Read my spark plug...

[theholycow]

The intake backfire is what I originally thought was going on. A couple times I think I've made it happen in the yard with the air cleaner off and seen flames or smoke come up through the carburetor.

I have to review this entire thread and make a master list of things I need to check or try. Don't worry, I won't post yet another list.

[theholycow]

Ok, I reviewed the thread and made a list of dozens of things to consider or try, including a 17 item short list.

[theholycow]

I had the day off from work today and perfect weather. I worked about 10 hours. Here's some of the things I did that are relevant to this thread...

Checked power brakes to see if they still held any vacuum assist after sitting overnight. A slight amount remained. I still need to check after only a half hour.

Completely inspected entire collection of vacuum hoses, drew diagrams as-built, compared to diagram on label on car, learned a few things, found+replaced some rotten hoses.

Setup stub vacuum hoses so I can easily switch between manifold, TVS, and ported vacuum instead of having to reach under/behind things.

Checked butterflies; they are completely closed when accelerator is at rest.

Checked distributor cap and rotor. Appears ok. Checked distributor for play; seems good; very minuscule play, may be entire engine moving for all I know. Checked spring-back of advance; works smoothly.

Checked the EFE for cold starting performance. It properly actuated as soon as the engine was started. At 8:30am I didn't even know what EFE was, nor what that doohickey was or why it had a metal line going to it...now I have a basic idea of how it operates, although I don't understand how it works. I'll google that.

Pulled spark plug wires while idling, one at a time. Surprisingly the effect was minimal, but it was consistent; each one caused a slightly rougher idle.

Checked for vacuum leaks more thoroughly than ever before, using starting fluid spray this time.

Finally found and replaced PCV valve. The old one was fine.

Tried manually actuating EGR valve with my fingers while idling. Engine did not stall but RPM dropped from 1500 to what sounded like about 400. If it was idling at a proper speed I bet it would have stalled.

[GasSavers_Erik]

Wow- very thorough testing- great job.

I am stumped as to why your idle would still be so high...

Does your quadrajet have idle mixture adjustment screws? If so, you might try adjusting them to verify that the idle mix is correct (screw it too far in = lean and the idle gets rough, unscrew it too far= rich and the idle gets rough). If you can't make it too rich or too lean with the screw, then maybe it is running too rich (to high of float level in the carb? sticking float? sunk float?) and that in combination with a vacuum leak could be giving you the extra air and fuel needed for such a high idle.

When you rebuilt the carb, did you shake the brass float to be sure it wasn't half full of gas? Sometimes they can have a tiny crack and fill with gas and then you end up running too rich because the gas level is too high in the carb- but this wouldn't explain the bucking (which means too lean).

If you had a little power assist left after sitting overnight, the booster is likely OK. You might try rapidly pumping the brakes and see if the idle changes.

If you unhook an manifold vac hose when the engine is idling, does the idle speed go up or down?

[theholycow]

Not thorough enough! You and others provided plenty of other things I need to check, and eventually I will.

I never did rebuild the carb. I will probably do that. I hate to invest in it when I'd rather invest in converting to fuel injection, but it may be more realistic.

If I unhook the vacuum hose that leads to the distributor, my idle speed goes down. It's the same result as running ported vacuum: Idle is between 400 and 1100, somewhat randomly, but usually 900 or less and much more tolerable. Idle is supposed to be 550 in Drive, and I think 650 in Park/Neutral.

However, last night I found out something. I ordered the OEM GM service manuals and received them yesterday. Good bedtime reading, right? I learned exactly what the DS/VMV is for.

From the 1980 Buick Advance Chassis Manual:
The DS-VMV limits the distributor spark vacuum to a calibrated value until ported vacuum applied to the control port of the DS-VMV is greater than the calibration value. When the ported vacuum is greater than the calibration value, the distributor spark vacuum is equal to the ported vacuum signal.

In short, it provides whichever vacuum source is greater (assuming that "calibrated value" means "value of the other vacuum source").

That means it always provides that vacuum source that I first thought was manifold vacuum but has a check valve. I need to double-check that against the as-built vacuum diagram I made yesterday and see if I'm missing a tee or something. I'd check now but I left my as-built at home.

Even if the DS-VMV is in good working condition and I put it back together the way it belongs, it still may not be sufficiently prepared for manual transmission driving. If I put it back together properly and it has low idle, no bucking, but still can knock, then I may need to retard the timing.

Side note: I would need two vacuum pumps to test the DS-VMV. Quantity of vacuum pumps I own: Zero.

I wrote this whole post thinking that the DS-VMV, properly hooked up, would give me the no-advance low idle I need, but reviewing it I see that it will actually provide plenty of advance at idle with manifold vacuum.

I should try using the DS-VMV in different combinations with real manifold vacuum, ported, and check valved.

[Jay2TheRescue]

Originally Posted by theholycow

Side note: I would need two vacuum pumps to test the DS-VMV. Quantity of vacuum pumps I own: Zero.

I see a trip to Harbor Freight in your future

[GasSavers_Erik]

Some auto stores will loan out vacuum pumps along with lots of other tools.

Is there any way to tell if the manual trans model of your car also used the DS-VMV?

If your idle speed randomly varies from 400-1100rpm and it is smooth and for sure not missing on any of the cylinders at each end of that range- then maybe it is an intermittant vac leak or some variation of fuel level in the carb float bowl.

Does the gasket that seals the top part of the carb ever look like it is wet?

With the engine warm, if you quickly floor it while idling (no need to go above 2500 rpm), does the engine speed immediately rise? Or is there a half second hesitation and sometimes a small pop? (this means it is running lean)

A stuck float can sometimes be "released" by simply tapping on the fuel bowl region of the outside of the carb with a wrench.

[theholycow]

Even with the factory service manual I can't tell for sure that a manual trans model of my car ever existed outside of the sales brochure. I am pretty sure it was never offered with my engine either way. Anyway, I found more documentation that may conflict with what I read about the DS-VMV so I guess I need to continue reading until I confirm exactly what it is supposed to do.

Carb bowl/float issues seem plausible and should be fixed if I ever do that carb rebuild. The gasket between the carb and the air cleaner is old and nasty and I would have to replace it before I could say if it ever looks wet.

I'll have to try quickly flooring it while warm in neutral. I'm pretty sure I've done it but I'll do it again. It never hesitates or pops (except if you count the sound of backfiring through the intake as a pop).

[theholycow]

Ok, I'm more sure of what the thingy is now. It's very similar to a DS-VMV but is actually a SAVM. See my detailed post:
http://www.gassavers.org/showpost.ph...53&postcount=6

It provides 7 inches of vacuum at idle, ported vacuum at part throttle, and manifold vacuum at heavy throttle (when hooked up normally).

As-built before I removed it, assuming it was working, it would provide 7 inches at idle and heavy throttle, and ported at part throttle. That should have given me no skip/buck, no knock, but fast idle.

What would probably be best for my car: Ported at idle, manifold at part throttle, 7 inches at heavy throttle. I've spent the last half hour drawing charts trying to imagine how to accomplish that, and I can't figure it out. It's probably very obvious...

[GasSavers_Erik]

If you currently have a problem with spark knock under heavy load, then I'd select the option with lowest vacuum advance under heavy load but highest at idle =manifold vacuum

Post #6 here is a good read: the author makes the case that ported vacuum advance at idle was primarily for meeting emissions standards.

So, it seems like maybe your high rpm idle problem might be caused by extra ignition advance and with that extra advance, your engine is running so much more efficient that you can't reduce the idle to where it needs to be That assumes that the carb was designed with an idle air port a certain size with the low advance in mind.