How to use vacuum gauge for FE?

[theholycow]

Originally Posted by Erik

Here's a pretty good site with animations of what problems a vac gauge can diagnose: http://www.secondchancegarage.com/public/186.cfm

Sometimes, or maybe all the time, my needle shakes like the movement demonstrated in scenario 3 (but not at that level). Does that mean anything?

[GasSavers_Erik]

It might be a cylinder missing. Purposely pull off a spark plug wire and see if the gauge acts the same way/gets worse.

[theholycow]

Originally Posted by pgfpro

Usually they will have
5 in./Hg orange spring
6 in./Hg black spring
8 in./Hg yellow spring

Example a 4 in./Hg gold spring will over come the engines 4" or less of vacuum and add fuel through the enrichment system.

So I've been reading up on Quadrajets. I've got more material from different sources to read yet, but I did come up with this bit:
"On some Q-jets, two springs are used beneath the power piston. The smaller (primary) spring seats in the center of the piston. A larger diamater (secondary) spring surrounds the smaller spring. It exerts added force upward on the power piston. It starts the first stage of power enrichment at 8-10 in.Hg manifold vacuum. The center spring gradually continues enrichment until it is completed at 5-7in.Hg manifold vacuum."

I'll eventually find out if mine is that type. All the reading is because I'm studying to rebuild mine.

[theholycow]

I just found this, from another manual, in my notes:
"On M4ME models a remote vacuum source is used to operate the spring-loaded power piston instead of the conventional manifold vacuum source. A tube in the front of the throttle body (center) connects the remote vacuum source (power enrichment control valve) to a passage to the power piston vacuum channel."

I'm not so sure about that, I don't think I've seen it in any vacuum hose diagrams.

[PeterRRC]

Hello,

This is very interesting discussion for me since I also have installed vacuum gauge in my car (I have the old ODB port in my car so I can not use SGII) about month ago.

I do know BSFC chart for my car but I'm unable to tell - using vacuum gauge - when (during acceleration) I hit 75%-80% load.
Does it a constant value? I mean it will be valuable if someone with SGII (where is displayed load) AND vacuum gauge tell me that 75% load on SGII is when vacuum gauge shows eg. 7 or 8 In/Hg?
If not then how to determine 75-80% of Load? Any ideas?

[theholycow]

So, here's the strategy I've worked out and the information I know. It seems to work well.



At 10-7 in/hg, often I can back off just enough to reach 11 and still maintain enough speed. Occasionally I can downshift but usually that just makes it worse, as I have to get deep into the throttle to get the higher RPM of the lower gear.

By the time I hit 5 I'm sure I'm as deep in power piston territory as possible and backing off won't suffice, so I might as well go wide open and pulse until I am going fast enough to back off to 11. Sometimes in the 10-7 range I may need to just give up and open it up for a pulse too.

The power piston section assumes that I have the two stage power piston that was in some Quadrajets. I'm not sure of that. I'll find out, if I can ever get my hands on a core to rebuild (or if I give up and accept some downtime while I rush through a rebuild with the car parked).

Any thoughts on potential improvements?

[palemelanesian]

I see 75% LOD on the Scangauge matches up to 12-12.5 PSI MAP. Since 14.7 is ambient that means the throttle is mostly open and free-flowing and causing very little vacuum due to restriction.

That converts (wikipedia) to 25 in HG. Since this is a vacuum reading, subtract from 29.92 = 1 atmosphere to get ~ 5 in HG.

This site confirms that "about 5" number.

[theholycow]

Hmm...so, do you think I might do better to stay at 5in/hg than to open it up the rest of the way? There is a definite increase in power, and power piston level is the same, but I can't say for sure whether efficiency increases or decreases.

I imagine that the AFR stays pretty steady, possibly leaning slightly, going from 5 to 0, putting the most rich mixture at 5, hence why I have been avoiding 5.

[palemelanesian]

I know little-to-nothing about carburetors. What I do know is you want to stay as close to ideal air-fuel ratio as you can. Whatever conditions take you lean or rich are to be avoided.

[theholycow]

I ought to add O2 sensors...otherwise it's all just guesswork.