Some troubleshooting ideas please.

Actually I plan to do as several have suggested. I’ll take her out and run her hard then pull over and pull a plug. Stand by

 

Put in new coil and test voltage, then new plugs and idle, my bet is still on SPARK!

All this wanting go drive and then check won’t solve anything , I’m betting because you’ll still see un-burned fuel until you fix something.

I’m gonna beat this SparkDog until you get 12v. At coil.

 

If it does not shoot lightning bolts 1/2 inch it is the coil

 

If at this point the carb is still making for the black plugs then I would check this out about the carbs function.
Get the motor warmed up enough to have the choke off and having a stable idle.
Now look down into the primary side of the carb and into each throttle bore .

you should see no fuel dribbling out of the small angle cut brass tube in the top of each booster.

if you do then the carb still has a issue because the fuel that the carb idles on does not come from there.
If your seeing fuel from there then there’s a couple of things that could be at play here.

1) the power piston is hanging up in its bore due to crud and the engine vacuum is not enough to over come that and allow the primary metering rods to seat.

2) the power piston spring is the wrong one ( too much tension) or it got stretched out, or your motor has a vacuum leak ( low vacuum) or the cam has been changed and now the vacuum is too low for that power pistons spring tension and you need a softer one.

Q-jets have 3 different tension springs that can be had.

3) some passages in the the primary side of the carb are clogged .

PS.
if you can with the motor warm run both idle mixture screws all the way in and the motor does not die, then your for sure getting extra fuel from the carb at idle that the motor does not need.

 

With the Lectric Limited conversion they specify the resistive ignition wire must be used and they want to see 8 volts after the motor starts. Mine is about 8.5.

 

Put the points back in..... Or hop it up to 14.4vts.

 

So, I ran all the carb tests suggested and seems the carb is OK. Power piston is all the way down at idle and p0ps up when the throttle is zinged so the vacuum goes down. Turned upside down the needle seals fuel from getting past the float. No obvious dripping in the primaries and idle is smooth. Vacuum is steady at 14.5 HG.

But, I may have gotten some water in the tank during my fill-up the other day. I added several bottles of HEET to the tank and, after calling Hagerty to make sure I have towing on my policy, I went out for some high speed operation. Ran up through the gears several times to about 5800 rpm and put about 15 miles on the car. On the last pass up into the triple digits I pulled into our Publix Grocery parking lot and shut it down. Pulled the #1 plug and here's a comparison of the same plug after idling cold in the garage vs. after my drive today. Never knew plugs could load up that bad sitting still.


So, just in case there is an intermittent problem in the future, I'll be buying both a spare QJ and a spare distributor. And the distributor won't be the Lectric Limited single wire. I'm thinking of either the MSD Pro Billet or one of the Pertronix offerings. But, for now the problem is gone.

 

There's something seriously wrong. First guess would be idle fuel mix, although it could be something else.

 

That plug is too hot of a heat range, you need to drop down one step.

 

I’d change your oil too if you haven't done so.

 

Ray, I changed the plugs in my 69 375 hp l78 chevelle which is all stock except for the pertronix in the distributor. Those engines came with a dual line holley not a qj. You know how far it is from my house to the restaurant that we ate at "4 miles with some hwy speed and some city. Straight there and Straight back.
The plugs look exactly like your idle plugs.

 

Forgot, the needle and seat were put in new prior to changing plugs.

 

Just a point of clarification, Ray bought a carb that Fred Catlin had rebuilt/modified from a board member. Ray asked me to go back through it and inspect it. I cleaned it the best I could and jetted it for what I thought his combo needed. I also ran it on my GS and had no issues. I sent a couple videos to him, idle and wot. I also sent a second (larger seat with the carb). Per his request I mailed to the address he gave me. A week or so later I was told it was taken apart, cleaned and rejetted. I have no idea of its current configuration.

 

Rob is correct. I had him mail the carb to Jim Weise who built and knows my motor well. Jim ran it on a similar motor as mine on his dyno to tweak the jetting a bit and sent on to me. It has run fine on my motor ever since. It actually resolved some idling problems I had been having with the carb that was on my motor originally. This was about a year and 1/2 ago. I don’t believe the carb had anything to do with the current issue for which I began this thread and have every confidence that both Rob and Jim’s work on the carb was top notch.

 

Why would you do that?
That’s a lot of money tied up in parts that aren’t being used for a “what if” situation.
May as well have a spare of everything, LOL
My point is, these older cars are just as reliable, maybe even more so than newer cars, and they will break, just like newer cars, fix it, and drive it.
I don’t even carry a screwdriver with me in my GS
Last major malfunction I had on my GS was the shift cable popped off the trans. linkage on my way to the racetrack, who woulda thought of a breakdown like that and be prepared for it???
I put it in drive from under the car, and drove it 40 miles home and fixed it.

 

What’s the Voltage from the coil to the distributor? And the Ohms at the coil?
My Petronix 3 requires a different coil and produces 40,000 v. Instead of the stock coil which makes only 28,000 v.

The 70 Black GS was fouling plugs and would not run a 1050 cfm on a built 470, put on a 950 same problem, even tried changing jets, we found the culprit was the coil only getting 8.5 volts and spark looked like yours very weak, we changed coils and the spark became a lightning bolt nice and blue and big, solved it.

 

That white plug looked too hot
www.dragstuff.com how to read plugs good stuff there to look at on plugs.

 

Thank you Ray. Rereading our texts I saw where we discussed and I cautioned you about seat size and fuel pressure. You may want to confirm which seat you are currently running since I sent two and it was taken apart. My opinion is any seat > 0.140” used with a RobbMC might be problematic.

 

Thanks for your comments, Larry. My last two GSs have had TA aluminum heads and I have never used anti-seize on either. As you said, it's not necessary any longer with plated threads.
I haven't experienced any issues pulling a plug on a hot motor. At least that's my experience but, I seldom do it except in situations where I want to take look after a high speed pass.

As for my choke, I have it set to only engage on the smallest edge of the cam and it goes off very quickly. But, I think I'll disengage it just to see how the motor runs when it's cold.