Compression Test Results

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perko

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Hi all,
Thought I'd run this past the heads on this forum and get a proper opinion. I'm tracking down a miss in my 1973 318 Duster. I replaced the cam a while ago because it was backfiring out the carb horribly and the #6 exhaust lobe turned out to be flat. The miss started immediately after the swap, so I'm inclined to think that its not the cam going again. I think that it was there before the swap, I just couldn't hear it over the machine gun carb backfire. Anyway, I did a compression test because I suspected a valve train problem.

Cylinder: Dry/Wet
1: 100/115
2: 90/115
3: 115/115
4: 115/120
5: 115/couldn't reach
6: 60/70
7: 110/couldn't reach
8: 115/115

Clearly there is a problem with #6. I was thinking that the valves or seats got rocked when the cam went. Whats everyone think?
 
maybe the #6 exhaust valve is not closing completely?
that would pose the question, how does this correlate with the #6 exhaust lobe going out?

if the two are related your new cam is at risk

did you remove the valve cover with the engine running to see how the movement on that rocker looks?
 
It also looks like you have a ring/bore issue on #2

If you isolate them by even and odds, you see the odd bank is pretty close at 100-115 where there is much more variation on the passenger side (right ) bank....
 
Apply air to #6 with both valves closed and you'll get the answer.
 
X2 What Rusty said.
I would also do #2 as it has a problem too which krazy stated before.
Heck do all of the even bank.
 
So... I guess my next step is leak down test on #6 and see what happens. I should hear air coming from the carb and/or exhaust if there is a valve not sealing issue? From there i suppose I'll have to have the heads repaired. I've got a set of old 920 heads that need work anyway that I could use.

How concerned should I be about the #2 cylinder? This thing isn't intended to be a race car. I just want it to hold together long enough for me to learn how to rebuild a nicer engine or end up with another project. How worried should I be about the new cam dying? Do you think that inspecting the lifter faces would be enough to convince me that the cam is ok or shot?
Thanks
 
Hey all,
Sorry for dredging up a dead thread but i need some advice. My landlord sold our place so we had to move and the duster took the back burner. I took the intake/rockers off so that I could check the cam so see if the move damaged it but I think it looks ok. While I was there I did a leakdown test. I don't have a real gauge, just a way to pressurize the cylinder, but I figured that I would just listen for air and that would help me diagnose the problem. However, every cylinder that I tested had air leaking out of what sounds like the crank case. Even the bores that were the best in the compression test. Am I doing something wrong? Or should I assume that there are ring issues all around?
 
Are you doing the test right? Is the piston at TDC?
 
No, i'm not doing the test just as you should, but since I have the rockers off I didn't think that it would make a huge difference since all the valves are closed.

I don't have a real leakdown tester I just wanted to listen for air coming out one of the valves, but its not working out like that. Is it possible to have rings busted to the point that it would cause a misfire and not be apparent on a wet compression test?
 
Since you have the rockers off, get yourself a piece of brass bar stock.
You can hammer a piece of brass water pipe flat if you are broke.
Place the brass over each valve stem and smack the brass with a hammer.
It may knock enough carbon loose to let the valve seal.
 
My problem seems to be that the valves ARE sealing when I expected them not to after reviewing the compression test results.
 
How long has the engine been sitting? Might be the rings / bores are worn or have a problem, or might be there isn't enough oil to seal.

There is nothing wrong with doing a leak down both at TDC and BDC
 
Rotate the engine around to different positions while you do this. Standard procedure for engines that have been sitting for a while. But that all does not do anything for you, because you can't do a valid test without the gauges for leakdown rate. All you know is that they all leak to some extent.... which is not any surprise, and you're not getting any real info to properly diagnose anything from that.

And of course, the chance that the valve train that is causing the problem has been removed.....so you are not really getting anywhere. Since there are multiple causes for the misfire, you need to do some valid tests... putting the rockers back on is a start.

And busted rings would readily show up on a dry compression test.
 
Might as well pull every lifter, one attatime, and flip it over for a look see. I'm assuming you put all new ones in with the new cam.
But hey, before you do that, what type of carb are you running? and have you considered it as the source of the miss? Or even a bad sparkplug, wire or cap?
 
perko said:
would the position of the piston really make a difference if all the valves are forced shut anyway?
Click to expand...

What you are trying to do is isolate the cause of the leakage. With the piston at the top of the bore, the rings are sealing on a different part of the cylinder than bottom. So if the bore is cracked, or damaged such as a gouge from a broken ring or foreign material, it might give you a clue. The valve leakage does not matter, top or bottom, if the rocker gear is off.

Not clear to me if you actually have a leakdown tester or just air. You can EASILY rig a leakdown tester, all you need is a regulator (you can use the one on your compressor) build a .040 orifice, and a gauge. Google it. You don't need two gauges. You can set the "input" pressure by plugging the output

And, you don't really need an .040 orifice, either, it's just that it was the original spec when these testers "came about" A 1/16 drill is .0625 and that will work for "relative" results. Any "real" hardware or fastener store sells "numbered" drills. A no 60 drill is .040

https://www.google.com/search?q=hom...cD7iUETUM:&usg=__oCxbgSob4oevwd48ta9cYsMY8H8=

https://www.google.com/search?num=2...0....0...1c.1.64.hp..21.20.1957.0.gPSu0u0O3DU
 
Sure it will. It's SOP on small piston aircraft engine when doing annual inspections is to rotate the engine around; you will find spots of more and less cylinder leakage. Especially if they are worn some....but since the OP is not doing a real test, it is a moot point anyway.
 
Well its actually raining here which is unusual, and i no longer have a garage so we'll see what happens. Thanks everyone, I appreciate it.
 
You are confusing yourself. You said it was skipping on number 6, correct? Then concentrate on THAT cylinder. You are making this more difficult than it needs to be.
 
nm9stheham said:
Sure it will. It's SOP on small piston aircraft engine when doing annual inspections is to rotate the engine around; you will find spots of more and less cylinder leakage. Especially if they are worn some....but since the OP is not doing a real test, it is a moot point anyway.
Click to expand...

For testing with air, it does not make not one hill of beans difference where the piston is in the bore, if both valves are closed.

The only exception to that would be if there was a crack or hole in a cylinder below where the rings are at TDC.

Even then, if the piston was at TDC and the hole under the rings, the leak would still show up, since the rings are not a complete seal.

I can sure tell some of yall have never done these tests on an engine before because you argue totally moot points. This is something that I have done and done and done and done and..........done.

If he does it as I outline, unless he is doing something WRONG, he WILL FIND the problem.

He doesn't need complex leakdown crap. ALL he needs to do is be able to apply air to the cylinder that's not firing. That's all. It's really that simple.

If you cannot make out where the air is coming from, you're doin it wrong OR you don't have enough air pressure. End of story.

This ain't rocket science, folks. Stop makin it so damned difficult for him to find an obvious EASY problem by arguing stupid points just to try to make yourselves sound smart. It ain't workin.
 
Yeah that's what I was thinking. I just wanted a notion of where the problem is coming from. I don't have a space to re-ring/rebuild an engine right now. messed up valves or engine swap, that I could take care of.

Is there a specific PSI I should be applying to the cylinder to ensure that the rings seat? Does it matter at what rate the pressure is applied? As of now, looks like there's something hinky with the bottom end of cylinder 6.
 
My thinking is that 70 p.s.i. would be adequate to get an idea of where your leak is. The one thing that concerns me is that you had a lobe go flat on your cam. If it occurred in a short period of time then it put a lot of metal into the engine. I think it would be a wise to take your oil filter off, cut it open, and inspect it. If the filter is relatively clean then you are good. If you find metal in there then you will have to tear it down and clean it. If you don't have access to a filter cutter then contact someone who can cut it open for you. Most circle track guys do this as a preventative measure and could help you inspect it.
 
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