Cylinder compression check numbers and spark plug reading inconsistent idle
1972 Dart Swinger 318
109K miles stock, probably original everything.
Carter 2 barrel bbd
New spark plugs 1500 miles ago
about 6 weeks ago I had it in the best tuned state since I got it in October. It idled great, took off great and all was well in the world. I was driving it almost daily, then we had a cold streak and when I went to drive it again it ran good for the first couple of miles and then it died at a stop light. It would still run ok, take-offs were good and it ran down the highway fine.
I started messing with the idle settings but it would only keep running at about 1300 rpm, obviously not a good place to be. I let her sit for about the last two weeks.
Yesterday I took the carburetor apart to see if it was clogged or anything. Except for part of the accelerator pump rubber laying in the bottom of the bowl nothing was wrong. I put a new pump in and reassembled and started to tune it again.
Pretty much the same story, with all of the vacuum ports plugged it would read about 25 steady on the vac gauge at 1000. It would idle but if you flicked the accelerator it would stumble and die.
I have about 20 steady degrees of advance set with the vac advance unplugged. I couldn't find any vacuum leaks around the carb or intake manifold with the carb spray method.
So tonight I finally decided to pull out the compression tester and got these results:
#1 - 120
#3 - 135
#5 - 135
#7 - 85
#2 - 140
#4 - 140
#6 - 130
#8 - 130
#7 spark plug was the only one with oil fouling. Not sure how to read the look of the other plugs but they seem fairly normal considering I've sunk a float on these plugs - twice.
I put about an ounce of 10W30 down #1 and #7.
#1 went up to about 155
#7 went up to 125 on three cranks.
So it seems like rings on #7. Would this cause my issue?
I put about 3 oz of sea foam into #7, cleaned the spark plug up and put everything back together. I was able to get a good tune within 3 minutes. I got it to idle smoothly at 785 with 19 degrees of advance. I could flick the throttle and it would settle back down, I got in the car, put it in drive and sort of power braked it without spinning the tires. Again it would settle down without dying.
I let it run about another 6 minutes driving it back and forth in the driveway. Then it started idling badly again. Right back where I was. I wonder if the oil I put down #7 gave me enough compression to get it tuned up and when it warmed up completely and the oil burnt off it made it run rough again.
Mostly just wondering if I am on the right track thinking my idle troubles are from #7's low compression, is 85 low enough on one cylinder to cause this? Also wondering what would cause it to do this all of a sudden.
Here are some pictures of the spark plugs, I'm interested in any theories or additional information.
close-up of number 5 & 7 plugs for comparison.
1972 Dart Swinger 318
109K miles stock, probably original everything.
Carter 2 barrel bbd
New spark plugs 1500 miles ago
about 6 weeks ago I had it in the best tuned state since I got it in October. It idled great, took off great and all was well in the world. I was driving it almost daily, then we had a cold streak and when I went to drive it again it ran good for the first couple of miles and then it died at a stop light. It would still run ok, take-offs were good and it ran down the highway fine.
I started messing with the idle settings but it would only keep running at about 1300 rpm, obviously not a good place to be. I let her sit for about the last two weeks.
Yesterday I took the carburetor apart to see if it was clogged or anything. Except for part of the accelerator pump rubber laying in the bottom of the bowl nothing was wrong. I put a new pump in and reassembled and started to tune it again.
Pretty much the same story, with all of the vacuum ports plugged it would read about 25 steady on the vac gauge at 1000. It would idle but if you flicked the accelerator it would stumble and die.
I have about 20 steady degrees of advance set with the vac advance unplugged. I couldn't find any vacuum leaks around the carb or intake manifold with the carb spray method.
So tonight I finally decided to pull out the compression tester and got these results:
#1 - 120
#3 - 135
#5 - 135
#7 - 85
#2 - 140
#4 - 140
#6 - 130
#8 - 130
#7 spark plug was the only one with oil fouling. Not sure how to read the look of the other plugs but they seem fairly normal considering I've sunk a float on these plugs - twice.
I put about an ounce of 10W30 down #1 and #7.
#1 went up to about 155
#7 went up to 125 on three cranks.
So it seems like rings on #7. Would this cause my issue?
I put about 3 oz of sea foam into #7, cleaned the spark plug up and put everything back together. I was able to get a good tune within 3 minutes. I got it to idle smoothly at 785 with 19 degrees of advance. I could flick the throttle and it would settle back down, I got in the car, put it in drive and sort of power braked it without spinning the tires. Again it would settle down without dying.
I let it run about another 6 minutes driving it back and forth in the driveway. Then it started idling badly again. Right back where I was. I wonder if the oil I put down #7 gave me enough compression to get it tuned up and when it warmed up completely and the oil burnt off it made it run rough again.
Mostly just wondering if I am on the right track thinking my idle troubles are from #7's low compression, is 85 low enough on one cylinder to cause this? Also wondering what would cause it to do this all of a sudden.
Here are some pictures of the spark plugs, I'm interested in any theories or additional information.
close-up of number 5 & 7 plugs for comparison.
