Low compression in one cylinder

[Abchoppers]

1956 Mercury 312, around 61,000 miles. I don’t know if engine has ever been rebuilt. I pulled a plug to see how they look and it had oil on the threads. I pulled all of them and all ended up having oil on the threads. That led me to perform a compression test. All but one cylinder had good compression :
1 155
2 150
3 150
4 145
5 150
6 135
7 90
8 145

I did the test on a cold engine. I removed the valve cover on the drivers side after getting the number 7 cylinder reading of 90. All of the rocker arms were moving and there was plenty of oil.

So what can cause one cylinder to be low? I’m sure there are a few things. Does any of this explain the oil on the plug threads?

The car runs great, plenty of power but does have a slightly rough idle sometimes. It has a 57 intake and distributor, a PCV valve, new Holley 4160 carb.
Thanks all,
Ryan

[cadillac512]

Usually a valve isn't sealing properly. I'd check the valve adjustment paying particular attention to #7. Likely an exhaust valve but not always. If it's not a too-tight valve, the valve may be leaking due to a burned face.

It's also possible the low compression is due to a ring problem, but then you should see excessive blow-by from the crankcase breather (and it would pulse with the weak cylinder firing).




Terry

[cadillac512]

The oily threads isn't necessarily cause for concern. It happens fairly often on engines that are just fine.



Terry

[Flathead Fever]

Squirt some oil in the cylinder and if the compression comes up it's the rings, the oil helps make a temporary seal. While its running pull the plug wire and if you notice an rpm change that cylinder still has enough compression to fire. Do not pull the wire if it has electronic ignition, only if it still has points. It can damage an ignition module. If it's still firing I wouldn't worry about it. You can also get the piston up on TDC and take the compression tester hose and hook it to a compressor and pressurize the cylinder. Listen for escaping air int o the exhaust pipe, carb or crankcase. Make sure the valves are opening and closing the same amount as the other cylinders. Also put a vacuum gauge on the engine and check for a steady needle.

[Abchoppers]

Originally Posted by cadillac512

Usually a valve isn't sealing properly. I'd check the valve adjustment paying particular attention to #7. Likely an exhaust valve but not always. If it's not a too-tight valve, the valve may be leaking due to a burned face.

It's also possible the low compression is due to a ring problem, but then you should see excessive blow-by from the crankcase breather (and it would pulse with the weak cylinder firing).




Terry

I wanted to check the valve adjustment anyway so I’ll do that a definitely pay attention to number 7
Thank you

[Abchoppers]

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Originally Posted by cadillac512

The oily threads isn't necessarily cause for concern. It happens fairly often on engines that are just fine.



Terry

Good to know, thank you

[Abchoppers]

Originally Posted by Flathead Fever

Squirt some oil in the cylinder and if the compression comes up it's the rings, the oil helps make a temporary seal. While its running pull the plug wire and if you notice an rpm change that cylinder still has enough compression to fire. Do not pull the wire if it has electronic ignition, only if it still has points. It can damage an ignition module. If it's still firing I wouldn't worry about it. You can also get the piston up on TDC and take the compression tester hose and hook it to a compressor and pressurize the cylinder. Listen for escaping air int o the exhaust pipe, carb or crankcase. Make sure the valves are opening and closing the same amount as the other cylinders. Also put a vacuum gauge on the engine and check for a steady needle.

I read about the oil trick and I will try that next. It does have electronic ignition. As far as I can tell it is firing on all cylinders. My vacuum reads at a steady 20 pounds.
Thanks for the tips

[dmsfrr]

Originally Posted by Abchoppers

... My vacuum reads at a steady 20 pounds. ...

That should rule out a leaky intake valve, or the vacuum would pulse when that cyl fired.

[Abchoppers]

I did the compression test again on cylinder 7 after adding oil, no change. I did it a couple of times just to be sure. Like dmsfrr mentioned, since the vacuum is pretty steady at 20 to 22 inches, I’m leaning towards an exhaust valve like Terry first suggested. My next step is to check the valve lash and hopefully #7 is too tight. So here’s my questions -
Is it bad to drive the car with an exhaust valve not fully seating? If my valve adjustments are all good, is the only way to fix this is by pulling the head?
Thanks guys

[wellcraft17]

I had a 55 ford yblock with the exact same issue as you back in my college days, late 60s, 7 cylinders with 155-160 psi and one at about 90. I had bought the car from an older gentleman who had parked the car for approximately 10 years due to he could no longer drive. It ran ok with a slight miss at idle which really bothered me . So one day talking to an old ford mechanic explaining what my issue was he says that cyl probably has something stuck between the valve and head. Hooked up an air hose to the cyl and I could hear air leaking out of the exhaust valve, rusty carbon valve seat or burnt valve? He told me the cheap fix, I had no money nor the time to remove the head, would be to remove the valve spring, hook up a drill and spin the valve with an up and down motion with the hope if something was on the valve seat It would clean it off. That’s exactly what I did, reinstalled the spring and my compression came up to approximately 150 psi cold. Cheap fix, couple hours labor drove the car until the late 80s and sold it

[cadillac512]

Originally Posted by Abchoppers

I did the compression test again on cylinder 7 after adding oil, no change. I did it a couple of times just to be sure. Like dmsfrr mentioned, since the vacuum is pretty steady at 20 to 22 inches, I’m leaning towards an exhaust valve like Terry first suggested. My next step is to check the valve lash and hopefully #7 is too tight. So here’s my questions -
Is it bad to drive the car with an exhaust valve not fully seating? If my valve adjustments are all good, is the only way to fix this is by pulling the head?
Thanks guys

If adjustment doesn't cure it, the best way to be sure it's a valve is to pressurize the cyl with air and see where it comes out (piston at exactly tdc or valve adjusters on that cyl backed all the way off so valves can't open). Tail pipe, carburetor, or crankcase. ( though you'll always get some from the crankcase). If it's a valve there is a possibility it's as wellcraft said above, although the adjustment on that valve should have been loose when checked if something was on the seat. If it is coming from a valve and all easy efforts don't fix it, then yes the head must come off.
As for driving it, if a valve is beginning to burn it will continue to get worse until you notice a dead miss on that cyl. Usually it only damages the valve but the seat can suffer as well sometimes.



Terry



Terry

[Abchoppers]

Originally Posted by wellcraft17

I had a 55 ford yblock with the exact same issue as you back in my college days, late 60s, 7 cylinders with 155-160 psi and one at about 90. I had bought the car from an older gentleman who had parked the car for approximately 10 years due to he could no longer drive. It ran ok with a slight miss at idle which really bothered me . So one day talking to an old ford mechanic explaining what my issue was he says that cyl probably has something stuck between the valve and head. Hooked up an air hose to the cyl and I could hear air leaking out of the exhaust valve, rusty carbon valve seat or burnt valve? He told me the cheap fix, I had no money nor the time to remove the head, would be to remove the valve spring, hook up a drill and spin the valve with an up and down motion with the hope if something was on the valve seat It would clean it off. That’s exactly what I did, reinstalled the spring and my compression came up to approximately 150 psi cold. Cheap fix, couple hours labor drove the car until the late 80s and sold it

I love hearing stories like this! If it’s not a valve adjustment issue I hope that trick will work on mine. Thanks for sharing

[Abchoppers]

Originally Posted by cadillac512

If adjustment doesn't cure it, the best way to be sure it's a valve is to pressurize the cyl with air and see where it comes out (piston at exactly tdc or valve adjusters on that cyl backed all the way off so valves can't open). Tail pipe, carburetor, or crankcase. ( though you'll always get some from the crankcase). If it's a valve there is a possibility it's as wellcraft said above, although the adjustment on that valve should have been loose when checked if something was on the seat. If it is coming from a valve and all easy efforts don't fix it, then yes the head must come off.
As for driving it, if a valve is beginning to burn it will continue to get worse until you notice a dead miss on that cyl. Usually it only damages the valve but the seat can suffer as well sometimes.



Terry



Terry

I’ll try a pressure test after I check valve adjustment. Getting a remote starter delivered tomorrow to make the job easier. My 11 year old daughter is great at cranking for compression tests, but I think she’s getting tired of it haha
Thanks for the help

[miker98038]

Reminds me of trickling water down the carb at high idle to let the resulting steam clean the carbon out. Which might also be worth a try. Great story.

[Abchoppers]

Originally Posted by miker98038

Reminds me of trickling water down the carb at high idle to let the resulting steam clean the carbon out. Which might also be worth a try. Great story.

I actually just read about that for the first time yesterday when searching different threads, I thought it was a joke at first. I’ve also heard about using sea foam treatment as well. Hopefully I don’t need it but you’re right, might be worth a try
Thanks

[Flathead Fever]

I enjoy challenges like this, especially on someone else's engine. This is war, you never let the vehicle win no matter how much it costs and sleep you lose. That's good that the oil trick did not raise the compression, better a valve issue than a ring one. I bought a Case walk behind trencher that would not run. I squirted oil in the cylinder, and it fired right up and ran great. It ran until it stopped smoking. The oil was making a seal until it burned off. Bought a new piston and rings and that solved it.

A small head gasket leak could cause it, it usually causes white smoke when you first start it, and the exhaust would have a sweet smell to it. The plug would be shiny clean, the water steam cleans plug. It might run just fine and not overheat with a tiny leak. It would be losing coolant over time. The white smoke, sweet smell at fist startup and shiny plug would be the clue. A very small leak runs into the cylinder while itis sitting and then burns off when you first start it.

It could have a flat intake cam lobe not opening the valve as much as it should. Also, a bore scope down the intake to check for carbon deposits on the back of the valve would be a fun test. If the valve was leaking the vacuum gauge needle should flutter but you said its steady. Can you here any popping in the tailpipe or carb?

Are there any noisy lifters? a flat cam lobe would increase the clearance a lot and it would be making noise with a solid lifter. With hydraulic lifter engines the lifter takes up the clearance and you don't hear the noise. Yoo have to watch the rockers moving to see if one does not move as much. Something to watch if you are going adjust the valves.

Every time a piston creates vacuum it moves the vacuum gauge needle, together they are taking turns creating vacuum so fast that it shows up as a steady reading on the needle. If one cylinder fails to do its job the needle drops for a split second. Your cylinder is still creating enough vacuum to keep that needle steady. It's a lot harder to diagnose stuff like this that have not totally failed yet. I would get engine misfire codes on the Dodge 5.2L and 5.9L engine on the phone trucks and vans. They ran fine, I could not find any problem. Nothing showed up on a scope. compression was great. Everything looked and tested fine. You would never know there was a problem driving it except for that stored misfire code. I pulled the valve cover and rocker arms to the cylinder with the misfire code and put a straight edge across the valve tips and there was one valve sticking up a lot higher than the others, the straight edge rocked back and forth on it. The valve had sunk into the head so far it cracked the head, but it still ran fine. They did not have harden seats. If one cylinder is not producing the same power the rpm slows for a split second and the computer can see this. It knows where #1 is, between the crank and cam sensors it can calculate which cylinder caused the rpm to slow down. That speeds up the diagnsing time. We sent the Dodge heads out to have the cracks welded and hardened seats installed and they last longer than they did originally. At first I did not trust welding heads, but we never had a problem with any of them, better than they were new. We had so many of those engines and it started happening to all of them. If we got a misfire code, we pulled the valve cover first and that was almost always the problem. Even though the engines ran fine we had to fix them if we had a light on and code when we did the CA State Smog tests every two-year's. We were a licensed fleet test station ( I couldn't do my own vehicles which sucked, only the fleet vehicles) We kept a pair of heads and gaskets ready to go so we could swap them out at night and not have the vehicle out of service the next day. They were really big on not having more than 1% out of service at any one time in a yard with 400 vehicles.

Either the air is leaking out of the cylinder or there is not enough flowing into the cylinder to compress it to 150 lbs.

[Flathead Fever]

Cylinder Leak tests


Pull the plugs on either side of the cylinder you are testing. If you don't have a leak down gauge, you can take your compression tester hose and remove the shrader valve from it. Hook shop air to the hose and listen to where it goes This good way to check an engine out of a car your thinling of buying.

https://www.youtube.com/watch?v=og9nXfXSaoY&t=27s

https://www.youtube.com/watch?v=yhHv-AwbFiY

https://www.youtube.com/watch?v=NbxW6HRYHAo

[Flathead Fever]

Originally Posted by Abchoppers

I actually just read about that for the first time yesterday when searching different threads, I thought it was a joke at first. I’ve also heard about using sea foam treatment as well. Hopefully I don’t need it but you’re right, might be worth a try
Thanks

Carbon buildup on moder engines will raise the compression and cause a vehicle to fail a smog test for excessive NOX readings. Excessive pressure and heat combine the oxygen and nitrogen to create a NOX molecule, the brown haze in photochemical smog. A non-working EGR valve is usually the problem but when it was not, we poured water down the running engine to the point of almost drowning the engine from running (phone company trucks who cares). You don't want to pour so much you hydraulic the engine and break something. That steam cleaned off the carbon and would bring the reading down enough to pass. This engine we are working on here has a cylinder with low compression, excessive carbon on the piston is not the problem, it could use more carbon.

[Gene F]

#2, and add some Marvelous Mystery Oil to the crankcase, and take her for a spin.

[Abchoppers]

Originally Posted by Flathead Fever

I enjoy challenges like this, especially on someone else's engine. This is war, you never let the vehicle win no matter how much it costs and sleep you lose. That's good that the oil trick did not raise the compression, better a valve issue than a ring one. I bought a Case walk behind trencher that would not run. I squirted oil in the cylinder, and it fired right up and ran great. It ran until it stopped smoking. The oil was making a seal until it burned off. Bought a new piston and rings and that solved it.

A small head gasket leak could cause it, it usually causes white smoke when you first start it, and the exhaust would have a sweet smell to it. The plug would be shiny clean, the water steam cleans plug. It might run just fine and not overheat with a tiny leak. It would be losing coolant over time. The white smoke, sweet smell at fist startup and shiny plug would be the clue. A very small leak runs into the cylinder while itis sitting and then burns off when you first start it.

It could have a flat intake cam lobe not opening the valve as much as it should. Also, a bore scope down the intake to check for carbon deposits on the back of the valve would be a fun test. If the valve was leaking the vacuum gauge needle should flutter but you said its steady. Can you here any popping in the tailpipe or carb?

Are there any noisy lifters? a flat cam lobe would increase the clearance a lot and it would be making noise with a solid lifter. With hydraulic lifter engines the lifter takes up the clearance and you don't hear the noise. Yoo have to watch the rockers moving to see if one does not move as much. Something to watch if you are going adjust the valves.

Every time a piston creates vacuum it moves the vacuum gauge needle, together they are taking turns creating vacuum so fast that it shows up as a steady reading on the needle. If one cylinder fails to do its job the needle drops for a split second. Your cylinder is still creating enough vacuum to keep that needle steady. It's a lot harder to diagnose stuff like this that have not totally failed yet. I would get engine misfire codes on the Dodge 5.2L and 5.9L engine on the phone trucks and vans. They ran fine, I could not find any problem. Nothing showed up on a scope. compression was great. Everything looked and tested fine. You would never know there was a problem driving it except for that stored misfire code. I pulled the valve cover and rocker arms to the cylinder with the misfire code and put a straight edge across the valve tips and there was one valve sticking up a lot higher than the others, the straight edge rocked back and forth on it. The valve had sunk into the head so far it cracked the head, but it still ran fine. They did not have harden seats. If one cylinder is not producing the same power the rpm slows for a split second and the computer can see this. It knows where #1 is, between the crank and cam sensors it can calculate which cylinder caused the rpm to slow down. That speeds up the diagnsing time. We sent the Dodge heads out to have the cracks welded and hardened seats installed and they last longer than they did originally. At first I did not trust welding heads, but we never had a problem with any of them, better than they were new. We had so many of those engines and it started happening to all of them. If we got a misfire code, we pulled the valve cover first and that was almost always the problem. Even though the engines ran fine we had to fix them if we had a light on and code when we did the CA State Smog tests every two-year's. We were a licensed fleet test station ( I couldn't do my own vehicles which sucked, only the fleet vehicles) We kept a pair of heads and gaskets ready to go so we could swap them out at night and not have the vehicle out of service the next day. They were really big on not having more than 1% out of service at any one time in a yard with 400 vehicles.

Either the air is leaking out of the cylinder or there is not enough flowing into the cylinder to compress it to 150 lbs.

You’ve given me so much good info here, much appreciated the time you put in! I’m glad it’s pointing to a valve rather than rings. I don’t see any of the signs that you said about a head gasket either. Definitely no smoke at all, no sweet smell and no super clean plugs. I tested the vacuum again and it holds steady at 20 inches still. I do have what I think is a slight miss in the engine, very slight. My father was the one who noticed it actually. I’m leaning towards an exhaust valve but need to run the test to be sure. I think I do have noisy lifters, another reason why I wanted to adjust them. It doesn’t seem to be just one, but there is an overall light ticking noise while running.

I like the vehicle never wins. With 2 jobs, 3 preteen girls and a big house, time and money are both short. I was worrying about this, hoping it wouldn’t snowball into something big. I have to look at it as a challenge, one that I’ll win! Thank you guys for the help

Originally Posted by Flathead Fever

Cylinder Leak tests


Pull the plugs on either side of the cylinder you are testing. If you don't have a leak down gauge, you can take your compression tester hose and remove the shrader valve from it. Hook shop air to the hose and listen to where it goes This good way to check an engine out of a car your thinling of buying.

https://www.youtube.com/watch?v=og9nXfXSaoY&t=27s

https://www.youtube.com/watch?v=yhHv-AwbFiY

https://www.youtube.com/watch?v=NbxW6HRYHAo

I watched all 3 videos, thank you!

Originally Posted by Flathead Fever

Carbon buildup on moder engines will raise the compression and cause a vehicle to fail a smog test for excessive NOX readings. Excessive pressure and heat combine the oxygen and nitrogen to create a NOX molecule, the brown haze in photochemical smog. A non-working EGR valve is usually the problem but when it was not, we poured water down the running engine to the point of almost drowning the engine from running (phone company trucks who cares). You don't want to pour so much you hydraulic the engine and break something. That steam cleaned off the carbon and would bring the reading down enough to pass. This engine we are working on here has a cylinder with low compression, excessive carbon on the piston is not the problem, it could use more carbon.

The thread I was reading was exactly this, in fact after it was steamed the compression actually lowered in the cylinders that were high. Interesting stuff for sure

[Abchoppers]

Originally Posted by Gene F

#2, and add some Marvelous Mystery Oil to the crankcase, and take her for a spin.

I’ve read that some people have good luck with that stuff, sometimes it’s worth a try!
Thanks

[Flathead Fever]

I take my five-year grandson to taekwondo, and they teach perseverance, winners do not give up, they do not tap out. That's me when working on cars, more like that used to be me. Lately my body doess not have that same spirit. I was watching an MMA fight thinking about hoiw that doesn't look that hard and then I had to practically tap out reaching for the remote with my torn shoulder (third time it's tore) so I could fast forward through the commercials. It hard working on this stuff as I get older and I'm only 61. I have the knowledge but not the body to lean over a fender for hours is gone.

I was mechanic for 30-years. Seventeen ASE certification. Master Mechanic. Master Heavy Truck Mechanic, CA State smog License. We fixed 100% of everything in house. There was no passing it on to someone else to figure out. It was coming back to you util you fixed it right. They kept them for over fifteen years so we saw everything break that could. I had the advantage of having duplicate vehicles to look at and even swap parts from that you don't have at home. Tools you don't have at home. I worked with some pretty smart guys and a few not so smart ones. I learned from both groups what to do and what not to.

Because I was a fleet mechanic and scored well on the Stats exam test I was invited by the State to be on the team of ten-guys that wrote the smog test questions for the new smog mechanics. They wanted dealer mechanics, small repair shop owners, college automotive professors and one lowly fleet mechanic on the team. I was on the bottom of the barrel in that group of guys. When we developed those test questions for the State of CA, we were told to create two ridiculous answers, a distractor answer and the real answer. When I have a tough problem, I always think back to those test questions, wouldn't it be nice if there were only four possible causes to an automotive problem and two of them were stupid choices/ Engine has low compression on one cylinder? Mechanic "A" says, it's because the radio is tuned to political station that differs from their own view. Mechanic "B" says, old cars never did have good compression in all cylinders when they were new. Mechanic "C' says, the valves have high performance valve springs with more pressure. Mechanic "D" says it could have a head gasket leak. Those tests are designed for people just starting out as mechanics. The difficulty level was designed for a beginner mechanic. They won't be good mechanics until they have years of experience. We were told 15-years ago not to create any carburetor test questions, that there were students in automotive classes that had never even been in a car with a carburetor. Some of this old technology although fairly simple will become a lost art.

My dad had old Fords, so I started with a '65 Mustang fastback I turned into a fake Shelby. Eventually I bought a real '66 GT-350. I've had fifteen early Mustangs. I had a '57 T-Bird my mom bought new. I have a bunch of unfinished flathead Ford hot rod projects. When you work as a mechanic all night long. raise two little girls during the day while building a house, the last thing you want to do is work on your own cars at home. Now, that I have all these project cars and time to work on them (somewhere around seventeen vehicles that are not finished) I can finally start working on them. I collected cars and parts for 40-years. I've had two failed back surgeries (fused vertebrates), three shoulder surgeries and I'm in constant pain. So, it is hard to get out there and work on this stuff. That's part of the reason I like sitting here helping people with their problems, it's easier on my body, until I go to stand up. Oh my god does that hurt. The longer I make my rambling answers the longer I can put off trying to stand up. My most important automotive advice, do not become a mechanic!

[cadillac512]

Flathead Fever said:


"My most important automotive advice, do not become a mechanic!"



...Now you tell me...only 50 years too late!


Terry (I agree, mostly)

[Flathead Fever]

Running compression test. This would be a test you might do if you have good compression on a static test. It might still be interesting to see what it shows on your low cylinder. This test is rarely done. This video is longer than it needs to be you can stop watching after 10-minutes.

https://www.youtube.com/watch?v=YbR7Ja2E28s&t=766s

You want to see the modern way of doing a running compression test using a pressure transducer. I couldn't cut it out in the real automotive repair world anymore, the technology has left me in the dust in just a decade.

https://www.youtube.com/watch?v=cGM6H5dz7g8

[Abchoppers]

Originally Posted by Flathead Fever

I take my five-year grandson to taekwondo, and they teach perseverance, winners do not give up, they do not tap out. That's me when working on cars, more like that used to be me. Lately my body doess not have that same spirit. I was watching an MMA fight thinking about hoiw that doesn't look that hard and then I had to practically tap out reaching for the remote with my torn shoulder (third time it's tore) so I could fast forward through the commercials. It hard working on this stuff as I get older and I'm only 61. I have the knowledge but not the body to lean over a fender for hours is gone.

I was mechanic for 30-years. Seventeen ASE certification. Master Mechanic. Master Heavy Truck Mechanic, CA State smog License. We fixed 100% of everything in house. There was no passing it on to someone else to figure out. It was coming back to you util you fixed it right. They kept them for over fifteen years so we saw everything break that could. I had the advantage of having duplicate vehicles to look at and even swap parts from that you don't have at home. Tools you don't have at home. I worked with some pretty smart guys and a few not so smart ones. I learned from both groups what to do and what not to.

Because I was a fleet mechanic and scored well on the Stats exam test I was invited by the State to be on the team of ten-guys that wrote the smog test questions for the new smog mechanics. They wanted dealer mechanics, small repair shop owners, college automotive professors and one lowly fleet mechanic on the team. I was on the bottom of the barrel in that group of guys. When we developed those test questions for the State of CA, we were told to create two ridiculous answers, a distractor answer and the real answer. When I have a tough problem, I always think back to those test questions, wouldn't it be nice if there were only four possible causes to an automotive problem and two of them were stupid choices/ Engine has low compression on one cylinder? Mechanic "A" says, it's because the radio is tuned to political station that differs from their own view. Mechanic "B" says, old cars never did have good compression in all cylinders when they were new. Mechanic "C' says, the valves have high performance valve springs with more pressure. Mechanic "D" says it could have a head gasket leak. Those tests are designed for people just starting out as mechanics. The difficulty level was designed for a beginner mechanic. They won't be good mechanics until they have years of experience. We were told 15-years ago not to create any carburetor test questions, that there were students in automotive classes that had never even been in a car with a carburetor. Some of this old technology although fairly simple will become a lost art.

My dad had old Fords, so I started with a '65 Mustang fastback I turned into a fake Shelby. Eventually I bought a real '66 GT-350. I've had fifteen early Mustangs. I had a '57 T-Bird my mom bought new. I have a bunch of unfinished flathead Ford hot rod projects. When you work as a mechanic all night long. raise two little girls during the day while building a house, the last thing you want to do is work on your own cars at home. Now, that I have all these project cars and time to work on them (somewhere around seventeen vehicles that are not finished) I can finally start working on them. I collected cars and parts for 40-years. I've had two failed back surgeries (fused vertebrates), three shoulder surgeries and I'm in constant pain. So, it is hard to get out there and work on this stuff. That's part of the reason I like sitting here helping people with their problems, it's easier on my body, until I go to stand up. Oh my god does that hurt. The longer I make my rambling answers the longer I can put off trying to stand up. My most important automotive advice, do not become a mechanic!

You’ve had an interesting career and your knowledge has helped many I’m sure, definitely helped me. It’s great how so many of you guys with real skills are willing to help out. I’m a backyard mechanic at best and I’ve already learned a ton here.
Thanks again

[Abchoppers]

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Update - I’ve adjusted all of the valves, compression on number 7 is still the same. Next I did the air pressure test. I decided to loosen both the number 7 valves until they were both closed with no rockers touching to be sure. This is where it gets tricky. I can hear the air pretty loud, but it’s very hard to determine exactly where it’s coming from. I’m using a rubber hose up to my ear. I hear a little from the exhaust tips, but also a little from the carb. Very faint on both. The oil fill I hear it the loudest with some light air blowing out, but I know that some air will always escape there. With the valve cover off, I can also hear it coming out of all of the pushrod holes. I thought it would be a lot more obvious to decide where it’s coming from, but it’s not. Maybe I’m doing something wrong? I’m using an actual leak down tester hooked to my compressor.
Thank you

[dmsfrr]

Part of the reason you're hearing the exhaust leak in several places may be the exhaust crossover inside the intake manifold.
Cyl's 2-3 and 6-7 have a common outlet in their head that is connected to the other side of the engine thru the center of the intake manifold.
The exhaust crossover also contains a heat exchanger tube for the choke mechanism on the carb that can leak into the carb if the small tube has failed from corrosion.

[Flathead Fever]

Borderline problems are difficult to diagnose. That was a good idea backing the valves off all of the way. If the exhaust valve was leaking just a little bit, it would be difficult to hear it all the way back at the tailpipe. Try pressure testing a good cylinder to see how much leakage you have from the oil fill cap compared to the bad cylinder. The rings shouldn't be leaking if that wet compression test made no difference, unless you did not get enough oil in there. Maybe gently wiggle the valve assemblies and see if that changes the valve leakage, I've never tried that one. Try the same tests on a good cylinder compared to that bad one.

When we pulled heads to verify our leaky valve diagnosis, we used to put them on their sides on the bench and fill the ports with water to see if it ran out of the valves. No way to do that on your engine.

[fordor41]

Originally Posted by Flathead Fever

Borderline problems are difficult to diagnose. That was a good idea backing the valves off all of the way. If the exhaust valve was leaking just a little bit, it would be difficult to hear it all the way back at the tailpipe. Try pressure testing a good cylinder to see how much leakage you have from the oil fill cap compared to the bad cylinder. The rings shouldn't be leaking if that wet compression test made no difference, unless you did not get enough oil in there. Maybe gently wiggle the valve assemblies and see if that changes the valve leakage, I've never tried that one. Try the same tests on a good cylinder compared to that bad one.

When we pulled heads to verify our leaky valve diagnosis, we used to put them on their sides on the bench and fill the ports with water to see if it ran out of the valves. No way to that on your engine.

I've also filled the ports but I use gas. lot of times gas will leak where water will not.

[Flathead Fever]

To be honest with you I don't t know what kind of volume of air a leak down tester puts into a cylinder. It might not be what a compression tester hose with the Schraeder valve removed, and hooked to a compressor would put in there. Maybe that would be a larger volume of air that was easier to hear.

There is another option. Originally automotive repair shops had smoke machines to find leaks in the evap system. The charcoal canister, gas cap and all the plumbing that must be a sealed system. If there is a small leak it will set a check engine light or fail the manual evap test on the smog machine. Usually, it's a bad gas cap or somebody did not tighten the gas cap all of the way on, but sometimes it's a cracked hose on top of a gas tank where you can't see up in there. A smoke machine hooks to the battery and heats mineral oil and pumps it into those hoses under very low pressure, so it doesn't damage anything, then you just have to watch for where the smoke comes out. It will stink you out of a shop. The same tool can be used to pump a cylinder full of smoke although I have never done it. Usually at the phone company when cylinders had low compression it was because the piston was in the oil pan Those drivers were pretty hard on stuff. We had a compression test we called the dipstick test. If a cylinder had zero compression, if you could take the dipstick and check the oil through the spark plug hole it failed the dipstick test. A valve job was not going to fix it.

Smoke test video. Fast forward through this guys' video to about the 15-minute spot. This is around a $280.00 smoke tester. They make them as cheap as $75.00, and you will use it once in your life at home at it still might not find the problem.

https://www.youtube.com/watch?v=48AD0P9RsHs

There are some build you own smoke machine video. Thise one is pretty clever.

https://www.youtube.com/watch?v=tixozqgSwN0

[Flathead Fever]

Originally Posted by fordor41

I've also filled the ports but I use gas. lot of times gas will leak where water will not.

I had always heard to use gas instead of water. At the phone company the fire department would not allow us to store any gas cans with fuel in them in the shop. We had overhead lube reels in each bay, each stall had a water hose so that was what we used to fill the ports. If you filled a gas can you had to empty into a vehicle (not your own) before you put the can back into the cabinet.

[Ford blue blood]

That's kind of funny? An empty gas can is far more dangerous then a full can.

[Flathead Fever]

Originally Posted by Ford blue blood

That's kind of funny? An empty gas can is far more dangerous then a full can.

We couldn't have a full gas can in the shop, but we could have six stalls with vehicles with gas tanks full of gas.

[Abchoppers]

Okay fellas, so I checked another cylinder that had good compression with the air test. Basically the same level of air noise from same places. I also did use the compression tester without the shrader valve and had the same results. I’m putting roughly 100 psi into the cylinders so it should be enough air.

When rockers are completely backed off and both valves of number 7 are fully closed, they both look like they are the same exact height. I’m thinking that the drill spinning trick might not work for me since one should appear lower if it was stuck open at all. I tried hitting both valves with a rubber mallet lightly and it didn’t seem to change anything. I’m Going to try and rig up a smoke machine because that would really show me the leaking area.

So I’m thinking the plan for now is to just drive the car. It’s definitely still
Combusting in the cylinder and overall runs really good. After summer is over my plan is to pull the head and really examine it, have it machined if needed and a new head gasket.

I’m definitely open to any other ideas, I just don’t want to keep taking up everyone’s time. You guys have been beyond helpful. I really mean it when I say I’ve learned so much and am grateful.

[cadillac512]

Excellent plan. For now, drive the car, enjoy it and monitor it's performance. If anything gets worse you can deal with it then. It'll give fair warning.
Terry

[Abchoppers]

Originally Posted by cadillac512

Excellent plan. For now, drive the car, enjoy it and monitor it's performance. If anything gets worse you can deal with it then. It'll give fair warning.
Terry

Thanks Terry, my girls would kill me if I tore the engine apart now, they’re loving it way too much!