Dear all,
I took the '67 Deville for a drive yesterday. Car started normal after sitting a few weeks. Drove for about ten miles to the gas station to add a out 10 gallons. Then drove it another 30 miles or somwehere she ran perfectly fine.
I parked it to go and het something to eat. It sat for about an hour. When I came back to drive home, she started fine, but the idle was a little lumpy. Opened the hood, removed the air cleaner to see what was going on and when blipping the throttle she backfired through the carb. Weird. Drove it back home, but idle stayed lumpy like it is running on 6 or 7 cylinders when idling. Off-idle and power is normal. I floored(Italian tune up) it to clear out a potential blocking of an idle port, no luck.
I am wondering what's going on?
Things that i've tried:
- Remove spark plug wires one by one to see if there was an idle change: it appeared that the idle did not change when removing spark plug wires of the 3rd and 5th cylinder. When removing 1,7,2 or 8 there was a clear change.
- Checked for vacuum leaks but there appear to be none.
- Covered the front of the carb to see if the idle would increase, indicating a vacuum leak. No change. Only when almost completely covering the intake with a gloved hand would make it change and eventually stall.
- checked the dwell angle to check if something was going on with the points, but the dwell was at a perfect 30 degrees.
Here is a link to a video where it is idling : https://drive.google.com/file/d/1MWpByfKOZBF22mdLzyyAADafiaes3ps2/view?usp=drivesdk:
I am somewhat stumped. What is going on all of a sudden?
I would be looking at Spark Plug Numbers 3 and 5 to see if there is any fouling.
Plus, putting a Vacuum Gauge in a Vacuum line and see what the movement of the needle is. Below is a link to the diagnosis chart of the internet.
Bruce. >:D
Try Seafoam. Treat the oil and clean the carburetor per instructions.
I might suggest trying a dose of Techron. I'll circuits in the Q-jet seem to be the first to clog up
Greg Surfas
Update:
I checked all the spark plugs and they all looked fine. Checked all the gaps while I was at it, and they were all within spec.
Did a compression test as well, just to rule things out. All but cylinders 4 and 6 showed between 165 and 175 psi of pressure. Numbers 4 and 6 respectively showed 190 and 205 psi.
This seems a bit odd, but I am not sure if it could cause bad idling.
Took off the carb and took it apart. Carb looked clean inside, no gummed up fuel. Gaskets were all in very good shape. including the gaskets between intake and carb.
I rebuild the carb a couple of years ago. I used a can of brake cleaner to blow through all the (idle) passages. The only thng I did not do was take out the idle mixture screws, because I did not want to mess with their setting.
Tomorrow I will put the carb back on, start her up and check with a vacuum gauge to see if that will give any insights.
When doing the Compression Test, did you do it just dry?
If so, then you need to do the Wet one to rule out Leaking Valves and Rings.
Bruce. >:D
Bruce: "Plus, putting a Vacuum Gauge in a Vacuum line and see what the movement of the needle is. Below is a link to the diagnosis chart of the internet."
Vacuum gauge is a must use tool, especially with this era Cadillac.
I've discovered vacuum leaks that amazed me... vividly comes to mind is the repurposed tin can (vacuum reservior) that developed a split due to contraction and expansion.
Another challenge was a small plastic vacuum junction that developed a tiny split and thus a leak. Another vivid frustration involved clogged idle jets
If your jets don't affect any change in operation, then there's a good chance that you've got fuel separation gunk in the idle passages. Using compressed air
blasted through those idle jet adjustment screws worked to remove the gunk...sprays didn't work! ALso, check your
vacuum advance --- most overlooked part in an ignition system, and easily checked for operation. Your compression is excellent... uneven readings may have to do with
how you performed the test...i.e. warm engine, throttle wide open. Those are
good numbers. Hope this helps, James
One thing to check is the checkvalve on the brake booster mine cracked one day and it ran like crap. All the standard vac checks failed and I never thought to look there until I had a rock hard pedal-- But that didn't show up immediately.
Whats the firing order and cylinder layout of the 429?
Do you have an endo/bore scope? Maybe since you have the carb off try to look down the intake to see if maybe some crap loosened up and is keeping a valve or two from closing?
Maybe do a quick check of the timing chain? Pull the distributor cap and rock the crank back and forth while watching the rotor to see how long it takes it to respond to direction changes? Did these have the plastic covered gears or did that start the next year? Even if its not plastic I have seen worn ones suddenly ride up and do further damage and suddenly get a lot looser. The popping though the carb means a valve was open when it should not have been so that's either cam timing or something keeping the valve from fully closing/seating.
And not related to the car, Bruce was the last person I would have suspected to do a link wrong and mess up the formatting. Can you please fix it? Those long links make it super difficult to read and reply to the thread.
I just had this happen with my 69 deVille and it ended up being the points.
Quote from: TJ Hopland on July 25, 2025, 12:27:24 PM......And not related to the car, Bruce was the last person I would have suspected to do a link wrong and mess up the formatting. Can you please fix it? Those long links make it super difficult to read and reply to the thread.
Oops, sorry for that. But have fixed it. And chastised myself severely.
But, in my defense, when I compiled it, and before I hit the Post button, it was in its' compressed state. Don't understand why it automatically expanded.
Bruce. >:D
Update (and I think fixed):
So, after checking the carb (which was clean), the compression (with a new gauge as the old one fell on the floor and broke the glass; dry and wet: all cylidners were in the 165/170 range dry and 180/190 range wet except for cylinders 4 and six which had higher compression (around 180 dry and over 200 wet) which I believe are good compression rates).
Hooked up the vaccuum gauge, ran her until she was warm, and low and the vacuum reading was low (hovering between 14 and 16). In the meantime also checked ignition timing which was at about 6 degrees btdc.
I put the vacuum gauge in the brake booster vacuum line, so I could rule out that as a cause. Next step was to unplug PCV valve (and block that port with a bolt in the hose), plugged the vacuum advance, and the connections to the vacuum canister and to the transmission. She started to run better with a vacuum reading of about 19.
I found that the vacuum hose at the transmission was quite loose and had come off. So I cut a small piece from it so there is a "fresh" connection to the vacuum switch in the trans.
I adjusted the mixture screws on the carb a little bit and got to 20 inches of vacuum on the gauge with a good idle.
So I believe the problem was as simple as a loose vacuum hose....
Stupid of me, I should have checked that first before taking off the carb. Oh well, learning by doing.
Thanks for the help anyway! :).
Thanks for the update and closing the loop on this.