Misfire at cruise, popping/afterfire at 3000RPM

[AJ/FormS]

DO NOT TAP the valves with the piston parked at TDC!
Ok, at the top of the compression stroke you can, but be careful that you do not tap a valve deep enough to smash into a piston.....
To do a LD test, the piston has to be at the top of the bore, and on the compression stroke so that both valves are closed.
If you have a solid-lifter cam, there has to be at least some lash evident.
When you inject the air, it likes to blow the piston down, so; the piston has to be almost exactly at TDC. Furthermore, the LD test is usually done at a low enough pressure (30psi) that you can control that pressure surge.
I inject 80psi because it gives me a reasonably accurate result; but
I installed a valve on my tester that allows me to bring it in slowly, so the dang piston stays at the top. And my tester has only one gauge on it so that I only have one error to deal with.
So, then I deadhead my gauge, and adjust the regulator so it reads 80psi. Then without moving anything, I open my valve to exhaust the gauge, then slowly bring the pressure back up. If the piston moves, I back it up and start over. If the piston stays, then the gauge will read less than 80psi, because some of it always leaks somewhere.
So then, if the gauge reads say 76 psi, then the pressure loss is;
(80 less 76)/80= 5%LD

Had you tested with the typical 30psi;
then 5% would be (1 less 5%) x30=28.5psi. But most of us would not be able to read the .5 psi on the stinking little gauge supplied. Maybe not even a full psi. So we might round it to something like 28 or 29 psi and call it done. But 28psi is ;
(30 less 28)/30=6.7% LD, and
29 is; (30 less 29)/30= 3.3% LD
Both numbers of which are way out to lunch, compared to the correct number of 5%.

So the bottom line is to always test with as much pressure as you dare,
BUT
Always make sure there is NO BAR on the balancer to slam thru the rad, or smash your hand...
I am comfortable with 80psi, which, on a 4" piston, is 1005 pounds of downward force;
that's a lot of force.
If you ever can't unscrew your balancer-bolt, now you know the secret.......
30psi is 377pounds of force, a relatively tame-looking number, in comparison. Don't let it fool you; you cannot hang on to the bar very long after it starts moving; so I highly recommend that you don't even try..

To avoid all the rig-maroll,
you can unscrew the rocker shafts, and do the LD with the pistons anywhere; the pressure will push them to the bottom, and then the LD number will stabilize. With the pistons at the bottom, you can rap the valves to your hearts content.

 

[Righty Tighty]

Thanks @AJ/FormS . I’ll apply that tomorrow, and it’s good news that there’s a method that is slightly more fool proof.

The good news: got her back together after the compression/leakdown tests and cleaned up the wire routing. Replaced the fouled and damaged plugs. Initial start, idles very smoothly, and at a slightly higher RPM than before. I dialed back the timing to 16 advanced and checked to see what it was at higher RPM. With the vac can disconnected, I got 37 degrees at 2800 RPM, but the engine stumbled and stuttered at anything higher than that.

Took it on the road, and a light cruise revealed no misfires. Little heavier on the throttle, good to go. But again, anything over 3/4 throttle or say, 2800-3000 RPM, stumbles and falls on its face. No more afterfires, though.

 

[67Dart273]

One way to insure you are compression stroke. You have a socket/ breaker to fit the front crank bolt? Remove no1 plug stick your finger in and bump engine until you start to feel compression. Now bring timing marks up to TDC. This is no1 on compression "ready to fire. Now mark the wheel just visually you don't have to get "accurate" at every 90*. Now just rotate 90, rotate 90, etc, and go right down the firing order

If you have the valve covers off, you can look at the valves, and feel for compression with your finger same as no1

 

[Righty Tighty]

That's pretty much exactly what my process was. I started with a piston stop to ensure TDC, because I wasn't confident in the balancer's accuracy. Then once I was at #1 TDC compression, I divided the balancer into quarters using a paint pen.

When I got to #4 and found the leak, I backed up the crank to see any difference, and then moved forward. The best I could see was 85% leakage between cylinders 8 and 4, and between 4 and 3.

I had the valve cover off, but for some reason it wasn't as obvious as it was with my 318. All rockers looked pretty equal.

 

[Bewy]

I would replace the coil.

 

[RustyRatRod]

Just keep it under 3000. Problem solved.

 

[halifaxhops]

Hey you have another box you can change out? Just had a blue one fail similar condition.

 

[Mattax]

Not that anyone above is off the mark, but no amount of ignition fiddling will fix a mechanical problem, then again no amount of mechanical fiddling will fix an ignition problem either!

Very true!
But wrong ignition can cause mechanical damage.

I got 37 degrees at 2800 RPM, but the engine stumbled and stuttered at anything higher than that.

Took it on the road, and a light cruise revealed no misfires. Little heavier on the throttle, good to go. But again, anything over 3/4 throttle or say, 2800-3000 RPM, stumbles and falls on its face. No more afterfires, though.

So we still have little idea what the timing looks like, but now know it was 41*BTDC at 2800 rpm.
Yes that could damage at full throttle.
Whether it did cause mechanical damage or not, definately want to avoid going close to full power with too much timing lead.

 

[Brooks James]

Barely visible carbon tracking in the distributor cap ?

 

[Righty Tighty]

Is this a new build, or did it just recently start misbehaving?

If it were me, I'd pull the valve covers and rockers shafts/pushrods. It wouldn't take long and would tell you real quick if something is fubar up top. I've seen issues like this before turn out to be a bad cam, bad rocker arms, failed lifters, etc. The issue being present when revving in neutral is what makes me suspect valvetrain.

These types of issues are often easy to spot with the valve covers off, and especially if you crank the motor over (with ignition disconnected!) and watch the rockers.
You could also set TDC on #1 and watch the springs as you remove the rocker shaft and see if any of the valves are being held open somehow. At any rate, lots to see and determine under those valve covers.

Not that anyone above is off the mark, but no amount of ignition fiddling will fix a mechanical problem, then again no amount of mechanical fiddling will fix an ignition problem either!

Well, I bought the car a few months ago, and according to the guy I bought it from, the engine has about 500 miles on it. He had the 440 built and dropped it in to replace a 318. He said he put some miles on it to break it in, but started having trouble tuning it. Got frustrated and pushed it into storage, where it sat for 20 years.

I'll pull the valve covers off today and have a look.

DO NOT TAP the valves with the piston parked at TDC!
Ok, at the top of the compression stroke you can, but be careful that you do not tap a valve deep enough to smash into a piston.....
To do a LD test, the piston has to be at the top of the bore, and on the compression stroke so that both valves are closed.
If you have a solid-lifter cam, there has to be at least some lash evident.
When you inject the air, it likes to blow the piston down, so; the piston has to be almost exactly at TDC. Furthermore, the LD test is usually done at a low enough pressure (30psi) that you can control that pressure surge.
I inject 80psi because it gives me a reasonably accurate result; but
I installed a valve on my tester that allows me to bring it in slowly, so the dang piston stays at the top. And my tester has only one gauge on it so that I only have one error to deal with.
So, then I deadhead my gauge, and adjust the regulator so it reads 80psi. Then without moving anything, I open my valve to exhaust the gauge, then slowly bring the pressure back up. If the piston moves, I back it up and start over. If the piston stays, then the gauge will read less than 80psi, because some of it always leaks somewhere.
So then, if the gauge reads say 76 psi, then the pressure loss is;
(80 less 76)/80= 5%LD

Had you tested with the typical 30psi;
then 5% would be (1 less 5%) x30=28.5psi. But most of us would not be able to read the .5 psi on the stinking little gauge supplied. Maybe not even a full psi. So we might round it to something like 28 or 29 psi and call it done. But 28psi is ;
(30 less 28)/30=6.7% LD, and
29 is; (30 less 29)/30= 3.3% LD
Both numbers of which are way out to lunch, compared to the correct number of 5%.

So the bottom line is to always test with as much pressure as you dare,
BUT
Always make sure there is NO BAR on the balancer to slam thru the rad, or smash your hand...
I am comfortable with 80psi, which, on a 4" piston, is 1005 pounds of downward force;
that's a lot of force.
If you ever can't unscrew your balancer-bolt, now you know the secret.......
30psi is 377pounds of force, a relatively tame-looking number, in comparison. Don't let it fool you; you cannot hang on to the bar very long after it starts moving; so I highly recommend that you don't even try..

To avoid all the rig-maroll,
you can unscrew the rocker shafts, and do the LD with the pistons anywhere; the pressure will push them to the bottom, and then the LD number will stabilize. With the pistons at the bottom, you can rap the valves to your hearts content.

My LD tester is a NOS Harbor Freight model - seems a bit better quality than the newer junk, but maybe it's just older junk...LOL. I like the idea of installing a valve to control the air flow. What I did was zero the test gauge at 30 psi and go from there. Once connected, I referred to the second gauge which shows leakage, and thats how I got my numbers.

Regarding the method of unscrewing the rocker shafts, help me understand. Wouldn't the air pressure also push the valves shut, causing a false reading? This seems like a much easier method since you don't have to fuss with finding TDC, I wonder why more people don't do it this way. I also may not understand what you mean...

Edit: When I tapped the valves, I tapped pretty lightly. Light enough to where I don't think I was even overcoming the springs. I just thought that maybe a little bit of energy would settle the valve, if that's what it needed. Hopefully that sets your mind at ease a little...haha.

I would replace the coil.

I have a newish MSD Blaster 2 that I had installed briefly, and while that may not be exactly when I noticed the problem, the problem definitely existed using that coil.

 

[Righty Tighty]

Hey you have another box you can change out? Just had a blue one fail similar condition.

I have another blue one that I got as a spare - I'll try that one.

Ignition systems are so mysterious to me, and it seems the more I read, the more confused I get. Example, I read that the coil/ECU/ballast (if required) all need to match. If this is the case, how do I confirm that?

Barely visible carbon tracking in the distributor cap ?

I'm going to pop the cap off today anyway to inspect the reluctor and pickup, I'll report back.

Thanks everyone for the help so far. I really did try to figure it out on my own...promise!

 

[Righty Tighty]

So we still have little idea what the timing looks like, but now know it was 41*BTDC at 2800 rpm.
Yes that could damage at full throttle.
Whether it did cause mechanical damage or not, definately want to avoid going close to full power with too much timing lead.

So it was likely at 45*BTDC when my initial was at 20, am I following here? And with the vacuum advance connected, likely even higher (assuming the vac advance is functioning)? I have an FBO limiter plate kicking around, thought about installing that. BUT, I think I'd rather troubleshoot and find the actual problem first.

I'm definitely worried about causing damage, so when it starts to stumble and act up, I immediately let off the throttle.

 

[512Stroker]

Well, I bought the car a few months ago, and according to the guy I bought it from, the engine has about 500 miles on it. He had the 440 built and dropped it in to replace a 318. He said he put some miles on it to break it in, but started having trouble tuning it. Got frustrated and pushed it into storage, where it sat for 20 years.

I'll pull the valve covers off today and have a look.


My LD tester is a NOS Harbor Freight model - seems a bit better quality than the newer junk, but maybe it's just older junk...LOL. I like the idea of installing a valve to control the air flow. What I did was zero the test gauge at 30 psi and go from there. Once connected, I referred to the second gauge which shows leakage, and thats how I got my numbers.

Regarding the method of unscrewing the rocker shafts, help me understand. Wouldn't the air pressure also push the valves shut, causing a false reading? This seems like a much easier method since you don't have to fuss with finding TDC, I wonder why more people don't do it this way. I also may not understand what you mean...

Edit: When I tapped the valves, I tapped pretty lightly. Light enough to where I don't think I was even overcoming the springs. I just thought that maybe a little bit of energy would settle the valve, if that's what it needed. Hopefully that sets your mind at ease a little...haha.


I have a newish MSD Blaster 2 that I had installed briefly, and while that may not be exactly when I noticed the problem, the problem definitely existed using that coil.

Well it has become apparent that your problem is 20 years old that is why the Guy you bought it from parked it.

Some things way off on your leak down readings if you had that much leakage going on you would have other issues. Remove the rocker arms/shafts and start over. Keep the push rods in order and check each one to see if some are bent, just roll them on a clean flat surface.
Does it blow blue smoke out the exhaust and/or out the valve cover breather?

 

[Righty Tighty]

Well it has become apparent that your problem is 20 years old that is why the Guy you bought it from parked it.

Some things way off on your leak down readings if you had that much leakage going on you would have other issues. Remove the rocker arms/shafts and start over. Keep the push rods in order and check each one to see if some are bent, just roll them on a clean flat surface.
Does it blow blue smoke out the exhaust and/or out the valve cover breather?

Haha, I guess if you put it that way.... He never mentioned this specific problem, he said the problem he was having was a very high idle. That issue did surface for me, but has been since been resolved. That's not to say the issue at hand didn't exist as well.

I'm going to conduct another leak down today and will report back with new (and hopefully better) numbers, and while I'm at it, inspect the push rods. I have the stamped steel rockers, and was half expecting to see one of the rods punched through a rocker. Haven't seen that yet, but I still need to pull the driver side cover.

The engine doesn't blow any smoke at all, and doesn't seem to consume oil.

 

[Mattax]

So it was likely at 45*BTDC when my initial was at 20, am I following here?

I must be misisng something. How you're getting 45*?
I got 41 by adding the 4 degrees difference between the initial of 20 and the initial of 16 to the 37* you measured at 2800 rpm.
In doing so I assumed the 16 and the 20 degree initial was measured at the same rpm. That's admittedly a fairly big assumption.

And with the vacuum advance connected, likely even higher (assuming the vac advance is functioning)?

No. With your foot bringing the throttle 3/4 to fully open there isn't enough vacuum.
Vacuum advance is for light to moderate throttle lean conditions.

I have an FBO limiter plate kicking around, thought about installing that. BUT, I think I'd rather troubleshoot and find the actual problem first.

Good thinking.

I'm definitely worried about causing damage, so when it starts to stumble and act up, I immediately let off the throttle.

Great!

 

[Mattax]

My suggestion is to deal with things methodically.
At some point it will be neccessary to measure the timing as I suggested.
That said, I think the initial focus should be on the mechanical and why the leakdown test showed #4 so far off.

 

[Righty Tighty]

My suggestion is to deal with things methodically.
At some point it will be neccessary to measure the timing as I suggested.
That said, I think the initial focus should be on the mechanical and why the leakdown test showed #4 so far off.

Thanks, I will definitely get to the timing once I've gone through the mechanical stuff.

I understand now how you got the 41 number. I thought for some reason you were saying 41*BDTC @ 16 initial, which I didn't understand, so I thought maybe you added 4* from the cam. Which in hindsight, wouldn't make sense because nobody knows what cam is in it. And it also doesn't make sense because adding 4 from the cam would give us 40. AT ANY RATE....haha... 41 is a lot, yes?

And yes, the 16 and 20 were measured at the same RPM.

 

[Mattax]

. 41 is a lot, yes?

Considering the factory spent a lot of time coming up with an advance that would maximize power, how close does 41 at 2800 rpm come to the acceptable range? Even considering the initial timing could be set a couple degrees more advanced if local conditions and fuel permitted.

Another comparison you can make is to the Direct Connection/ Mopar Performance recommended starting point when using their distributors. With that distributor, 38* at 2800 rpm would generally be OK.

 

[AJ/FormS]

When I got to #4 and found the leak, I backed up the crank to see any difference, and then moved forward. The best I could see was 85% leakage between cylinders 8 and 4, and between 4 and 3.

I don't understand this between business.
Nor is it possible to "back up the crank" with pressure in the chamber unless you have a really long bar, and then you cannot go very far until your bar hits something.
So you are looking at EIGHT different TDC-pressure tests, with each done at TDC-compression. There is no between 8 and 4 business; nor between 4 and 3; nor between anything.
To do a LD test, each piston in turn must be at TDC on it's specific compression stroke.
If your engine had the leakage you are reporting.......... it would not even start.

 

[Righty Tighty]

I don't understand this between business.
Nor is it possible to "back up the crank" with pressure in the chamber unless you have a really long bar, and then you cannot go very far until your bar hits something.
So you are looking at EIGHT different TDC-pressure tests, with each done at TDC-compression. There is no between 8 and 4 business; nor between 4 and 3; nor between anything.
To do a LD test, each piston in turn must be at TDC on it's specific compression stroke.
If your engine had the leakage you are reporting.......... it would not even start.

I understand that the piston needs to be at TDC on the compression stroke, so when I saw how much leakage the gauge was showing, I thought maybe I wasn’t at TDC and I had a valve slightly open. So, I turned the balancer backwards a few degrees to see if the leak changed. And did the same forwards. Maybe at 30 psi it’s easier to turn the crank? I don’t know, but it turned by hand. It wasn’t easy, but it turned. Or what if that cylinder truly is leaking that much? It definitely wouldn’t build enough pressure to make it hard to turn. That’s just me spit balling.

 

[George Jets]

Disconnect the 2 secondary carb linkages and just run it on the center 2bbl. Then test run it up to 3000 rpm again.

If running on the center 2bbl only straightens out your stumble, then you know the engine is working proper and either one or both of the secondary 2 bbls are plugged and they are bringing in a lean condition as they are opening up, adding more air but not the needed extra fuel.

Multiple carbs, multiple problems especially if the vehicle has been sitting for a long time.

 

[Righty Tighty]

Disconnect the 2 secondary carb linkages and just run it on the center 2bbl. Then test run it up to 3000 rpm again.

If running on the center 2bbl only straightens out your stumble, then you know the engine is working proper and either one or both of the secondary 2 bbls are plugged and they are bringing in a lean condition as they are opening up, adding more air but not the needed extra fuel.

Multiple carbs, multiple problems especially if the vehicle has been sitting for a long time.

I ran it off the center carb only while I was rebuilding the outboards, had the same issue.

 

[RustyRatRod]

Didn't this engine sit for a LONG time? I might suspect valve springs if it was mine.

 

[AJ/FormS]

Your engine has FOUR important timing milestones. In order of importance, they are;
1) Power-Timing
2) Stall Timing
3) Idle-Timing
4) Cruise Timing
All except #4, ARE DONE/MEASURED WITH THE VACUUM ADVANCE CANISTER DEFEATED

Step-1
When you check/set the Power-Timing; you rev it up in Neutral/Park , while watching the TDC mark. Keep revving until it stops advancing. Record the rpm. Now, keeping the rpm, at the number or higher, set your Power-Timing to 34*. I say 34 because at your pressure, it might be dangerous to run any more,as regards detonation.
At this point for now, all the other timings have to take a back seat. But at 34* at least you shouldn't be breaking stuff. And if your rough and poor running issues were due to detonation, that should now stop.
Step-2
Your idle-timing is now "fixed". Not fixed as in repaired, but fixed in position. It is what it is because it takes a backseat to Power-Timing. Whatever it is, it may not be right for your engine. @Mattax is really good at showing you different curves and how they play out.
The right Idle-Timing for your engine, at your cylinder pressure, will be between 12 and 18 (with an automatic). Do not compare your engine to anybody else's. Your engine will tell you what it wants if you let it.
To do that; the short story is; at idle your engine is idling on fuel that it is being pulled from the transfers together with what the engine can pull from the idle-discharge ports. Those two should be the only sources of idle-fuel. Furthermore, the ONLY source of air, should be what is coming in thru the primaries. All other throttles should be closed up tight but not sticking. All vacuum taps on the intake must be proved not to be leaking. And the intake manifold must also be proved leak-free. The only exception is the PCV valve, which must be plumbed to enter just beneath the primary throttle valves.
So, then,
you set the mixture screws to, in the center of their adjustment range. Then adjust the speed screw to give her what other fuel she needs, for a smooth idle. If the IDLE-timing is too much, the throttle will have been too far closed. So now with you tweaking the speed screw, the rpm will rise up and be too fast. The only way to slow it down, is to take timing out. And this is the correct thing to do.
However,
when you do this, you are simultaneously destroying your Power-Timing. This is where the FABO timing plate come in. You install it at whatever position gets you both numbers. Such as 14 at Idle/34 after 3000.
If you have an automatic trans, your engine does not much care about any timing between idle and stall. So let the engine tell you what it wants and just make her happy at idle. Do not force any more on her, it will just give you other problems.
Now, if your engine has a big cam in it, say [email protected] or more; you are gonna get into an unhappy place where the engine is asking for say 14*, but you just can't get her to smooth out, and the exhaust stinks so bad it makes your eyes water and hurt. If this happens to you, the engine is wanting some idle air bypass; that is air that enters below the throttle plates, like where the PCV valve enters. Or you can do what most of us do, drill tiny holes in the primary throttle valves. Either of these two methods, IMO, is acceptable.
Step-3
So now, having established the two basic points, of 14*@under 1000, and 34* after 3000rpm, we have to connect the dots as quickly as possible, yet not so quickly that the engine gets into detonation. This is handled by the springs in the Distributor (hereafter just "D").
At your pressure, you may have to delay the "all-in" timing to well after 3000. I'll guess 3600 .. So you are gonna need a tighter than usual advance spring set.
And, you will want a stable idle, so the timing can't be jumping around under say 1000rpm. So now you have two data points; 1000 and 3600. Go find out what yours is doing and how close it is. Maybe what you have is close enough.
Lets say it comes in exactly as above. Then your curve will be 14 to 34, at 1000 to 3600. By the math, that is 20* over 2600rpm so; 20/ (2600/100) =.77 degree per 100 rpm.
So now you can figure a few things from that. Firstly, say you have a 2400 rpm stall; your timing at 2400 should be 2400 less 1000, divided by 100 and multiplied by .77 equals 11degrees which you add to the 14 Idle-Timing, equals 25* of stall-timing. So now, if your engine doesn't detonate on that, then you can try to speed things up a bit. But if it does, then you have to slow it down. This is done with different springs. But if you have a 3000 stall, then at 3000rpm your timing will be;
(3000 less 1000/100) x .77 plus 14= 29.4...
and so on.
Eventually, you may find that my initial guess of 34* at 3600 was too conservative, and so you can change your timing-plate........ and start over. Or as your tuning skills improve, maybe you can run more Idle-Timing. If you change either of those, then you have to re-examine your springing. lol, ain't HotRodding grand.
Step-4
Ok now we can figure out your cruise timing.
Firstly we need to know what your Cruise-rpm is, so lets take it at 2400.
Previously we found that the timing at 2400 was 25*. Now, is the time to hook up the V-can and map it. Suppose at 2400, your can is able to bring in 9 degrees. Your Cruise-Timing then, will be 25+9=34 degrees.
But is that correct?
Here's how you figure out what your cruise timing should be; For this test, the rpm must be kept at Cruise-rpm/2400 in this example. And the V-can must be working.
So then rev it up to 2400, while simultaneously dialing in some advance. Don't even look at the timing marks. Just keep advancing it until the rpm no longer increases at 2400 with additional advance. Now, read the advance at 2400 with a light.
Suppose you got 53*. That is what she likes at 2400 in Neutral. She just told you so. Now take out 3* for load compensation, and end up with 50* as your target.
But, but, but we figured out mathematically that 34* was all you were able to get. So your 9* can has got to go.... Hang on; firstly, if it's a Mopar can with a hex shaped reservoir, it's adjustable. So go find a 5/32 hex-key (allen-wrench), remove the hose and stick your wrench in there to find the socket. Then crank it CW until it meets resistance. Do not force it past the resistance. Now it will advance as fast as it can, to as much as it can.
Next; there is a number on the arm, like 7 or 9, or something. This is the max number of degrees your can is able to advance, in distributor degrees so, you double it to read crank degrees. So a #9 is 18* crank.
On the arm, you will find two stops. When vacuum is applied, the stops hit the back of the can, and stop it from advancing any further. You can cut some material off the stops, until she hits about 22/24 degrees. But leave the stops on there to not tear your diaphragm. So now, say you get 22* out of your can.
Add that to the 25 mechanical, and you get 45* is the best you can do. If you are not looking for max fuel economy, just leave it. And if your engine does not overheat in traffic, just leave it. Those last 5 degrees you can think about later.

Ok so where are we?
Oh yeah
You have to get your timings at least close, before you start throwing fuel at it. ....... which you will have to do eventually.

But when you speak of the engine not wanting to rev past a particular rpm, no matter what the throttle setting is;
and the advance mechanism is responding,
and the strobing is consistent, then;
One of about three things is happening;
1) you have hit a rev-limiter, lol, or
2) you have a mechanical problem in the valve gear, or
3) your cam-timing is out to lunch.
4) or your exhaust is plugged, but with open-headers, obviously that one does not apply.

Lack of timing does not act this way.
Fueling does not usually act this way, until the fuel-bowl is close to empty.. then it stalls.

For purposes of tuning; disconnect the linkage from the outboard carbs and wire them shut.

 

[Righty Tighty]

@RustyRatRod It did, for the better part of 20 years from what I was told.

So, if I follow your logic, if an engine sits for a long period of time, some of springs would be compressed while others aren't, and some in between. And are you saying that springs compressed for long periods may experience problems? It makes sense to me, as in perhaps the compressed spring retains a memory and doesn't open up as much as the rest, leaving that valve susceptible to being left slightly open.

@AJ/FormS , give me a sec to read your post.