I was using a vacuum gauge to set my idle a few days ago and got a reading that the books tell me is worn vavle guides. The needle bounces rapidly from about 6-10" at about 1000 rpm. If I bring up the throttle the needle steadies. I have a rough idle and I do go through oil fairly fast (no external leaks). But I don't have ANY smoke at start up (or at any other time for that matter), and never have (motor only has about 1500 miles or less on it). I haven't checked the plugs yet but I will soon. When I built the engine I had it decked so the pushrods are alittle long. I wouldn't think it would waste my guides this fast, but what do I know? Is there any possibility that this could be from my idle not being right and not valve guides? The thought of pulling the heads and having that done isn't very appealing right now.

Thanks

 

Would a vacuum leak cause this reading? That would also explain my rough idle. Much better thought than bad guides. Ideas?

 

if your running a lumpy cam then i wouldnt sweat it. Lots of lift + duration = low irregular vacuum.

 

I have the LT4 Hot camin my 383. 218*/228* dur at .050 lift and 112 sep. .525 lift. Is that lumpy enough to cause that?

Thanks

 

yeah... that would cause a nice lumpy idle.

 

Wow that's a relief. I figured I'd get alittle bumpy idle but the vacuum gauge scared me some. Thanks again.

 

The needle can also bounce if you have a dual plane manifold and the idle mixtures aren't the same on each side.

 

...Or if you have sticky valve or bad PCV valve... I'm running a .510/.520 cam on one of the LT1s, and idle vacuum is a little low but rock steady.

 

6-10 is too low for that engine. It may need some adjustments at idle, from fuel to timing to idle speed. I have a bigger cam than that and pulling a solid 12 at idle.

 

Max,

I probably interpreted his post differently. I thought it indicated that the reading was swaying from 6-10" Hg, not reading at that level (i.e., about 18" Hg but swaying 6-10).

Now, I'm not sure.

 

It is reading between 6-10" in gear (about 650 rpm not 1000 like I said at first, sorry). In park it will steady at 15" at about 1000 rpm. I am at 4000 ft elevation if that makes any difference.