: A vacuum test



csbuckn
03-31-09, 10:20 PM
So I just did a vacuum test from the intake manifold. The book says it should be around 18hg and it was. The issue is that the needle was moving back and forth very, very fast between 17 and 19hg. The book says it should be steady. It doesnt say what the very rapid movement means. It gives diagnostics for low readings and slow movement but not very rapid movement. Any ideas? I'm trying to correct the sparatic idle. This is the only issue that needs correcting since I put the motor in and its driving me crazy.

sven914
03-31-09, 10:57 PM
It sounds like worn valve guides. This would normally be indicated by excessive oil consumption. With the vacuum gauge on, slowly increase the engine RPMs. If the pointer becomes steady, then the valve guides are worn .

Another thing it could be is weak or broken valve return springs, but usually the fluctuations would be from 10-21HG. They would get more severe as RPMs increase.

csbuckn
04-01-09, 08:31 AM
When I increased the RPMs, the needle still moved back and forth very rapidly. I think I'm gonna have to take off the TBI and intake and reinstall.

cadillac_al
04-01-09, 09:14 AM
Google is your friend:

http://www.secondchancegarage.com/public/186.cfm

I was a little fuzzy on vacuum readings and figured I could google it faster than I could get up and grab a manual lol.

sven914
04-01-09, 09:15 AM
If there was vacuum leak around the throttle body or intake manifold, then the vacuum reading would be much lower and the needle would be steady. Pretty much the only things that will cause the needle to fluctuate are valve-train related problems. Ignition misfires could cause some gauge fluctuation, but you would know if the engine was missing.

If this is "BUILT" 350 you're having a problem with, than the performance cam could be causing your grief. Performance cams are not designed to give a smooth idle, and can cause weird vacuum readings. I have a friend who builds performance cars and he has a 72 Monte Carlo with a .400 under the hood; he put a high lift cam in it and it won’t idle at all. He has to flutter the throttle to keep it running.

csbuckn
04-01-09, 10:14 AM
That page makes me fell better, my scenerio is #3. I do have a cam in there but its only an RV/torque cam. But I also have to two-foot-it at lights. Maybe it was destined to be a two footer, makes green lights interesting

VortecCaddy
04-01-09, 11:38 AM
I have a friend who builds performance cars and he has a 72 Monte Carlo with a .400 under the hood; he put a high lift cam in it and it won’t idle at all. He has to flutter the throttle to keep it running.

That sounds like its not getting enough fuel from the carb if he has to give it gas just so it idles properly. Does he build engines or performance engines?

cadillac_al
04-01-09, 02:50 PM
Hey csbuckin, how tight are your valves? Most manuals say to give them 1 turn after lash but everybody I know uses 1/4 turn after lash. They just run crappy when using 1 turn in my opinion. It should still have enough vacuum to idle though. good luck.

csbuckn
04-01-09, 10:23 PM
I dont remember how much I turned the valves but probably a 1/4 turn. Next thing is to run new gas lines and put a clear fuel filter under the hood. I've had to replace the fuel pump, filter and redo the connection at the engine so maybe something came loose even though it looks good.

sven914
04-03-09, 10:28 PM
I have a friend who builds performance cars and he has a 72 Monte Carlo with a .400 under the hood; he put a high lift cam in it and it won’t idle at all. He has to flutter the throttle to keep it running.

That sounds like its not getting enough fuel from the carb if he has to give it gas just so it idles properly. Does he build engines or performance engines?

He builds performance engines. He checked the fuel flow and it's getting plenty and it runs fine at high RPMs. He just put in the wrong cam.

csbuckn
04-04-09, 10:00 PM
So it was the fuel line connection at the fuel filter. fixed it and she runs with no stumbles or anything. Now the drivetrain is at 100%. Wonder how much HP and Torque she has now.