Miles71

Spark Plugs, Originals?

I had a slight issue with rough idling. On a hunch I replaced all of the AC-Delco R45S plugs with new Champion spark plugs (RJ18YC .040). The old plugs looked very good when I removed them. Just the right amount of brown color was present.
5 of the plugs had a gap of .030"
2 plugs were gapped at .035"
And 1 plug was gapped at .018"

I think the .018 plug was causing the engine to miss a little while idling. I monitored the 800 RPM idle with two different tacks (Analog and Digital). While idling, I would see the RPM jump around a bit 760-800 randomly. I could hear the engine pattern change a little during the RPM dips (the drift was approx 40-60 RPM).

After the new plugs were installed I noticed the Digital tack had smaller drift at 800 RPM, typically +/- 10 RPM. The overall idling in Park (800) and Drive (600) seems smoother.
I readjusted the Idle-Mixture and Idle speeds screws for best vacuum / idle again. Hot and cold engine starts are good. Highway performance is good. No issues with Off-idle bog.
Current settings:
Stock 350, 8.5:1 CR, 4BL, Mixture 4.0 turns, Idle Speed Screw, 1 and 1/8 turns.
Vacuum 17.5 at 800 RPM Park, Hot engine, single exhaust.

The old plugs looked in good condition. The anode was nice and sharp, not rounded. The plugs may only have 20k on them. The plug on the left is gapped at .035", the plug on the right is gapped to less than 0.018". Looks like someone dropped the plug on the ground before installing in the car 41 years ago.

Here is a close up of the bad plug. The measuring tool wire closest to the plug is the .018" wire. This wire just barely fits in the gap.

Miles71

Plug images
Here is a close up of the bad plug. The measuring tool wire closest to the plug is the .018" wire. This wire just barely fits in the gap.

69 Ragtop

Your picture shows newer R45S's (1975 or so, and beyond, but I don't remember the exact year). The ones in '71 had green lines on the ridges and the "AC Delco" was just a block "AC" (not script). See pic. In 1971 the division was still "AC Spark Plug", not merging with "United Delco" until 1974.

Also, if the carbon builds up on the top of the piston it can hit the electrode, closing it slightly, or on rare occasions there can be debris in the cylinder; however, in the latter case there is usually evidence of micro-damage from the debris hitting stuff (little nicks). I'm inclined to believe the plug was dropped, as you said, but just for fun I'd put a mark on the plug where the electrode is, then insert it in the cylinder it came from. If the electrode lines up in the position closest to the piston I'd get a 'scope and look in there for build up.

Yes, the electrode should be in fantastic shape if it wasn't firing correctly. Definitely no hot spark to do the eroding. This is another indication that the electrode was closed during the tune up, not later.

Miles71

Carburetor update 2,000 miles later:

It has been 2,000 miles since I last touched my 650 Quadra Jet. The engine runs great, hot and cold starts are usually the first try (30F to 95F). Acceleration is smooth from Stop to highway speeds (using Gulf 87-octane).

Original problems: Low Vacuum, Off idle Bog/Hesitation and Backfires.

First I had the carburetor rebuilt at Hobs Carburetors in Pelham NH. The Carburetor had 20k original miles and had never been opened in 41 years. The carburetor sill had the plastic caps covering the Idle Mixture screws. The rebuild gave me a good stable starting point.

Backfires:
This was caused by a damaged sparkplug. One of the plugs installed in the late 70's had the wrong gap. Seven of the plugs were gapped correctly between .030 - .035. The bad plug was gapped at .018.
I installed a new set of champion plugs gapped at .040 and have not had any backfiring issues since. Still running a stock points and original coil.

Low Vacuum:
All of the vacuum hoses were 41 years old. I replace about 80% of the vacuum hoses under the hood. I had the typical problems including:
Hard Brittle and cracked hose ends.
Loose fitting hoses.

It is hard to measure if there were any notable improvements after the hose replacements. The air cleaner "Thermal air intake vacuum sensor", starts to leak above 5 inches of vacuum. I tried two different sensors, with same results, so I assume this is normal.

Off idle Bog:
It took me a while before I figured out that the (2)-Idle Mixture screws and (1)-Idle-speed-screw work tougher as a Ratio, like a see-saw. If you do not get the ratio correct, the system is not balanced.
I will cover the changes I made to the screws in a separate post.

After I found a good setting for all three screws I ended up with a Vacuum of 17.5 at 750-800 RPM idle (Park, engine stock 350, and 8.5:1 CR, Single exhaust, 180 HP Net).
The car accelerates smoothly from Idle to speed without any hesitation.

Millage:
I do not have very many data points in this area since I am doing mostly city driving and a frequent WOT to enjoy the sound of the Quadra-Bog.
On the first drive (before the carburetor rebuild), I drove from New York to Boston in one evening, 250 miles (99% highway). Speed 55-70, Air conditioning on 50% of the time, Millage 16 MPG.
Last week's millage test with the current carburetor settings, delivered 15.5 MPG. This was a short drive 150 miles, 80% highway, with Air conditioning on 75% of time. I did 1 or 2 WOT runs of 10-15 seconds each.
With the limited data, I can assume that with the Air conditioner OFF and if I keep the RPM's below 2,500, I should be able to get 16 highway easily.

Thanks everyone for your suggestions.