So, this is gonna be hard to describe, but here goes. I was tuning my engine this evening because I finally got around to installing my long tube headers. That's all it had on it, because the full exhaust isn't being made until it goes in the shop next week. So I slapped the headers on and got her warmed up to start dialing the idle settings back in on the carb (yeah, I know, a dirty word 'round here).

So here's where it gets not so good. The engine was fully warmed up, about 195* on the thermostat. It was idling at 850, a tad rough but I was working it out. 32 pounds of steady oil pressure. Everything seemed good. But then as I was walking around to hit the other side idle screws the exhaust note changed. Remember, open long tube headers so it was ear-splittingly loud, but there was a shift. Instead of 8 sharp exhaust note cracks, there were suddenly 7 loud exhaust note cracks and one somewhat quieter exhaust note "chuff".

For the old timers and serious gearheads, this one cylinder reminded me of the sound a Model A ford makes on all 4 cylinders. A softer exhaust note, which I'm pretty sure is caused by low compression. Now, this engine has barely 150 miles on it, this new noise is present at all rpms (although you can't hear it past 1800 because the engine is too loud). What is most likely here? I'm gonna check compression on all 8 cylinders tomorrow evening. I'm hoping it's a lifter issue. It came on all of a sudden, and hasn't gotten any better or worse in the few minutes more that I kept tinkering.

Overpressurized lifter hanging an exhaust valve perhaps? Or collapsed lifter on an intake valve? I'm just really hoping it's not more serious, but I don't see how it can be. There doesn't appear to be any accompanying rod knock or piston slap. No sudden exhaust smoke or anything. They're Speedpro Hyper-pistons BTW, and in my experience they usually fail suddenly and completely when they go. Anyone ever had an engine do something similar to this before? Hopefully the compression check will tell all.

EDIT: I suppose it could just be a blown exhaust gasket, since these are new headers, but this is really more of a "chuff" than the hiss I usually associate with a leaky header. I'd be rather surprised, in fact, if I could hear a leaking header gasket over the sound of the other 7 cylinders pumping through open long-tubes.

 

Jon,
Sure it is not an plug wire came off or is loose? Or did the headers burn a plug wire?

 

John,
Check your injectors or your spark to all 8.
It sounds like the compression is there, but no fire, or no fuel on one cylinder.

 

Hmmm. It's quite possible I fouled a plug tinkering with the fuel like that. I'll pull and check 'em all this evening, along with the wires. I was in the driveway and it was dark, so I think I'd have seen a burned wire arcing, but there again I could be wrong. Hopefully it's just that and not something more serious.

Coupeguy, I'll have a check at spark, but being a carbed motor I know I'm getting fuel to all 8 cylinders.

 

First thing that came to mind...chances are one is grounding on a hot tube

 

This is where a point and shoot IR thermometer is great.

 

Ok, checked the engine last night. None of the plug wires were on the headers, none have any melted spots. I didn't test them individually, but they are as new as the engine and no evidence of problems. I pulled all the plugs, all were a dark tan color. A bit on the rich side, but none were wet or fouled so they were all firing. Compression test yielded ~150psi on each cylinder, no problems there. Valve cover inspection holes off and watched all the pushrods and valves, no valves appear to be hanging. Plenty of oil to all the valves, etc. Not really sure what this means now.

My buddy thinks it's just a weird resonance with the new Tri-Y headers. But I tuned that engine for an hour the night before last, and I swear it didn't start making this noise until the last few minutes before I shut it down. When you rev it up, you can no longer hear the different noise after about 2k rpms, but this is likely just because the engine is so loud at that point.

It could still be an exhaust gasket, but I checked all the bolts and they're not loose at all. No obvious evidence of a gasket leak, but I guess it's possible. I'm going to wait until the exhaust is finished up in a week or two and see if I can still hear it then.

There's no noise when cranking, so it's not a rod or something bad. If it were serious I wouldn't have great and consistent compression. I figure I'll just hold off on driving it much til the exhaust is done and see if I can still hear it.

Anyone got any other ideas?

 

Exaust manifold/header gasket is most likely shot.