Vacuum vs timing question

[chaseincats]

Vacuum theory question for you.

I was looking at the atlanticz vacuum measurement page and gave their test a shot.  At idle with 10 degrees btcc timing per the FSM I'm sitting at around 16.25 lb of vacuum.  The only way to get it into the 17-22 range (in this case to 17) is to advance the timing to 15 degrees at idle while configured to run at 800 rpm at idle.

My question is, is vacuum lbs something to pursue or is it just a measurement.  Basically asking should I leave my idle timing at 15 degrees so I can have 17 lbs of vacuum or put it back to 10 because vacuum is just a measurement for engine health and not a tuneup configuration point in the FSM like timing, valve lash, etc?

[Yarb]

Have you done the remove the oil cap test and see if it wants to stumble a bit?

[chaseincats]

It will stumble/sometimes die if I do that which is good.  I've also smoke tested the engine and its sealed up.  It does have worn piston rings which is probably the main issue.  Is 17-22 lbs a goal for vacuum in terms of having the engine run well or is it just a test to know engine health though.  Basically should I return the timing to 10 degrees and 16lbs of vacuum or leave it at 15 so i have 17lbs?

[Yarb]

My opinion is to roll it back to 10. Is it running good?

[Zed Head]

Intake vacuum is a diagnostic tool, not a target.

You should know your distributor's vacuum and mechanical advance curves to help decide where to set your initial timing.

[chaseincats]

20 minutes ago, Zed Head said:

Intake vacuum is a diagnostic tool, not a target.

You should know your distributor's vacuum and mechanical advance curves to help decide where to set your initial timing.

Gotcha, the car has the original distributor so in that case I'm assuming I should put it back to 10?  I came to 15 degrees because the car's idle changes periodically ever so slightly and twisting the dist to 15 (but not above) was the timing where the idle change happened the least.

The idle change is so slight that the tach only barely moves if at all and the only way to notice it is using a vacuum gauge or your ear.

[Zed Head]

Really depends on what your goal is. 

I'd still take the breaker plate off and have a look inside just to be sure you know what you're working with.  I collected quite a few and most of them had a stuck breaker plate (no vacuum advance), and crusty gummed up centrifugal advances.  Might be why your idle speed is changing.

  

[chaseincats]

4 minutes ago, Zed Head said:

Really depends on what your goal is. 

I'd still take the breaker plate off and have a look inside just to be sure you know what you're working with.  I collected quite a few and most of them had a stuck breaker plate (no vacuum advance), and crusty gummed up centrifugal advances.  Might be why your idle speed is changing.

  

If I pop off the hose from the TB and suck on it, when I let go you hear the click of it snapping back in place.  You also can't blow through the hose so the vacuum diaphragm is still holding air.  I also put the timing light on it and pushed the throttle and you can see the timing advance immediately so they dist should be good.

[Zed Head]

You're using precise numbers on initial timing, 10 and 15, but no numbers on the rest of the curve.  And your test doesn't tell you about anything parts sticking.  

Sticky throttle plates are common.  A new path to follow...

[chaseincats]

6 minutes ago, Zed Head said:

You're using precise numbers on initial timing, 10 and 15, but no numbers on the rest of the curve.  And your test doesn't tell you about anything parts sticking.  

Sticky throttle plates are common.  A new path to follow...

Should I pop the top off the dist cap and spray some silicon lubricant in there and scoot the plate around a bit?

[Zed Head]

👀 first.

[Yarb]

My opinion is to roll it back to 10. Is it running good?