Cylinder #7 down on compression

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gtgto

gtgto

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I drove the car all season and it felt like power was down some. Not sure where this winter project is going to go but today I was off and decided to do a compression check. Engine cold and all plugs out 7 of the 8 cylinders were low of 137 and high of 155.......Cylinder 7 was at about 110PSI.......Not good at all.......engine is 70 340 matching number to my Duster. I am not burning oil so I am thinking valve issue. Checked springs on #7 and pushrods and they look good. Where do you recommend I go next? Should I pull the intake and look for cam/lifter issue? Can a leak test be done with heads still on engine in car? No idea how to do a leak down test but I willing to give it a shot.
 
What kind of fuel do you use ?

The reason I ask is that buildup on the back side of valves can cause the same thing .
It will ever so slightly cause the valve to leak .
It will eventually burn a valve if left long enough .

Tommy
 
No engine expert here, but I would do(and have) a leakdown next. They are done with head on. Chk boobtube out to make a LD tester. Basically uses same pieces as a compression tester in a different configuration. Good luck!
 
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gtgto said:
No idea how to do a leak down test but I willing to give it a shot.
Click to expand...
If you are a newbe, and you do not have a LD tester;
here's what I recommend.
1) Take off the driver's side VC, and look for a sloppy pushrod preload, on the low-pressure cylinder, as compared to another cylinder with good pressure; which may signal a cam-lobe going down. Of course the checking has to be done with the lifters on the base-circles. If you suspect a loose fit, more testing is needed.

As to the LD test
2) Next, remove the radcap, and fill the rad right up. Cuz;
Later, as you put pressure to the cylinder for this type of LD test, the piston will fly to the bottom; and the chamber will pressure-up. Keep your knuckles away from the fan! Then; Right away, check the coolant in the rad for bubbling. As the pressure comes up, 100psi can do damage in the rad if the headgasket is leaking; So if you get bubbling, or the water blows out; stop the test!
If no bubbling, then for the rest of the time, keep one eye on the filler-neck.
3) Next, remove the rocker gear. This will allow the cylinder to seal up, as good as it can get. What you need to do now is to inject air into the plug-hole, and see where it comes out.
But
If you hear air coming out the carb, or air coming out the tailpipe, then, one or both of the valves are leaking. Since the piston is at the bottom, you can now bop the valves without fear of bending them on the pistons. I smack them with a dead-blow hammer, but you can use whatever. Don't go crazy cuz it is possible to kick a lock out.
After every bop, listen for a change or decrease in the hissing, and/or a change in the sound of the valves smacking back onto the seats. As you continue, if you get a sharp "pop", and the hissing more or less quits then stop the test. What you are doing is smashing the carbon off the seat, and simultaneously blowing it out.
If this works, then after a few bops, any hissing left is going into the CC past the rings, which is sortof normal.
But if the hissing never stops
You may have a bent or burned valve.
5) Put the valve gear back on, with the rocker shaft oriented correctly, and correctly torqued to IIRC 15 ftlbs for 5/16ths bolts. Then
6) redo the compression test.
If the numbers are same as before, you still don't know if it's bad or stuck rings, or a scratch in the cylinder wall, or really could still just be valves; so now, you gotta go rent an LD tester, and repeat the test with the Piston at TDC/Compression, minus the valve bopping..
 
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gtgto said:
Can a leak test be done with heads still on engine in car? No idea how to do a leak down test but I willing to give it a shot.
Click to expand...

Absolutely, the spark plug hole entry is done with a fitting with flexible hose which comes with the tester. The test will give you a bunch of information.
 
Piston to the bottom of the cylinder and put air pressure to it, listen for where it's leaving the ship. Intake, exhaust or breather.
 
1970, it's 54 years old and depending on the miles on it it's probably tired. But a leak down test would be a good place to start but don't be surprised if it's due for a rebuild.
 
Look at the spark plugs, if one or both are different, snap a coupla pix of them and all, in order as a bunch, if poss. tia.
 
@AJ covered it. If you don’t want to remove the valve train. You can spin the motor over in direction of rotation with a large breaker bar and socket on the balancer nut until the valves are close for the cylinder you are checking, and then do the next cylinder in the firing order. Leave the plugs in the other holes Will reduce the chance of the motor spinning when you hit it with air from the LD tester.
 
I do not see much benefit doing a LD test. At least one cyl is low so at a minimum, the heads need to come off. Problem may be readily apparent: burned/leaking valve or failed head gasket.

If the engine has high mileage, a ridge forms at the top of the bore where the ring travel finishes. The top ring can hit the ridge....& break.
 
Doesn't anyone put a little oil in the cylinder & retest anymore? One tablespoon of oil, spin the motor over to spread the oil & retest... If the compression goes up the rings are the problem.. If not valves or cam are the likely cause.... 110 PSI isn't a burned valve, it could be carbon but a burned valve would typically be less than 30 PSI...
 
Inertia said:
Look at the spark plugs, if one or both are different, snap a coupla pix of them and all, in order as a bunch, if poss. tia.
Click to expand...
Here are the plugs. I’m planning on going to Advanced Auto to rent/borrow a leak down tester tomorrow morning.

IMG_6108.jpeg


IMG_6107.jpeg
 
Inertia said:
Thnx for the pix, - I woulda guessed #5 was the issue, maybe the head gasket between the 2.
Good luck .
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what o you see in #5 plug? What causes the electrode post to get discolored? I'm off today and I'm going to give a leak down test a try. Will post what I find later.
 
gtgto said:
what o you see in #5 plug? What causes the electrode post to get discolored? I'm off today and I'm going to give a leak down test a try. Will post what I find later.
Click to expand...

I was taught to look at every spark plug I took out, instructed that
you could tell an awful lot about an engines running condition, and lotsa other stuff, like valve guides failing, ring failure, detonation, intake gasket leak, bent pushrod, worn cam lobe, timing, - all in a glance.
I was taught to look for a difference, then figure out why.
Your #5 plug is different, from all others, yet #7 is low compression.
My conclusion is #5 plug came outta #7 cyl, and got mixed up, previous to you knowing the importance of order, - or got exchanged at an earlier time.
Just experience and work habit .
Still needs to come apart, regardless.
Good luck, you'll be okay, lottsa help here.

PS. #5 plug looks kinda wet, and seems to have an irregulaty/protrusion toward the center porcelain, might just be the pic.
Is there something goofy there ?

The center electrode is discolored by contaminants, (oil sucked up), incomplete combustion, ( low compression), etc
 
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The plugs are NGK Power 4 I think. What do you recommend I use cause when I put the engine back together I will be replacing the plugs most likely. I did the leak down and #7 exhaust valve is the culprit. I got the head off today and brought it to a shop and he took the #7 valves out. The valve is worn very thin but was not burnt yet. He’ll have the head done probably next week. Said seals and
Retainers are good stuff and definitely itely not original. Pistons in car I. Need to clean up to have a better look but they do not look to be factory 70 340 pistons. Cam and lifters look to be in great shape too.

IMG_6117.jpeg


IMG_6118.jpeg
 
gtgto said:
The plugs are NGK Power 4 I think. What do you recommend I use cause when I put the engine back together I will be replacing the plugs most likely. I did the leak down and #7 exhaust valve is the culprit. I got the head off today and brought it to a shop and he took the #7 valves out. The valve is worn very thin but was not burnt yet. He’ll have the head done probably next week. Said seals and
Retainers are good stuff and definitely itely not original. Pistons in car I. Need to clean up to have a better look but they do not look to be factory 70 340 pistons. Cam and lifters look to be in great shape too.

View attachment 1716329174

View attachment 1716329175
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That looks like an 8.5:1 piston. A 4 or 5 in an NGK should be ok.

The plugs look screaming lean and the pistons (and probably the exhaust ports) look pig rich.

If you can you really need to remove the shell from the plug so we can see the entire porcelain.

My guess is there is so much heat being added to the intake charge you are vaporizing the fuel too soon.
 
What should I clean up the top of the pistons with. is a wire brush on a drill to much? I would like to get some numbers off of hem. Why would someone put 8.5 to 1 pistons in a matching number block that came originally with 10.5 to 1's
 
gtgto said:
What should I clean up the top of the pistons with. is a wire brush on a drill to much? I would like to get some numbers off of hem. Why would someone put 8.5 to 1 pistons in a matching number block that came originally with 10.5 to 1's
Click to expand...

Because people have been building engines with no compression for decades for no reason, other than the nattering naybobs said you couldn’t do it.

I’ve never 10.5:1 pistons with all those notches in them.
 
I like to smear a layer of axle grease about 1/2 inch down the bore, in all cylinders. Then bring whichever piston up to the top, the grease is to prevent chit/carbon getting between piston and wall.
This is most important cuz any debris, carbon, whatever you scrape off that gets down the crack, stands a chance of getting blown behind/beside the ring on first explosion, and when the ring goes down the worn tapered wall, compresses that chit behind, and can/does jam the ring. When the piston moves down after scraping/cleaning wipe the debris and grease off the cylinder wall, residual grease can remain in the gap, then wipe all before assembly.
I use a sharp scraper, the carbon is usually pretty soft, a wire brush on drill is useful in the valve reliefs.
There is still something funky about cyl #5 difference in appearance.
Maybe scrutinize the intake gskt for vacuum leak, head gasket for coolant loss, not sure from here, lol.
Good luck.
 
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