with just that or should I pull the balancer, line it up with with TDC, and get out the light?

Thanks again, this board is great!