Re: How to use vacuum gauge for FE?
 

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?

Re: How to use vacuum gauge for FE?
 

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

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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?

How am I doing?
 

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

https://i51.tinypic.com/2ql8wmp.jpg

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?

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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