This isn't the first time this has happened. I start hearing a slight popping in the exhaust systemthat eventually gets louder and more frequent until the motor begins to stumble very badly (takes a few days to progress usually). I HAD a possible contributing factor in that I found a badly collapsed rubber fuel line back at the tank. I have since replaced the entire fuel line (went from the original to a 5/16 alluminum line). The other possible contributing factor was poorly adjusted valves. I had a fair amount of rocker arm noise, but have since had them adjusted.

Something I've been battling for some time that may or may not be a part of this is an odd combination of plug readings. I've got a Holley 650 double pumper setup with 69 primaries, 74 secondaries, and a 10.5 powervalve (like 15+HG of vac at idle). The car is responsive with this setup. However...my plugs have me baffled.

#1 and #2 on the driver's side and #1 on the passenger side are getting VERY sooty. I mean CACKED black. #1 driver's side is also crusty black.

All the other plugs are PERFECT. I mean look up a photo of what you want your plugs to look like...that's them. nice toasty brown, no soot, no scarring. Really nice.

SO..with all that said...

Is the popping in the exhaust likely related to the plugs?

What would cause plugs to read out like that and how do I check/fix it? It was suggested to me that I lean the front jets down a step to 68's and go richer on the secondaries to like 76's AND drop down to a 4.5 powervalve. Could the PV be to high and open at idle causing the fouling?

I probably could use a little more advance in the timing (not much)...could that be part of the popping?

Should I have the valves gone through again?

Thanks tons all!

There could be a problem in those particular cylinders but it's more likely that there's a problem with the ignition circuit in those particular cylinders. I would start by replacing all the plugs AND inspecting your wires. If the wires show any sign of damage, or if they just look old and you don't know how old, replace them. The same goes for the distributor cap and rotor. The popping in the exhaust is likely unburned fuel from one cylinder meeting the hot gasses from a properly firing cylinder. Improper timing, combined with fouled plugs (and/or bad wires), could be a cause. In that scenario, inadequate advance would make it even more likely for a mis-fire.

Jerry is correct on where to start and should take care of it, but in the odd chance the problem is still there, I would recheck the valve adjustment, I had a engine that did the same thing and I had a few valves too tight.

Thanks gents. The wires are practically brand new (8mm MSD wires). I hooked an inline spark checker up on #1 and it's getting excellent spark (at least TO the plug). The cap and rotor are MSD parts and appear to be in good shape. I took them both off and cleaned out any carbon dust and wire-brushed the internal terminals on the cap to make sure they were clean. No difference...still popping. I also had replaced the plugs on the 3 bad cylinders 2 nights ago and still had popping that next day. I pulled the #1 again last night and it was black and crusty again.

Starting to sound more like valve adjustment? What is the best method for adjusting roller tip rockers on a roller cam?

Isn't it odd that the front 2 cylinders are the worst? Could it be a fuel delivery issue?

Could be bad news, cam lobe going flat.

Is it a high milage engine? Not uncommon for them to wear down, even less uncommon these days to wipe a lobe at startup on a new engine.

Turn the engine over and make sure all the rocker tips travel the same distance, if any don't move as much as they should look for a lifter/pushrod problem first but more likely a bad lobe.

Don't ask me how I know!:(

Yes, re-check valve adj. Is this solid or hydraulic roller? What are cams specs? How are you adj. the valves? What springs and how old are they, have you checked spring pressure while adj. valves with one of the spring checkers like Moroso or others sell? This is how I saved my engine from blowing up from just starting it up when found #1 intake spring virtually breaking apart. I think your idea of trying lower PV would be worth a try-Holley 950hp come with 6.5's so may try that or lower. I would try the old trick of pulling off plug wire while engine running to see if runs the same or drops in rpm! You get the idea right?! Double check for vacuum leaks-checks manifold bolts and hoses. Good luck, you'll get this thing solved!

Thanks again guys. the motor was a fresh rebuild when installed about a year ago...only a few thousand miles on it at this point. Unfortunately, I don't have exact specs from the builder. I know it's a '99 350 block with a 327 crank and domed pistons. I was told it was a full foller motor (but it only has roller tip rockers). The cam was said to be a "standard" late model roller for that block. It has no fuel pump lobe if that says anything. I don't have the lift and duration.

I haven't adjusted the valves myself but had one of my motor-head buddies do it (he's got some serious muscle in a few chevelles and a camaro or two..he knows what he's doing). I've been told (not by the guy that did the adjustment) with a roller setup, you back them off until the clack, then tighten them back down just barely until the stop, the a 1/4 more. Sound right? Same guy told me you can't use a feeler on a roller. True? How am I SUPPOSED to adjust them?

I have not checked spring pressure. i will.

The car idles very nicely at 700rpm. Rev's up nice...responsive. I can't imagine that any of the 3 cylinders with issues aren't firing at least somewhat, otherwise it wouldn't idle this well.

The carb size and powervalve rating are unrelated. The PV rating is tied to engine vaccuum (that's what opens it...when vac drops below the rating). Don't mean to tell you what you already might know... ;-)

did some more testing and tuning tonight...

compression check...clean. ~180 on all cylinders

i put an inline spark checker on the "bad" cylinders and it fires brightly. I guess that doesn't mean the plug is firing, but I'd bet it is

I turned the engine over by hand to try to find absolute (or reall close) TDC on #1 but had a hard time with the old screwdriver in the hole trick. I couldn't tell what the hell I was feeling. Felt like I was touching the SIDE of the piston. Well, I was able to get a pretty good feel for when it stopped going up and the timing mark was almost right on, so i went from there. It didn't hit me until i was done...I have DOMED pistons. DUH!

How do you feel for TDC with domed pistons? is there a BEST way?

Anyway, I used the timing light to set it at what looked like about 12deg BTDC but set there, WOT knocked like a mother. So, i just went by feel...I backed off the timing, drove it, and kept doing it until I could stomp on it with no ping. Results were about 8deg BTDC initial (didnt get a total advance reading). Car is still quite responsive and no more knocking.

Now, the plugs...scary question. If the plug was too long or screwed in too tight, sticking the electrode too far into the cylinder, could it make contact with the piston and assuming it was only barely contacting, would THAT keep the plug from firing correctly?! I think that MIGHT be an issue because when I pulled the plug, the gab was smaller than when i installed it (45 when it went in and about 30 when I removed it). What other than bottoming out against something in the cylinder would cause that? This is not the case on ANY other cylinder so I don't know for sure it it makes sense.

The block is a '99 350 block and I was assuming the heads were as well. WHere can I find castings on the heads to know for sure. The engine installer originally put Delco plugs in (short noses) and they were never quite right. i realized that he used Delco plugs for an old motor and I switched to NGK's for a '99...longer nose. All of the other cylinders are perfect so I thought maybe that was a successful change.

In the end...i think I've eliminated the pinging for now, even though I dont have as much advance as i think I could. I still have popping and I would venture a guess that tomorrow, those 2 cylinders will be fouled.

I'm REALLY starting to think I need to go the other direction with the jets. I've been dropping jet sizes to this point and was excited that the other plugs looked so good, but it's been mentioned more than once that too lean can cause a misfire and fouled plugs AND that too lean in the secondaries could cause WOT pinging.

Thanks for keeping up with me on this!

Just read this...could this be the culprit (plugs too long?)

The worst and most immediate problem created by an overly-long plug in an engine is that the exposed threads absorb a terrific amount of heat from the combustion process. This raises the plug-nose temperatures, and may take them up high enough to make the side electrode function as a glow plug. And when that happens you have the white-hot electrode firing the mixture far too early, like an over-advanced spark timing but worse because the early ignition causes yet higher combustion chamber temperatures, which causes even earlier ignition. This condition is known as "runaway pre-ignition," and if it is allowed to proceed it will wreck your engine.

I have to find the castings on these heads...where should I be looking?

Thanks!

Anyone maybe with a few more thoughts on this after my latest readings?

Thanks!!

I have a feeling that you may have a leaking intake manifold gasket. Like the gasket may have slipped and exposed an intake port to the oil in the lifter valley. That would explain the "crusty" plug. Its probably a long shot but it could be.

Ihe issue of an overly long plug is doubtful. I think you are probably referring to the "heat range" of the plug. I don't pretend to know every style plug GM uses in ALL of their engine but aren't your plugs the standard "gasketless" ones with about 1/2 " of thread. If so there is no way they will physically contact a piston. As to the gap change , detonation will do that. You say you backed your timing off so hopefully you'll no longet see a gap change.


Awesomely good looking car and engine bay!

I'll check the intake fit.

Does the heat range of a plug translate directly to the overall nose length? I know the Delco plugs I had in it before were shorter nose plugs. Is that the difference in heat range?

Thanks!