My 1980 LM-1 Engine with a stock bottom end, mild cam, Edelbrock Performer Intake and 600 cfm electric choke carburetor has a problem with the vaccum reading. It is jumping between 12 and 20 at idle. I was told it should spike itself about 20-22, right? Does this mean I have a vaccum leak? Could an exhaust leak cause this? Or how about the valves being torqued down too tight so that they don't close and open all the way, might I need to loosen them up a bit? It is possible I have the rockers too tight, I did adjust them the backyard way, I never used a filer guage on them. But at the same time I'm not sure if that would affect the vaccum reading. If you do think I have a vacuum leak, what is a sure fire way to find it, I would think it would be kind of a pain. Thanks everybody.

Renegade

If you adjusted them too tight you will hang a valve open. That can force air back into the manifold and cause wildly varying vacuum readings.

You need to go back through and re-do the lash.

Okay, I'll give that a shot and retest it. I'll let you know. Thanks Trainman.

Renegade

anyone else care to chime in? I could use some more info on what the vacume reading should be and look like? i.e. solid needle or shaking? mine shakes a ton and sits around 12 at idle as well, anyone?

My Blazer had the guage jumping around at idle when the gasket between the carb and intake adapeter was torn. I replaced it with on of those thicker ones and it's been fine since. I was at about 10 at idle, lean too.

Steady needle is good, jumpy you have a problem.

<font face="Arial,Verdana" size="2">Originally posted by sinner4:
Steady needle is good, jumpy you have a problem.</font>

DOH!

Guess I had better buy new carb base gaskets then....lol....

What else besides a leaky gasket would casue that ?

yeah what he said!!

<font face="Arial,Verdana" size="2">Originally posted by rscamaro73:
DOH!

Guess I had better buy new carb base gaskets then....lol....

What else besides a leaky gasket would casue that ?</font>

Burnt or bent valve(s), missfire, vacuum leak, miss adjusted valve lash, fuel puddling and a really out of wack tune (carb miss adjusted, clogged passages in the carb, anything that would make the carb not deliver fuel or air properly) can cause a shakey vacuum reading. Usually when an intake valve is burnt or bent you will also get a backfire threw the carb when accelerating the engine, same with a missed adjusted valve train though if its just a tad bit too tight you mightnot get any back firing.

Some engines with big cams will have this no matter what you do, it just comes with the territory of having a big cam that bleeds alot of compression at idle. My vacuum at idle in gear goes from 8 inches to 5 inches and back and forth. Although the guage follows the idle lope, when it lopes real low vacuum goes down, lope goes high the vacuum goes high.

[This message has been edited by Joekool (edited July 01, 2005).]

See this page for a lot of info on tuning with a vacuum gauge:

http://www.centuryperformance.com/vacuum.asp

thanks for the link trainman, good info!

If it was my valve hangin' up, wouldn't I have a backfire? Like I said before, my vaccum reading is really jumpy and I have no backfires, so is it safe to say that it probably is a leaky gasket? I doubt there is a problem with fuel delivery, being it is a new carb and I had it tuned, which I had the local tire shop put it on their analyzer and tune it, this is when they told me I had a vaccum problem, the guy said it is probably a sticky valve, but I fail to agree because if my valve was hanging open it would backfire. Please tell me if I'm wrong.

Oh, and get this, it is against that shop's policy to mess with valvetrain. they can hook up electrical, do brakes, mount tires, but not valves, it is sad really.

Thanks Everybody,

Renegade