Compression Test questions

[Jennys280Z]

Update:

Before starting the motor I retarded the dizzy's timing a little and filled up with pure 93 octane gasoline, and the noise is gone! Revved it up 30 or 40 times and never heard it once. I have to conclude that the awful noise was pre-ignition (shaking BBs in a coffee can) vs detonation ("tump tump tump"). And what I thought was a great job on the valves is looking likely again

I think it's interesting that after my AFM cover fell off and as I was putting it back on with the motor running, I noticed the hollowness of the potentiometer chamber which seems to resonate when the AFM cover is loose (e.g. not sitting flush on all edges) with the motor vibrations. The AFM from MSA is great, (it measured right what it should on the ohmmeter before I installed it) but the front cover has always been loose. That was nice at first while I was continuously going in there. But now that my mix is nearly ideal I need to think of a long-term solution to keep it from making noise or even falling off. What do you 280 folks do to secure your AFM covers?

According to the timing light, timing was retarded about one degree to ~ 9.0 advance. Not sure if I should stay here or return to 10 now that the Sea Foam is gone.

Bad news, cold idle isn't as high as it was. I used to warm up at 800-900 RPM and now I'm warming up at 700-800. Timing? The lack of Sea Foam? Or is this typical of S30s? No matter what, the motor sounded silky smooth and that's what matters most. Special thank you to Zed Head for responding to my noise and stopping the damage before it happened. Now for some homework on leak-down tests.

[Captain_Zeros]

Changing ignition timing will change your idle speed, it's normal.

[rcb280z]

Jenny, my cover is held in place with a zip tie wrapped around the AFM. I did this just in case I had to adjust again. Strange though...it should have come from MSA sealed with silicone. What is your idle after warm up? If it is lower than 800 rpm just adjust it with the set screw. My 77 cold idle is 1100 because I adjusted the AAR. And my warm idle is set to 800. I have the pinging problem if I advance timing 2 degrees and don't use premium also. But premium where I am is only 91 and almost $5 a gal! So I usually run at 10 BTDC. Nice job on the valve adjustment.

[Jennys280Z]

I thought 100 RPM for one degree was excessive but what do I know ....I'll advance my timing back to 10 deg and hopefully be able to blame the Sea Foam when there's no pre-ignition.

fwiw the mechanical advance returned back to 9 quickly when I let off the throttle tonight so the plate isn't stuck.

I think I've had a successful outcome on this issue and would like to thank everyone again, classiczcars' guys and gal.

[Jennys280Z]

Jenny, my cover is held in place with a zip tie wrapped around the AFM. I did this just in case I had to adjust again. Strange though...it should have come from MSA sealed with silicone. What is your idle after warm up? If it is lower than 800 rpm just adjust it with the set screw. My 77 cold idle is 1100 because I adjusted the AAR. And my warm idle is set to 800. I have the pinging problem if I advance timing 2 degrees and don't use premium also. But premium where I am is only 91 and almost $5 a gal! So I usually run at 10 BTDC. Nice job on the valve adjustment.

I didn't have time to warm it all the way up tonight since going to 9 degrees so I'm not sure what it is, rcb. I'll go back to 10.0 degrees and no further just to be on the safe side. Thank you, it's really good to know I'm not alone. Thank you for the compliment. x

[Jennys280Z]

Warmed it up completely tonight driving to the mall. Idle speed warmed up is still 900+ RPM even with the ignition still retarded to 9 dbtdc. A few observations on my 2nd drive after the valve adjustment... my oil pressure gauge read higher than ever today reaching close to 30 on some acceleration up hills and a still-cool motor. It stays above zero at idle now too, comforting to know I actually have some oil pressure there.

To exonerate MSA, there is silicone on my AFM. I can't believe I never noticed it. It's clear and hard to spot. It's also hard. They maybe had this AFM on the shelf for awhile. I peeled the dry silicone off and the cover fits more snug now.

It drove beautifully! A little bit of gas pedal goes a long way now! Docile and smooth down low but very responsive, the car actually feels easier and more forgiving to drive now post valve-adjust.

On the way to the mall I heard no strange noises. Then suddenly in the parking lot on the way home from the mall, the noise returned. So I pulled it into the garage, got a sound recorder, stuck my head down to the motor and started trying to reproduce the noise and here's what happened. It sounds fantastic until I go back to the gas pedal at 1:00, I'm sticking the recorder out the window at 1:10-1:20 and the sound is clearly audible. Then once again after the 1:20 mark I'm back under the hood, revving and hearing nothing, so at 2:22 I captured the exhaust note. I never heard any weird noise for any of those revs either.

<embed width="600" height="361" type="application/x-shockwave-flash" allowfullscreen="true" allowscriptaccess="always" wmode="transparent" src="http://static.photobucket.com/player.swf?file=http://vid81.photobucket.com/albums/j219/amiamelodramatist/isis_garden/photos/audio2_zpsd69ed6a1.mp4">

When I'm under the hood on the driver's side the noise sounded like it was coming from the left, towards the front of the engine compartment. Could this be a bearing in the fan/clutch? I spun the fan by hand and it felt fine. Water pump going bad, maybe? Could it be one of the pulleys up front? Oil pump? It sounds like tires losing traction on pavement when in the car driving. It makes a tinny whooshing sound at 1:15-1:20. I don't think it's pre-ignition anymore and I don't really think it's the motor based on the way the car is running. I don't see how this sound can be a valve when the whole way to the mall and 80% of the time afterwards I can't even hear it. *sigh*

[jonathanrussell]

Use an automotive stethoscope (sears, auto parts stores, etc sell). They have a wand that you touch to various places and you should be able to hear where the noise is coming from. You can approximate this with a cut off broom handle etc but the stethoscope is best.

[LeonV]

Use an automotive stethoscope (sears, auto parts stores, etc sell). They have a wand that you touch to various places and you should be able to hear where the noise is coming from. You can approximate this with a cut off broom handle etc but the stethoscope is best.

A long screwdriver works really well as a stethoscope, just be sure to press the handle-end against your ear...

[jonathanrussell]

And....keep that hair away from the fan....

[Zed Head]

All I hear at 1:10 - 1:20, besides the engine, is beeping. Your first video had more weird noise.

[jonathanrussell]

It sounds like a seat belt beeper or similar.

[EuroDat]

I can hear it at 1:10 - 1:20 but I have no idea what it could be. Strange that it wouldnt do it when you was listening at the engine. Recording from outside the window makes it hard to determine where it could be coming from.

Do you notice anything with the engine, hesitation or drop in power?

Is the hood closed when it does it? Vibration could be making the hood rub on something. Just thinking out of the box....

A stethoscope could help a lot if you can get it to do it while testing it. Internal sounds get muffled a lot and a stethoscope can overcome that to a degree.

If you use a screwdriver, use one with a big handle so it covers your ear better. Be carefull because you need to get close to things and you tend to concentrate on finding the noise and not your own safety.

Chas