Z Noise

[djhedges]

Latest video I've taken of the engine running.

Background

ITS 260z car so yes it has to keep the flat top carbs.

Car was dynoed tuned last year I think and ran fine at the SCCA school (March). When we went to unload the car from the trailer it sounded horrible.

I'm suppose to be at the race track today.

Things Tried

• Pulled the headers and re-wrapped them to ensure there were no holes or cracks.
  
• New intake/exhaust manifold gasket.
  
• New spark plugs, wires, cap & rotor.
  
• Cleaned the carb slides. Also tried swapping the slides between the front and rear carbs.
  
• Compression test looked good. 155/145/150/130/150/150 (psi read from cylinder 6 to 1)
  
• Replaced a couple of gaskets on the front carb since it seemed like it was leaking. Float bowl wasn't stuck or anything.
  
• Adjusted the valves. They were pretty good to begin with.
  

Things Observed

• EGT probes on cylinder 1 & 6 display 300F and 600F respectively.
  
• Fuel level in the carb windows seem identical.
  
• By pulling 3 spark plug wires the car runs better on the rear 3 cylinders.
  
• Attemped to pull one wire at a time and it was difficult to notice a RPM drop. However when I pull 3 wires and plug one back in it's easy to notice a RPM increase. My gut says theres spark.
  
• Observed the slides moving up and down with the car running, airbox off and fiddling with the throttle.
  
• 3.5psi Fuel pressure gauge just before the carbs.
  

[Zed Head]

Bad gas? Today's fuel doesn't last very long.

[Mark Maras]

It sounds like valve train noise to me, I'd pull the valve cover & see if the oil tube is squirting, drooling or leaking. It should be squirting oil out uf 12 holes in the tube. The lower compression on #s 2 & 4 would have me rechecking the valve adj on those two.

[djhedges]

Gas is non-ethanol and was bought around March 22nd so it's only a couple of weeks old.

I pulled the tube off just to make sure my make shift gasket wasn't covering the oil feed line and it isn't. Hmy wife crank the engine while I watched the oil drip out of the oil tube. All 12 holes look good.

I also pulled the brake boost vacuum line and covered the hole on the intake manifold with my hand. No noticeable change in RPM speed.

After all of these the EGTs are now even which is a bonus I guess. I'll readjust the valves on 2 & 4 after the engine cools and I've had lunch.

[Zed Head]

Stuck timing chain tensioner? Just a WAG. Something that might stick while sitting, maybe. Didn't watch the video the first time.

[djwarner]

Horrible is not a very informative description. You may know what your EGTs normally run buy 300F sounds low. Could be too rich or too lean.

[Captain Obvious]

How do your float bowl levels look? With all that stuff missing from the intake tract, it ought to be easy to see the sight glasses. Maybe your front carb isn't feeding it's cylinders. What happens if you goose just one of the carbs? With the EGT's you're seeing, I'd guess that goosing the front carb won't do much, but goosing the rear one would raise the RPM?

And out of curiousity... You running the stock needles? You removed and plugged all the idle air circuitry and I'm thinking you'll be rich at idle because of that. But I guess that doesn't matter for a track car?

[djhedges]

Sorry by "horrible" I meant the sound in the video. It hasn't changed despite the work I've done. It's hard for me to describe my first thought was it was a really bad exhaust leak or valve noise. A friend of mine who's a experienced mechanic thought it wasn't running on all cylinders based on the video.

I like the suggestion about trying to feed one carb or the other. Both raise the RPMs. I'll snag a video of it after my phone charges a bit. The sight windows look pretty identical.

I don't think these are stock needles and it probably is running rich at idle. The new spark plugs looked rich when I pulled them. Usually the car is extremely hard to start when it's cold because the choke has been removed but since February it's been really easy to start. Prior to that I'd have to use starter fluid and cycle the throttle every second for about a minute trying to start the car.

[Captain Obvious]

Well let me back up a little and ask... Are you just concerned about a noise, or are you concerned by the way it's running? In other words, is it running as expected but sounding bad, or is it running poorly and sounding bad?

[djwarner]

Low EGTs can come from an over rich condition. The fact that starting without a choke has changed, the plugs indicate rich running and sudden onset, I'd think you have a troubleshooting path to follow. I'd suspect a stuck carb needle. Try inducing a vacuum leak by pulling off a hose or cap to a vacuum port. If RPMs increase, it indicates an over rich condition.

[bartsscooterservice]

worn out camshaft ? it's coming from the engine I think, not the carbs.

[olzed]

Sounds to me like a small end or piston noise. Broken ring. Piston slap.

I had an engine which broke a ring, and a couple of pieces ended up mashed into the top of the piston. It sounded similar to your video.