Mak

According to the service manual, base idle speed should be 500 rpm in drive, which puts mine at about 800 rpm in park... seems a little high to me. I raised it after I noticed a faint tapping sound at idle... which was set closer to 400 rpm at the time (I fixed an exhaust leak and then started noticing the tapping). The noise seems to be coming from the carb/intake area (I've got a Edlebrock 4 barrel & performer intake). It only happens at low idle (in gear)... and seems to go away, or is barely noticeable, once the idle speed is raised to 500 rpm. It just feels like the trans clunks into gear pretty hard with the idle set this high. Anyone have insight into the tapping sound?
Oh yeah, this is a fairly fresh rebuilt 425 (less than 10k miles). I had the valve covers off today, everything looked good... and I'm pretty sure the noise is not valve train.

MDchanic

1. You must have one of those experimental turbine engines.

2. Either 500 or 550 RPM in Drive should be right.

3. With idle at 550 RPM in Drive, you'll be at about 7-800 RPM in Park.

4. If you want it a little lower, you can do it, but you risk stalling on rapid deceleration to a stop.

5 The first thing I think of when I hear upper end tapping is a tapping lifter.
If it only happens at very low RPMs, I think oil pressure.
How's your oil pressure when you run it at that low RPM?

- Eric

oldcutlass

The tapping is probably a worn rocker assy, or as stated above lifter/s! How is your oil pressure??

The clunking could possibly be contributed to worn u-joints also!

Mak

[QUOTE=MDchanic;329044]

1. You must have one of those experimental turbine engines.

LOL... I fixed my post (it was late last night).
I guess I need to check my oil pressure. I changed my oil from 10w/40 to 20w/50 right before I noticed the noise, but I also fixed the exhaust leak at the same time, which may have been masking the tapping noise... so I'm really not sure if it was already there or not.
I ran the engine with the valve covers off and listened with a stethoscope, the noise is loudest at the intake manifold under the carb... all the rockers are tight and everything looks pristine. It's not the typical slow rhythmic tapping sound I'd expect from a lifter at idle... more of a fluttering tapping/rattle, like a marble in a coffee can. If I give it just a little throttle, or put it in park, it completely goes away. I'm wondering if it could be a light ping/detonation at idle? But that seems odd to me.

MDchanic

Detonation is wel nigh impossible.

I'd consider something loose and vibrating when the frequency's just right. Don't ask me what .
Maybe someone else has an idea... ?

- Eric

Mak

Motor's pretty fresh, professionally rebuilt, new oil pump. The rockers look good and the noise isn't loudest there (see my post above).
U-joints are new. The clunk isn't terrible, just a little harder than I'm used to. It may not be a big deal... I just don't want it to turn into one.
Thanks for the idea's, I appreciate the help.

Mak

That's what I thought too. I took alt/ps belts off to eliminate them. Lowered the idle to make it more audilble and touched everything that could rattle externally... nothin. The only time I noticed any difference in the sound is when I put my hand over the top of the carb... that muffles it. Wierd! Probably not a big deal... just something to drive me crazy cuz I can't find it. I can't figure why it would tap at 450, but not at 500? I'll just keep an ear on it and see if it gets worse. Other than that, the engine sounds and runs great! (especially since I fixed the annoying exhaust leaks). Thanks a million for the advise... I really appreciate it!

DreWRoolZ

to find noises my dad at his shop will use a hose put one end to his ear and put the other end around different places the noise might be coming from.. might help you hear it better. my 371 is tapping but i believe it is from a lifter

hookem horns

Longshot, but check the flywheel cover between motor/trans. Could have got dented or loose when working on the exhaust leak.

Mak

Yeah, I've got a automotive stethoscope with a metal tip at the end. I find it works better if I remove the metal tip and just use the hose, though. My noise is definitely loudest at the intake manifold just under the carb... but I guess it could be transferring from just about anywhere, lol.

DreWRoolZ

hope you find it and it's nothing too bad! my daily driver 91 civic makes this weird rattling sound and it irks me to no end but i have yet to locate the problem! every time we go to try and find it it stops rattling lol i know it's nothing bad just an annoying noise

Mak

Longshot? Yes... but worth checking. I did have it off/on recently. I'll check it. Thanks.

Mak

Yeah, rattles are annoying. I suspect this noise was probably already there, but I didn't hear it until I fixed my exhaust leaks. My hearing ain't what it used to be... as far as being able to tell exactly where it's coming from.

droldsmorland

Do you have exhaust heat-riser crossover plugs/block offs installed in the intake/heads? Those will rattle if not properly installed.

oldcutlass

This was going to be my next suggestion!!

w31man

Did you ever find out what that noise was? I was searching this site and i have the same noise seems like its coming from right under the carb. Just wondering if you figured it out?

Thanks Greg

StarGeneral

I'm in the same boat as you two. I too have a strange tapping noise that seems to come from below the carb. My engine is a 455 with a quadrajet.

droldsmorland

A k-thunk K-thunk K-thunk and a Tap-Tap-Tap are vary different animals. The Thunk is rods or mains. The rest is upper cam, cam bearings, valve train etc...Either way if you have ruled out all of the moving parts on the out side of the engine then its surgery time.

Rickman48

How about the choke door - slop in choke linkage while open??

w31man

I have an engine thats has 50,000 original miles on it, I took it out to regasket it all this winter. I also as long as i had it apart put in new timing chain and gears, oil pump, valve guide seals, pushrods. Ran fine before, now when i start it it has like a poping noise around the carb. If i put my hand on the carb i can feel the vibration of the pop. Seems to run fine besides that noise just started it in the garage so far didnt drive it yet. My car is a 72 cutlass with a 350 2 bl carb engine

Greg

Mak

I raised the idle speed slightly higher and don't hear the noise anymore. It only happens at a certain RPM, which leads me to believe it's a rattle and not an internal engine problem. I suspect the previous owner installed cross-over block off's when he replaced the intake manifold and that's whats rattling (as others have mentioned)... when I get around to pulling the intake manifold I'll remove them if they're in there.

w31man

Thanks for the responce, well i know i dont have the blockouts in the manifold so i guess i will have to go back to the drawing board to figure it out.

Greg