Fuel's plenty good enough.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Years ago, when I was a young lad, fairly new to slant sixes and working on cars in general, I was working my 76 Volare Premiere sedan I had bought out of a junkyard. The motor was finally running good, until one day it wasn't. It idled rough, made a chuffing noise, and power was down. I couldn't figure out why. I eventually tracked it down to a small bolt that had worked its way down the intake passage and wedged under one of the intake valves, blocking it open. Here's the mystery- it wasn't a bolt that would be used anywhere on a car, and the carb never came off the intake. I removed it, bolted the head back on, made sure the unbent pushrod that had fallen out of the way when the valve was held open was properly seated, and all was well.

The point of my story? Dan- did you lose a penny or something down the intake or cylinder the last time you had the head or carb off? Something rattling around inside the cylinder? You seem to have covered everything else.

Good luck!

Dan, I am probably the last person you would take advice from, but I'm with Reed on this. Being well aware of your knowledge on the SL6, are you absolutely certain it's pinging you hear, and not some other noise that can be mistaken for pinging?

The little cover between the engine and front of the 904 on my car came partly loose, and drove me almost crazy by vibrating at certain speeds only, and transmitted through the engine, it sounded just like pinging to me.

Usually you need a bit more massive part, containing enough metal to produce a clanging sound when hit, to make it sound like pinging, for example something like a solid stay between engine/gearbox and exhaust pipe, vibrating in tune with the engine most of the time, unless it's bolt is a bit loose...

My 2¢.

Olaf

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Aspenized

Given what you've covered so far, and if my plug colors weren't showing lean, I'd also consider a non-internal source.

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I have had suspected internal engine noises turn out to be exhaust leaks, loose torque converter bolts ticking on the torque converter dust cover plate, oil dipstick rattling on the crank or the air cleaner, etc...

I don't want to question your slant six savvy, but even the best of us overlook the simple things sometimes.

My 69 225 auto was doing this as well.ie pinging really badly at light throttle.In the end i pulled it apart for a rebuild it got so bad.
Found broken rings on 5 cylinders and a burnt exhaust valve and vacuum advance seized up.All good now goes like it should.
Not sure if this helps but you never know.
cheers

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Actually this was my Grandpa and Grandma's Slant Six

Thanks, guys (and yes, that includes you Olaf!). I'm not averse to "Are you sure you've considered…" advice like this; it's good, but not what's going on here. This is a gradual deterioration since I bought the car about ~9K miles ago in Autumn '09. Idle's been getting progressively rougher, power's been gradually dropping, ping (definitely ping and not anything else) on highway has been steadily growing worse. Multiple distributors, multiple carbs, different intake manifold (w/good gaskets, etc.), head's been off, different spark plugs, multiple valve adjustments, etc. Pretty sure timing chain stretch is to blame for most of it; it was a little over 1/4" or so when I bought the car, now is over 9/16". Pretty sure I've got a tired pony, just trying to dope out how come it's not behaving as I'd expect it to with the cam retarded ~5+° from its nominal (already retarded in '73) position.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

EGR failing.

Only if it's part throttle pinging.

With not enough EGR, there's too much advance, and the peak combustion temp is higher than expected.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Nope. EGR already covered in original post.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I think the only thing that will solve this mystery is a post-mortem once the new motor is installed. Spin the crank until the timing dots line up and see if the #1 piston isn't where it should be.

Not sure if I have much to add.

Did someone mention hot running, and is it doing so? Many things could cause this (incl chain slack) and thus pinging. Spun damper giving inaccurate ign timing mark? Progressively worse is the poser... I have some real oil burners that did not ping.

Maybe you're just driving uphill all the time??

Lou

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Home of Slant6-powered fun machines since 1988

Brand new Chrysler heavy-duty (truck) damper, no evidence of overheat, though this engine has been overheated in the past. I think the postmortem idea is on target.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

points gap can make it retard/advance..I do not know about eletrics..issues there?

check rockers..working properly w/valve cover off..visual inspect../pushrods /bent/ wrong adj gap can increase comp.

any intake leak...running lean...start up let idle spray engine digresser near intake gaskets..does idle change?

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What if the intake is just closing on firing or the exhaust stroke has built cylinder pressure when the exhaust valve opens creating a ping like noise? Maybe fuel igniting in the tailpipe?

I think it's an Olephaunt deficiency.

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