I'm afraid you're going to have to do some valve stem inspection to see if there is evidence of valve-to-guide seizure. This means that at a minimum the manifolds will have to come off and you stick a camera into the port. The alternative is to pull the head and disassemble it. You could put a camera down the cylinders and see if there are witness marks on the pistons, but the absence of such marks would not put you in the clear.

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Joshua

Yep, I'm prepared to pull the head again if need be. If I can't find any intake leak or issues with dizzy/ignition, then it points back to the valves and I'll pull the head and remove the valves to check for issues.

So far I'm lucky there wasn't any serious damage, I don't have any bent push rods, no pitting on the pistons, and the lifter surfaces are fantastic. Slants are tough motors, I've seen detonation immediately destroy pistons, bend pushrods, etc. in "lesser" engines. I just had a head gasket failure, burned right through between #4 and #3:

http://s785.photobucket.com/user/67dart ... sort=3&o=9


thanks for your helpful input.

Brian

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Late to this party..... Looking at the original head gasket failure, my first guess was warpage of the block or head. You said you checked it with a straight edge... but what straight edge did you use? It needs to be true to .001" or less because you are looking for flatness in the head and block of .005" or less along the length and less crosswise. I use a precision straight edge designed to true up the ways on lathes and mills; that gives you any idea of how good this edge has to be to be worth using for a check. Did you check both head and block? They both have to be true. And you have to use feeler gages; don't trust your eyes alone.

Also, you needed to check all the head bolts for failure. Measure the length to see if any are badly stretched and look for any necked down areas with a caliper in the shanks. These head bolts can be re-used, but if one starts to stretch, it has started to fail.

Your compression numbers look good, but what are they hot, and after the engine started acting up again?

Did the engine overheat at all before the first head gasket failure? (BTW, you gasket failure mode is not all that uncommon.)

As for the valve adjustments..... check the ends of the rockers that ride on the valve tips. They can get grooves worn in them that will throw off your adjustments.

You can sometimes work out stickiness in valve guides due to varnish and burned crud with some carb cleaner worked down the guides.

I can't say that my straight edge was true to 0.001" or less; where does one find a precision straight edge?

I didn't check the head bolts meticulously, but I did check the ends to see if there were any signs that they were bottoming out. I didn't see any such evidence.

My compression numbers were not hot or after the acting up episode; rather they were cold prior to road testing.

My adjusters were flat on the surfaces that meet the valve stems. I could see lines where the two piece of metal that make up the adjuster were joined together, but the surface that mates with the end of the valves was all in the same plane, no grooves that were as wide as the valve ends. Is that what you mean?

Brian

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I pulled the distributor, the plastic gear was pretty worn, but i would expect that with all the detonation. Otherwise there were no tracks from arcing in the cap, the reluctor gap was good on all 6 tips, no signs of undue wear or shaft play at all, just the plastic gear was worn. I felt the camshaft drive gear, it felt good, no sharp edges, all teeth, no gaps.

Anyway, I have another freshly rebuilt super six distributor, so I put that one in. My old one had an 11 OR vacuum can, this new one has an 8.5 can. Otherwise identical units including spring kit.

While I was at it, I pulled the plugs. These plugs only have about 5 miles on them (30 minutes of running time, mostly garage during diagnostics), but you can see that only #1 (far right) is brown, the other 5 are white, speckled.

http://s785.photobucket.com/user/67dart ... t.jpg.html

As a comparison, when I pulled the head to change the burned through head gasket they all looked pretty much like this (though #3 and #4 had a green tinge indicating lean mixture):

http://s785.photobucket.com/user/67dart ... v.jpg.html

I checked the AFR again, it is 12.5 at idle and 13.5-14 at high rpms, that indicates to me that the mixture is not too lean. Still, I can't ignore the vastly different look of plugs #2-5 vs #1. Though, recall, #1 had a compression of 150 psi, while #2-5 were reading 180 psi, could simply be the amazing (spicy) pressure in 2-5 that is the difference. I would think maybe I just need to run rocket fuel in her, but I've already put 5000 miles on this motor and it was pretty stable on pump gas but for this detonation issue which has been sporadic over a period of weeks and has gotten steadily worse (even though I've used various gas sources, always 91 octane).

Well, back to my saga, recall, last time after it was running amazingly in the garage, I took it for a drive and it started the detonation and I crawled home, it was badly missing. Today I put in new dizzy, changed out the plugs, fired her up, runs like a champ again, see:

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

I have the lash set pretty loose, so it's chattering more than usual.

This is how it was running the last time I changed the plugs, took it out and after about 10 minutes of driving, kaput, bad miss, no power, detonation, crawled home. Bad gas? Maybe I need higher octane for this engine, the DCR was calculated at 8:1, should run on pump gas OK, did for the first 5000 miles...except for the occasional detonation, but that could have coincided with bad batches of gas I suppose. Though, there was that one time on the highway that it had a big power fall off and was running rough, then picked back up and ran fine the rest of the day (my fuel tank is new, there is no gunk in my tank, the strainer on the sending unit is clean, checked all that).

Today, I took it out for a brief drive, it ran great, lots of power. Got about half way down my hill, turned it around and came back uphill to put it under load, I could still hear some detonation, not too bad, but I immediately brought her home. This is precisely what it did last time, it runs great for a bit then the detonation creeps in, then it runs crappy. I suppose it's fouling plugs, but it wasn't doing this for the first 5000 miles.

Oh yeah, I also checked the vacuum at the intake it is fine, I can't find any intake leaks. See: http://s785.photobucket.com/user/67dart ... o.jpg.html

I tried a different plug this time, I was running NGK ZFR5N's, but today I tried NGK BCPR5EGP's which I think are a colder plug (shorter tip). I couldn't tell any difference, that video above is how it was running last time before it acted up, same this time, sounded strong, no missing, in the garage at least.

I'm stumped. Anyone think this is simply bad fuel? I can't find anything mechanically wrong with it. If it is a valve sticking, surely it would do it in the garage when I'm testing it, but that's not happening, it's running strong as an ox in the garage. Would a valve stick only under load?

Brian

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This sounds to me like you are checking the high rpm mixture by revving it in neutral. That won't tell you anything. I don't know what you are using to check AFR but I would look at how the number changes when the detonation is happening.

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I'll hook up the wide band sensor tomorrow and take it for a drive. Good point.

The thing is, let's suppose that nothing mechanical has changed, and the electrical is also good/unchanged. Why would I be getting detonation now, but not before, and only sporadically before. Could it be as simple as bad gas?

B

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Maybe your ears are too good, or your exhaust is not loud enough? Two things to consider!

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12.70 @ 104.6

Hey, Rick, well, I have toned my exhaust down considerably. That's why I was considering back pressure as a possible cause of my detonation, but alas, there is no back pressure to speak of....and the detonation is a real issue as my head gasket was burned through between #3 and #4. I suppose I noticed it even more when my exhaust was quieted down, nothing left to focus on but that.

Anyway, it's progressed to the point that I lose power and performance within 10 minutes of driving, so it's pretty serious.

I'll have to put her under load and check the AFR to see if it's too high under that condition. in the garage it's well within safe range at high and low rpms. I question this as the source of the problem, as I have not changed my carb settings over time and yet this problem has progressed steadily over time. I still have this nagging feeling that its a valve sticking under load when hot, but I have no evidence to back this up. The bottom half is OK, lifter surfaces are pristine, no exhaust back pressure, the AFR seems OK, I've swapped out ECU (orange box), and swapped out the dizzy for a newly rebuilt unit. The plug wires, cap, rotor are all fairly new. It does seem to be fouling plugs, it gets bad misses and detonation, then I change the plugs and it runs great, at least in the garage.

It has really good compression (cold crank testing) which suggests its not a valve issue, but if not, then I'm stumped. maybe bad gas, I suppose I could try some octane booster, see if the detonation stops. The engine is a street build, but essentially a race engine. Not too aggressive, but the strongest slant I've ever owned. When it was not experiencing this issue, it was running like a solid 318, not a hyped 318, but still, I've never had a slant run like a solid 318, pretty nice and 19 mpg, even with dual 2bbl carbs. I have 2.76 gears and it would chirp second gear regularly (my dad's rocket 350 olds did that, but a slant six?!)

brian

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It's hard to imagine how the problem could be 'bad gas'. Old stale fuel does lose some octane but not a huge amount. If the fuel in your tank somehow got some diesel mixed with it that could certainly be a problem but it would not be intermittent.

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I live in the SF bay area, and they often play with "winter blends" and change up what's in the gas pretty regularly. I agree with you, though, I have a hard time believing it's suddenly a bad gas I'm getting. If I do the AFR test under load and it's still within acceptable range, then it points toward valve sticking. The fact that this problem was sporadic over time suggests bad gas batches, but it's gotten steadily worse. At this point I can't drive it more than 10 minutes without loud detonation and concomitant loss of power. The car is useless until I figure this out, looks and sounds great in the garage, but not road worthy by any means. I really want to move on to body and interior work; as interesting as it is, for the scientist in me, I am getting impatient. I have procured another head, it has a fresh valve job and has been milled, but needs a few areas cleaned up where somebody knocked it against something and there is some factory casting that needs to be deburred. If I can't pinpoint anything, I'll swap out the head and see if that does anything. There is little chance that two different heads will have the same valve issue (if in fact its a valve issue). If it still has the problem, then at least I've proven that it's not the (current) head and valves.

Brian

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This question is being asked in order to learn and understand, not to question the validity of a theory: How is it that a sticking valve can cause detonation?

I can imagine how a stuck valve can cause a misfire, as well as low power, but how does it create conditions for detonation to show up? And if it is would it be the exhaust valve causing the problem?

Also would a vacuum gage show a sticking valve, such as is being discussed here?

The vacuum reading on the intake manifold can indicate a sticky valve. My vacuum reading was steady,,so it didn't indicate a sticky valve. That said, I'm only seeing the problem under load. I'm guessing it would be an exhaust valve that would, when sticking, allow the air/fuel mix to detonate and escape through the exhaust without producing any power and it would be lean because it is open to the air in the exhaust port. So, not detonation in the conventional sense, which is where fuel mix ignites prematurely, pushing against natural movement of engine components. My dizzy gear was pretty worn, so there was at least some of that going on, but no bent pushrods.

It could be that I'm getting detonation in the conventional sense, as I have really good compression and if my fuel mix is lean under load, then it might detonate prematurely. The pistons were relatively clean, no deposits to speak of or any sharp edges to get hot, the head was worked to remove and smooth aall that. I'll test it today under load with a wide band sensor, get another data point.

B

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Ok, That makes sense. Thanks

OK, so I hooked up my wide band O2 sensor and took it for a spin. Under load it still is making the pinging noise, and the AFR is just about stoichiometric, a bit richer. I did both banks, same result. Here are video's of me crawling up the hill under heavy load.

Front three: http://s785.photobucket.com/user/67dart ... w.mp4.html

Rear three: http://s785.photobucket.com/user/67dart ... 7.mp4.html

If anything the rear is a bit richer than the front. So it doesn't appear to be that the mixture is too lean. On the highway, cruising at 70 mph, AFR reads 14.5.

I dialed back the timing, it did seem to help some, but I'm still getting the ping pretty loudly. I'll dial it back some more and try again, I ran out of time today.

Here is the AFR reading in park in the garage, at 900 rpms and revving it to about 3000 rpms. Engine sounds really good and strong. Doesn't seem to be missing like it was, but I've only driven it for about 10 minutes total since I changed the plugs and the dizzy.
http://s785.photobucket.com/user/67dart ... r.mp4.html

In my "spare" time today, I cleaned up another head, just had a valve job. I have no idea what valves or springs or whatever, but it's been planed and a fresh valve job. I'll hold off until I've done more testing; it seems a bit better with the new dizzy and the new plugs, maybe I can dial back the timing some more and it'll idle OK. Still, this is where my timing has always been on this engine, and it ran fine before, no pinging/detonation issue for the first 3000 miles.

Something's up, when it's cold, I took it onto the on ramp and kicked it down, it dropped into second gear and really took off, sounded great. AFR was in safe range the whole time. Once on the highway and warmed up, when I push on the pedal it is really sluggish, no power to speak of, seems like somethings holding it back, I get the pinging, but the AFR is between 13 and 14.5 the whole time, it has plenty of fuel in the mix.

Back pressure revisited: When I checked my back pressure it was in the garage, there was no back pressure to speak of at idle or at 2500-3000 rpms, needle hovered at zero the whole time. Could that change dramatically if it's under load? I can try that, see if that makes a difference.

Brian

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