Vacuum Gauge Readings

[kwoolard]

Guys,

This nice weather we had this past weekend forced me to get my boat out of winter storage. Over winter I bought a MityVac single brake bleeding kit that comes with a vacuum gauge you can use for engine diagnostics. Anyway, hooked it up to my engine and the only accessible spot, other than removing a plug on the manifold, was the PCV connection at the base of my carb.

I got good vacuum at idle 18" Hg, and revved the engine and vacuum increased to 23" Hg, give it a couple of throttle pumps and the vacuum drops close to zero back up to 21" and then steadies back out at 18" Hg.

The only thing that I noticed no matter what vacuum reading I was getting, the needle filckered very very fast(probably 10 times a second) over the small range of 1" Hg. As I revved the engine the flickering was still there but was so fast it looked like I had a needle that was wide enough to cover the 1" increment.

So my questions are:

1) Is the PCV port on the base of the carburetor a good source of continuous vacuum?

2) Is this flickering of the vacuum gauge needle a indication of leaking valve guides, or is this normal valve overlap, or what is this representing?

3) Is this flickering caused by the location of my vacuum port (PCV Valve). Is this PCV port connected to both sides of the manifold or just one side and that is causing this flickering?


Thanks and sorry for the long post.

Keith

 

[chiefalen]

Re: Vacuum Gauge Readings

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

How to use a vacuum gage. #3 might be you.

 

[kwoolard]

Re: Vacuum Gauge Readings

Good website, I've read that thing over many times before, but he doesn't show the reading that I am getting. My flicker doesn't cover 3-4" only 1" and is uniform.

Is the carb PCV port just plumbed to one side of a dual plan manifold?

{edit}
I guess scenario 3 would be what I am seeing, but mines flickers around 18" not a lower vacuum than normal.

 

[chiefalen]

Re: Vacuum Gauge Readings

Go back look at #3.

 

[kwoolard]

Re: Vacuum Gauge Readings

You beat me to my post edit.

I guess that could be the scenario, but I figured since this is not a high performance engine and most marine engines (at least I thought) had minimal valve overlap to reduce water ingestion.

Are my readings typical of a 1999 5.7L Alpha Gen II (250hp)?

Got anything on the PCV port?

 

[chiefalen]

Re: Vacuum Gauge Readings

You got a serious motor ain't no puppy dog. You read it all?? Read it all, couple of times.

Your if running the way you describe is right on the money.

If you go to the top of the forum" Adults only " You will find all kinds of good stuff plus the link i gave you here.

Good luck !

 

[kwoolard]

Re: Vacuum Gauge Readings

Is the PCV a good spot to check vacuum or should I tie directly into the manifold?

 

[chiefalen]

Re: Vacuum Gauge Readings

Yes it's a good spot he covered it in the link first passages.