If I think about it, it only has a finite amount of mechanical advance, so it must be that I didn't have the vacuum line pinched properly. The vacuum can is still working, the diaphragm hasn't ruptured.

Anyway, I'll be conservative and set it just a few degrees advanced, drive it to work this week and see how it goes. It's running fine now, no weird detonation noises or anything.

I suppose I was lucky with that adjuster moving so much. I should mark them with paint or something and check them next week.

Anybody know if sharpie will stay on or will it wash off with oil? There's still some red overspray on the pushrods from when I painted it, that stuff seems to stick pretty good.

Brian

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If your vacuum advance pod is running off of ported vacuum, just pull its vacuum line from carburetor, no need to cap off at carb as is only sees air above throttle plate not manifold vacuum.

If you have vacuum pod seeing manifold vacuum, than one must cap off the tap on carb as this would act as a vacuum leak when disconnected.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Again, immediately after I changed the head gasket, the engine ran very smoothly, but under any load the loud sound was there. Not unexpectedly, I found that the valve lash was too tight, after having just changed the head gasket. But, the sound went away immediately (on the next test drive) after I loosened up and set the valve adjusters. Problem solved, right? Nope. As I was driving it to work, I still heard a little bit of the sound, faint, some under load up big hills. But, alas, after a hundred or so miles, it's come back (loudly) full time and run rough again, like a dead cylinder.

I'm wondering if its a sticking valve. Adjusting lash after the head gasket change definitely made an immediate difference. I suppose the adjuster might have moved again, I'll check them this weekend, but I suspect that's not the issue.

I thought it might be an exhaust restriction, because I had recently installed a 2" perforated SS tube in about 2.5 feet of my 2.5" pipe, from say under the driver seat back to the muffler. I wouldn't think this would create any significant back pressure, but you never know with resonance and such. It did quiet the exhaust down a good bit. Having said that, I recall this sound happening before I installed this exhaust, so I don't think that's it, that's a red herring.


Here are some recordings, first one is going up steep hill near my house, second one is at idle in the driveway, no load but bad miss or dead cylinder:

http://s785.photobucket.com/user/67dart ... q.mp4.html

http://s785.photobucket.com/user/67dart ... o.mp4.html


Up hill sounds like a loud rap, like a bent push rod or connecting rod or wrist pin, bad bearing, and no power to speak of. Once on level ground it is quiet, just runs rough like a dead cylinder or bad missfire, no rapping noise at all. As I said, last time I pulled the head the lifter surfaces looked perfect, no bent push rods, oil was clean no obvious signs of metal wear. I think the sound is detonation while one or both valves are partially open. Also, as I was driving it home, mostly level ground, it started to lose power, at one point, I couldn't get it above 55 mph, and I could feel the engine shaking through the steering wheel. But then after about 5 miles, the power returned and it was smoother, not full power, but I could easily accelerate to 70 mph.

More context: it was smooth and quiet on my way to work on Monday, but after about 60 miles or so it was running a little rough, just after I kicked it down on the highway to test acceleration. At work, I opened the hood and saw some oil drops around my breather cap, which would suggest crankcase pressure spike perhaps Also when I pulled the head weekend before last there was some oil on the valve cover (my engine is usually oil free at least on the top half). These oil signs are pointing to crankcase pressure which could be valve related, backing in through intake system. It has deteriorated back to previous bad running, loud noise with any kind of load, just not under idle (but is missing at idle). I'll check back pressure on the weekend, check PCV valve, check intake vacuum, recheck lash, check the plugs. Something's up. Signs point to sticking valve I think.

All thoughts welcome.

Brian

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I will check for back pressure; and thank you for the factory manual procedure on that.

As it turns out, I have a much easier way to check. I have bungs in each down pipe for my wide band O2 sensors. I can check back pressure directly there with a pressure gauge. Typical, normal back pressure at idle is less than 1.5 psi, at 2000 rpm less than 3 psi range. Of course if varies by car, setup, but I'll see if it's out of ballpark or not. Also, I will check intake vacuum.

I'll do a number of diagnostics this weekend.

FYI, when it comes to things developing problems, I seem to have a knack of having the most weird problems for any system, be it a slant six or a computer or what have you. I regularly call my IT department with issues. They, understandably, assume that I'm computerally illiterate, but they inevitably end up saying, "wow, I've never seen that happen before...I'll have to check and get back with you."

BTW, another symptom that I saw when I changed the head gasket, was #4 and #3 plugs had a green sheen. This would normally indicate lean condition, which is consistent with detonation. But, since they were "talking" to each other via the burned through head gasket, there's no way to know if the lean condition wasn't related to a valve sticking or that cross-communication between cylinders (i.e. fuel detonation at the wrong time, more air getting in than normal, etc.). I'll check AFR, but I have a very solid idea that my carbs are working fine. I have checked AFR before rebuild, and after, and the AFR is fine, 14.5 at idle, and 12.5 at higher rpms, should be plenty of gas getting in there. It would be very strange for two cylinders, or one, to not be getting enough gas, while all the others are getting sufficient supply. I suppose that could happen, but it seems to me that this is something more obvious, like a sticky valve in one cylinder, causing detonation.

That all said, I can't say anything for sure at this point, except that I have a problem, I haven't solved it yet and I'm frustrated with slant sixes. I grew up with them being bulletproof, and I've had a ton of issues with them in the last 3-4 years.

b

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Plugs look great. They all look like this: http://s785.photobucket.com/user/67dart ... v.jpg.html

Not the ECU (orange box) sending too much advance (unless my new spare happens to be doing exactly the same thing).

Not exhaust back pressure, none to speak of at idle or at 2500 rpms (red herring proven as such).

I'll run a compression test tomorrow; check the dizzy for signs of wear or broken springs.

Brian

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Might be ignition cross fire . Cap vented , carbon tracks, ohm test wires, Drill hole in cap at number one check rotor position with timing light.

OK, engine has good compression, here are the readings in psi:

1-150
2-175
3-180
4-175
5-175
6-175

My only thought there was I wish #1 was the same as the others, but 150 psi still seems pretty good. I don't think I need a leak down test at this point. At least the cylinders are creating pressure, no valves stuck I suppose, at least cold.

Reassembled and rechecked the valve lash, that is good.

Next step is fuel supply, either an intake leak or the carb jets are partially clogged or something. I took a much closer look a the plugs, #4 and #3 had a very slight green tinge (had to use a magnifying glass and a bright light to notice it, felt like Quincy), but mostly tan colored. Still, that suggests lean mix in the rear runners, so the rear carb is my (current) prime suspect. If that is the case, it is intermittent, as it ran fine for about a 100 miles or so. But this would be consistent with that intermittent power loss on the highway. The bad miss at idle could just be fouled plug due to detonation.

Its just weird how the valve lash adjustment after the head gasket change coincided with the detonation going away temporarily...gremlins. I suppose a particle could have dislodged and got sucked back into a jet.

I'll swap out the plugs first, see if the miss goes away at idle, but then on to inspecting the fuel and air supply...getting there.

Brian

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It's good that I have dual carbs. If I had only one carb, then I'd see the green tinge on all the plugs and it might still be timing or fuel mix, but because I only see the tinge in the rear bank, that at least points to fuel/air supply over timing I suppose.

b

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Well, I carefully checked out the carbs, the linkage was a little off, the rear carb's secondary wasn't opening as fully as the front, corrected that.

Checked AFR, it idles best at about 12.5, anything leaner and it idles rough; must be the cam just needs that. At high rpms, it reads about 13.5/14, still way within safe range, not too lean. I can't get it any richer without bigger secondary jets. Anyway, both banks gave identical reading, the carbs are perfectly in balance, air flow is the same (checked with snail gauge), AFR readings the same at high and low rpms.

After all that tuning, and changing the plugs, I took it out for a test spin. It ran great, no misses, but up hill I still got a bit of detonation. Hmmm... Took it back to the garage and checked the timing. It was a little too far advanced, as the total mechanical was 35. Set the timing back to 10 degrees at 800 rpm, 30 total at high rpms. Boy it sounded good, I could rev it up really high, AFR was about 13.5 on both banks at high rpms, seemed really strong, no misses, I've never heard a slant sound so good and balanced; idle was very smooth.

Took it out for another test run, it had plenty of power. I have 2.76 gears (highway car) and still it was chirping the tires around corners and such. Took it on the highway, after a couple minutes (about 10 minutes of driving total, so fully warmed up) the loud detonation came back. It was idling OK, no miss that I could discern. Coming back up the steep hill to my house the detonation was really loud, no power to speak of - crawled home. Curious, about half way up my hill, just for a few seconds, the power picked up and it quieted down, then the loud detonation came back and the power waned.

So, I think this is pointing to valve sticking, but I'll try the distributor first. Seems to me that a bad distributor would show itself when I set the timing or when I was revving the hell out of it checking the AFR, it was running spectacularly in the garage...but when I took it out and put actual load on it...ugh.

Brian

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I'll check it on the weekend, no time during the week. Seems like it has to be ignition system or valves, one of those two. b

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You would not be the first to have a sticking valve. Unless it sticks really hard it won't bend a pushrod, but the valve will hang open and kill compression and power. With the valve partially open there will be a lot of valve lash and noise.

I haven't read everything you've written so excuse me if this question has been asked and answered. What valves and guides do you have?

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Joshua

I have over sized valves, not sure of the source on those or on the guides.

The fact that it seems to be on #4 makes me think valve or distributor. Also, I'll check again for intake leak, but I didn't find any last time I checked and I had planed the manifolds flat. Honestly, I've had those manifolds off and on so many times I can't recall the last time I checked, it wasn't when I changed the head gasket two weeks ago, that I'm sure of.

Seems like if I had a bad intake leak I wouldn't get such a smooth idle and my AFR would be higher on one bank over the other, they're identical (and in safe range, not lean).

Brian

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I'd be willing to bet that if the head was done recently (in miles) that the guides are too tight and that an exhaust valve is sticking when it gets hot. Stainless and bronze expand more than cheaper steels and iron.

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Joshua

Joshua,

The thing is, I read that sticky valves typically were stickier cold than hot, my issue seems to arise under load and after its warmed up. This is counter intuitive to me, as metal expands and a slightly bent valve would stick more hot than cold. I suppose what I read could be referring to gunked up valves, I can see how those would move better hot than cold. My engine is super clean inside, having been rebuilt only 5000 miles ago, no gum or gunk in there.

Anyway, I'm narrowing the field. It's not back pressure, lean mixture (at least from the carbs, I'll check again for intake leak), ECU (unless two boxes are both doing the exact same thing), and I have excellent compression on cold crank test, about ~175-180 psi per cylinder (#1 was only 150, but maybe I'll check again after loosening lash a bit).

Brian

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