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.

dartnlo said:

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.

View attachment 1715320702

View attachment 1715320703

close-up of number 5 & 7 plugs for comparison.
View attachment 1715320704

Click to expand...

"7 is a problem. The reason the plug in #7 looks the way it does is because # 7 in not firing. And yes the oil you put in #7 could have brought that the compression back up in the cylinder enough for it to start firing again. And now you are back to where you were. Dead hole on # 7. Gonna have to fix it. May as well freshen up everything while your there.

Rmoore said:

"7 is a problem. The reason the plug in #7 looks the way it does is because # 7 in not firing. And yes the oil you put in #7 could have brought that the compression back up in the cylinder enough for it to start firing again. And now you are back to where you were. Dead hole on # 7. Gonna have to fix it. May as well freshen up everything while your there.

Click to expand...

Thanks!
Good to have other voices backup the voices in my head. Looks like this is going to be happening sooner than I had planned. It needed a new motor mount anyway

#7 not firing has nothing to do with the sudden change in idling problems. It will miss but should just drop some RPM's at idle, not stop idling at low RPM's. If you pull a spark plug wire off of 1 of 8 cylinders, the engine will continue to run and idle at low RPM's.

My suspicion: It sounds like your idle circuits in the carb are plugged. If so, it would accelerate OK with moderate to hard throttle (on the accelerator pump, mains and power valve), and cruise OK on the main circuits. And running OK for the first few minutes is likely due to the choke being engaged and putting in extra fuel, to cover a plugged idle circuit. The idle circuits here include the transition circuits, which are used for the throttle blips when it dies.

I'd start by checking the float level. Not sure why it would change, so probably not that, but almost everything is effected by from the float level.

nm9stheham said:

#7 not firing has nothing to do with the sudden change in idling problems. It will miss but should just drop some RPM's at idle, not stop idling at low RPM's. If you pull a spark plug wire off of 1 of 8 cylinders, the engine will continue to run and idle at low RPM's.

My suspicion: It sounds like your idle circuits in the carb are plugged. If so, it would accelerate OK with moderate to hard throttle (on the accelerator pump, mains and power valve), and cruise OK on the main circuits. And running OK for the first few minutes is likely due to the choke being engaged and putting in extra fuel, to cover a plugged idle circuit. The idle circuits here include the transition circuits, which are used for the throttle blips when it dies.

I'd start by checking the float level. Not sure why it would change, so probably not that, but almost everything is effected by from the float level.

Click to expand...

Ok, thanks for the reply. A quick question about the transition circuit. Is there supposed to be a ball in the power piston cylinder under the spring or just under the accelerator pump and idle pickup tube/venturi cluster? I've noticed that when it is idling the gas coming out of the venturi is spitting fuel, I'm guessing that it is supposed to be more of a mist. So maybe I need to inspect the idle fuel pickup area. I'm still learning about these carburetors.

I believe there should be no fuel coming out venturies at idle unless choke is stuck on holding throttle slightly open or idle setting is holding the throttle open too far or float setting high correct me if I am wrong

teringer said:

I believe there should be no fuel coming out venturies at idle unless choke is stuck on holding throttle slightly open or idle setting is holding the throttle open too far or float setting high correct me if I am wrong

Click to expand...

Exactly

teringer said:

I believe there should be no fuel coming out venturies at idle unless choke is stuck on holding throttle slightly open or idle setting is holding the throttle open too far or float setting high correct me if I am wrong

Click to expand...

Yes, absolutely.... no fuel out of the venturis at idle. (They're 'auxiliary venturis' actually). It all should be a 'mist' out of the idle port and out of the transition port/slot at fast idle.... pretty much invisible.

Stuck float? Power/metering piston stuck in the up position or the vacuum source to the metering pistons plugged?

Here is a good idle circuit diagram that I think applies here:
2bbl Carter won't idle - Slant Six Forum

You're getting past my BBD knowledge now!

I didn't address your idle problem at all. Your idle problem is the least of your worries anyway. Your gonna have to fix the compression first. Im thinking once you freshen up the rings and valves all this goes away with your ldle problem anyway. Probably wants happening is the total condition of the engine is weak and all the rings and all the valves are not up to par. In total when one cylinder went totally dead that put enough strain on the rest of the engine. When you put oil in cylinder #7 it went to idling ok again. That's because the cylinder came back for a few minutes. The vac signal from the engine to the carb is weak thus causing the carb not to get the correct amount of vac signal thus causing the engine not to idle properly. The carb is not stopped up. How I know this you said it ran fine when you put oil in #7 to make it fire again. It's no big deal. 99% of the time a dead will not cause you a real bad idle. But I think in your case. I thank the total condition of your engine is causing you a to have a bad signal. Carb can not and will not operate correct if its not getting the right signal from your engine.

From what I can see, BTW< the #7 plug may be oil fouled... not quite a good enough pix. if so, then one temp fix for the plug oiling is an adapter that helps limit fouling. It doesn't fix the real issue (rings or bores) but is a way to make it fire better for a while. And BTW, 85 psi compression dry is not a 'dead' cylinder, but is getting pretty weak.

With this engine being all original, sticking piston rings is a possibility, Maybe a teaspoon of Marvel Mystery Oil in that cylinder, and sitting for a day, or an application of Rislone, might help. Probabilities are not high that this is the issue, but does not hurt to try.

BTW.... 25 in vacuum at idle is super high.... actually too high. Sure you are reading the gauge in inches of vacuum?

teringer said:

I believe there should be no fuel coming out venturies at idle unless choke is stuck on holding throttle slightly open or idle setting is holding the throttle open too far or float setting high correct me if I am wrong

Click to expand...

toolmanmike said:

Exactly

Click to expand...

nm9stheham said:

Yes, absolutely.... no fuel out of the venturis at idle. (They're 'auxiliary venturis' actually). It all should be a 'mist' out of the idle port and out of the transition port/slot at fast idle.... pretty much invisible.

Stuck float? Power/metering piston stuck in the up position or the vacuum source to the metering pistons plugged?

Here is a good idle circuit diagram that I think applies here:
2bbl Carter won't idle - Slant Six Forum

You're getting past my BBD knowledge now!

Click to expand...

The dribbling may be when I have the idle speed screw adjusted to keep it running.

Thanks for that diagram, I think that perhaps my throttle plates aren't closing all the way. I'll have to check when I get home tonight.

Rmoore said:

I didn't address your idle problem at all. Your idle problem is the least of your worries anyway. Your gonna have to fix the compression first. Im thinking once you freshen up the rings and valves all this goes away with your ldle problem anyway. Probably wants happening is the total condition of the engine is weak and all the rings and all the valves are not up to par. In total when one cylinder went totally dead that put enough strain on the rest of the engine. When you put oil in cylinder #7 it went to idling ok again. That's because the cylinder came back for a few minutes. The vac signal from the engine to the carb is weak thus causing the carb not to get the correct amount of vac signal thus causing the engine not to idle properly. The carb is not stopped up. How I know this you said it ran fine when you put oil in #7 to make it fire again. It's no big deal. 99% of the time a dead will not cause you a real bad idle. But I think in your case. I thank the total condition of your engine is causing you a to have a bad signal. Carb can not and will not operate correct if its not getting the right signal from your engine.

Click to expand...

I'll apologize for saying this as I do not want to offend: Not making sense to me..... 125-140 psi compression is pretty good for a stock '72 318. And if the vacuum is above 20" (questioned above), then the valves are sealing decently at idle at least. You are VERY likely correct that the engine is indeed well worn but, idle should be OK with those compressions and a high vacuum signal. BTDT on a worn /6 with almost .030" cylinder wear to #1, sticking rings, and massive sludge all over.... idled fine with the carb correct.

nm9stheham said:

From what I can see, BTW< the #7 plug may be oil fouled... not quite a good enough pix. if so, then one temp fix for the plug oiling is an adapter that helps limit fouling. It doesn't fix the real issue (rings or bores) but is a way to make it fire better for a while. And BTW, 85 psi compression dry is not a 'dead' cylinder, but is getting pretty weak.

With this engine being all original, sticking piston rings is a possibility, Maybe a teaspoon of Marvel Mystery Oil in that cylinder, and sitting for a day, or an application of Rislone, might help. Probabilities are not high that this is the issue, but does not hurt to try.

BTW.... 25 in vacuum at idle is super high.... actually too high. Sure you are reading the gauge in inches of vacuum?

Click to expand...

I'm going to try the stuck ring solutions just to see if I can limp along for the summer.

This is just a plain old vac/pressure gauge. I just know it is at 25, probably because my idle is so high. When I was able to get it down to 775 rpm it was closer to 20.

dartnlo said:

I'm going to try the stuck ring solutions just to see if I can limp along for the summer.

This is just a plain old vac/pressure gauge. I just know it is at 25, probably because my idle is so high. When I was able to get it down to 775 rpm it was closer to 20.

Click to expand...

Ok that might explain the high vacuum number... 20-21" makes sense for a normal, stock-engine idle, with no vacuum leaks and decent intake valve seating.

If you want to try the Rislone-dribbled-slowly-down-the-carb at a 2000 RPM idle trick to try to help the rings, the neighborhood kids will love you for smoking up the whole area LOL

nm9stheham said:

Ok that might explain the high vacuum number... 20-21" makes sense for a normal, stock-engine idle, with no vacuum leaks and decent intake valve seating.

If you want to try the Rislone-dribbled-slowly-down-the-carb at a 2000 RPM idle trick to try to help the rings, the neighborhood kids will love you for smoking up the whole area LOL

Click to expand...

My girlfriend was probably enjoying last night's shenanigans just from burning off the oil in the cylinder after testing. She had the front door screen open. Smelled great in the house!

Ok, so I did mechanic in a can and hot soaked the bad cylinder for 2 days.


After seeing this carb diagram I decided to try something. Carb cleaner on a new carb.


So I puffed the excess sea foam through the spark plug hole and put the plug back in and started it up. I mosquito fogged my neighborhood on my way out of town. I discovered immediately that it was running decent and idling in drive at stop lights but I couldn't ignore the obvious miss. I was worried that I had somehow hydrolocked the rings into worse shape. So about a mile into the test/smoke-clear drive I pulled over because I remembered that I didn't hook the spark plug back up. I hooked it back up and the miss went away.

I drove out on the highway for about 10 miles and headed back home. I checked the compression and it only mildly improved to a little over 95, but since it was idling I decided once again to tune it up. I actually got it to idle down to 600 even in gear. There is still something wrong with the carb in general. I guess I never said before but this is a carter clone from Amazon. It idled best with the idle mixture screws out about 8 turns... As far as my vacuum being so high, well I figured that out as well:



Not sure how or when that happened because it was working before. So using 6 as my zero my vacuum was showing about 19 at idle now.

Long story just a little longer, I would say the carb cleaner did more to fix my issue than the sea foam. I doubt that my 10% increase in compression on #7 gave me my idle back. I need to go ahead and get a decent carburetor.

Thanks everybody for your input. Thanks especially to @nm9stheham for the link with the carb diagram that really helped my understanding of what was going on in there.

dartnlo said:

Ok, so I did mechanic in a can and hot soaked the bad cylinder for 2 days.


After seeing this carb diagram I decided to try something. Carb cleaner on a new carb.
View attachment 1715321878

So I puffed the excess sea foam through the spark plug hole and put the plug back in and started it up. I mosquito fogged my neighborhood on my way out of town. I discovered immediately that it was running decent and idling in drive at stop lights but I couldn't ignore the obvious miss. I was worried that I had somehow hydrolocked the rings into worse shape. So about a mile into the test/smoke-clear drive I pulled over because I remembered that I didn't hook the spark plug back up. I hooked it back up and the miss went away.

I drove out on the highway for about 10 miles and headed back home. I checked the compression and it only mildly improved to a little over 95, but since it was idling I decided once again to tune it up. I actually got it to idle down to 600 even in gear. There is still something wrong with the carb in general. I guess I never said before but this is a carter clone from Amazon. It idled best with the idle mixture screws out about 8 turns... As far as my vacuum being so high, well I figured that out as well:

View attachment 1715321876

Not sure how or when that happened because it was working before. So using 6 as my zero my vacuum was showing about 19 at idle now.

Long story just a little longer, I would say the carb cleaner did more to fix my issue than the sea foam. I doubt that my 10% increase in compression on #7 gave me my idle back. I need to go ahead and get a decent carburetor.

Thanks everybody for your input. Thanks especially to @nm9stheham for the link with the carb diagram that really helped my understanding of what was going on in there.

Click to expand...

Your vacuum gauge is broken and shouldn't be trusted.

toolmanmike said:

Your vacuum gauge is broken and shouldn't be trusted.

Click to expand...

Right?! So much for this harbor freight purchase! They had good reviews...

should be a screw to adjust in to zero under the face

If you leave that type of a gauge in the Sun or somewhere really hot it will not be accurate anymore.

Live and learn

teringer said:

should be a screw to adjust in to zero under the face

Click to expand...

Yeah, I adjusted it back to normal and checked it with my vacuum pump that has a gauge on it as well.

Funny how those spark plug wires being connected helps cures misses LOL. Good story.

8 turns out on the idle mix screw? Like 8 FULL turns? Some thing is sure messed up...did I just win the Captain Obvious award? The idle mix screw is almost falling out, and may be admitting air if that far out. So almost like there is way too much fuel (power piston up and the rods all the way up, float way off, needle/set damaged or filthy, float gas-logged and sinking) or the idle air jets are messup.

nm9stheham said:

...wires being connected helps cures misses LOL...

Click to expand...

Been there!
FWIW, You might verify the firing order. I've seen 5 and 7 on a BB have the wires swapped and engine still run but with lack of power. Also make sure you don't have a bad plug wire on 7 as well.

nm9stheham said:

Funny how those spark plug wires being connected helps cures misses LOL. Good story.

8 turns out on the idle mix screw? Like 8 FULL turns? Some thing is sure messed up...did I just win the Captain Obvious award? The idle mix screw is almost falling out, and may be admitting air if that far out. So almost like there is way too much fuel (power piston up and the rods all the way up, float way off, needle/set damaged or filthy, float gas-logged and sinking) or the idle air jets are messup.

Click to expand...

I would say more like 8 - 3/4 turns. Just from memory I got to the point where I was just turning as far as my fingers would let me. About the time I felt less tension on the spring is where it got better. Funny thing is that what started me down the path to a second, yes second chinese "Carter" was I was rebuilding my original and the idle mixture screw snapped off and I couldn't extract it. These chinese Carter clones are barely decent. Both of them had floats that leaked and sunk within 3 weeks of use. I got this one because it had more correct ECS ports. I've noticed while tuning (and this may be normal) that as I reduce the idle speed screw, it starts to whistle or whine as the throttle plate closes. At that point opening the idle mixture screws stops that whine. Seems to make sense to me as at idle it has to pull air from somewhere.

I'm looking for a better carb right now

Daves69 said:

Been there!
FWIW, You might verify the firing order. I've seen 5 and 7 on a BB have the wires swapped and engine still run but with lack of power. Also make sure you don't have a bad plug wire on 7 as well.

Click to expand...

Luckily mine was only unplugged for about a mile.