strange off-idle clattering sound in my 350

[mattfranklin]

1978 truck motor in my '70.
Was told by the PO that the guy he bought it from said it had a "rebuilt" 1978 350 in it with a "mild cam" and had about 30k miles since the rebuild.

It seems to idle quietly.
Just off idle it makes what sounds like tappet clattering sounds.
At higher loads I can't seem to hear it.

I don't think it's spark knock, that would be a higher pitched clatter.
I don't think it's rod noise. That would be more of a boom boom boom, from my past experiences. But I could be wrong. It's happened before.

The plan to to test compression to make sure nothing is seriously wrong.
Then readjust the valve lifter preload (or whatever they call it with hydraulic cam). Then put in thicker exhaust manifold gaskets.

I have heard that exhaust leaks can cause a clattering sound. But I'm just not sure. It has newer shorty headers.

Any ideas or suggestions on an easier path to check things?

[geezer#99]

It could be detonation which is way early timing that sounds like that. Easy to check.
Plug the vac advance to the distributor to see if it changes. Or check your timing and retard it a few degrees.

[landy]

Could it be a solid lifter cam and lifters need to be adjusted?

[litew8]

Yeah, maybe the PO didn't go back in and re-adjust after break-in.

[SCHRUMGMC]

How do you know if your VA is going bad ?

[Ironangel]

Check the torque converter to flexplate bolts...

[trac209]

Originally Posted by Ironangel

Check the torque converter to flexplate bolts...

This I'd wager^^^^

[terry b]

Originally Posted by SCHRUMGMC

How do you know if your VA is going bad ?

It will start smoking and drinking and not come home at night.

[litew8]

Originally Posted by terry b

It will start smoking and drinking and not come home at night.

[mattfranklin]

I guess as long as it smokes only the tires and sips only a little gasoline it will be all good. ;-)

Originally Posted by terry b

It will start smoking and drinking and not come home at night.

[mattfranklin]

Good question... In all seriousness, I'll hook up the Mity-Vac to the distributor's vacuum can and #1, see if the spark timing changes when I apply a vacuum. And #2, see if it will hold a vacuum without leaking.

Originally Posted by SCHRUMGMC

How do you know if your VA is going bad ?

[Stocker]

^^ Yes.... or even a cracked flexplate, though mine had a deeper knocking sound, somewhat like a rod knocking.

[71K20chevy]

If your vac advance is ported vacuum, it could knock upon light/moderate acceleration. Like Geezer said, plug it and see if it goes away.

[mattfranklin]

One more detail... It has a manual 4-speed transmission. Could your ideas about loose torque converter bolts or cracked flex plate apply to clutch related parts?

[NoNeck]

It could be your fuel pump pushrod. I searched for weeks for a similar clicking sound and initially thought it was a tappet or rocker arm on the passenger side. Take a long length of garden hose and use it as a stethoscope. BE CAREFUL AROUND THE COOLING FAN BLADES AND MOVING BELTS, but while someone inside revs the engine, listen to areas around the valve train, fuel pump, distributor (cracked cap?), and exhaust manifolds (exhaust leak).

I hope this helps.

POP

[mattfranklin]

Compression test looked good.
Dry. WOT. All plugs out. Engine cold.

#1 165
#2 165
#3 160
#4 163
#5 160
#6 165
#7 160
#8 158
So basic engine health looks good.
(I wish my Honda had compression numbers as consistent as these.)

I thought I was onto something when I discovered that the plug in #6 was only finger-tight. But that didn't take care of the clatter. It's still just a light clatter. And I hear it mostly when I gas it to rev it a little. But it bugs me.

I also tried retarding the timing but it made zero difference. Still worth a shot, though.

Next up: valve lifter adjustments. Maybe next weekend.

[Steeveedee]

Put up a youtube video. You'd be amazed at the results. Beware of crazy comments, though.

[mattfranklin]

I finally got some time to adjust valve lash. Using this procedure: http://67-72chevytrucks.com/vboard/s...d.php?t=219649

[mattfranklin]

Most were about zero lash except for #5, which had one about two turns in. It seems to idle better now, BUT the off-idle noise is still there.

What's left? Rod bearings, fuel pump push rod, exhaust leaks? Motor mounts? Clutch/flywheel innards? Harmonic balancer?

[litew8]

Maybe go back and mess with the timing again.

[mattfranklin]

Originally Posted by litew8

Maybe go back and mess with the timing again.

I'm really hoping it's something fixable like this, but I was unable to verify actual timing as my timing light wasn't working. I did however retard it by maybe half an inch on the circumference on the HEI -- what's that, about 5-10-20 crank-angle-degrees? It didn't seem to make a difference in noise.

Also, unfortunately, the sounds it's making sound a lot like the rod bearing videos on YouTube. Although one commentator thought piston pin noise might be similar.

And when I had the valve covers off I noticed that there were broken gasket parts and sludgey bits clogging the oil returns in both of the heads. Maybe a sign of what's deeper in the engine?

On the positive side it makes very good oil pressure within seconds of cranking.

Sometime this summer the engine gets pulled anyway to swap to in a '71 crossmember, so at that time I'll probably flip it over and pull the pan for a look at the bearings.

[litew8]

A local auto store might loan one out idk it'd be nice knowing it's tuned in ok, but yeah plan/fear the worse hope for the best

[Taibo]

Originally Posted by mattfranklin

Most were about zero lash except for #5, which had one about two turns in. It seems to idle better now, BUT the off-idle noise is still there.

What's left? Rod bearings, fuel pump push rod, exhaust leaks? Motor mounts? Clutch/flywheel innards? Harmonic balancer?

All of the suggestions so far are good, but much less likely than a simple exhaust leak between manifold and cylinder head. Before worring about rod bearings and fuel pump pushrods, check for a leak. You can sometimes find them if there's soot on the manifold but if your engine is covered with crud looking for black soot on black burned oil and grease isn't going to help you.

[mattfranklin]

I changed the oil today and switched to a heavier 15W40, When it first started up, before the oil pressure came up, it made more of that noise. Then once the pressure was up, that took only a few seconds, it was nice and quiet. Unfortunately, when the engine warmed up the noise came back. Sounds like rod noise. Maybe now is a good time to pull the engine and take a look at things. And it may be a good time to get that forged crank and take it a step closer to LT-1/Z-28 specs.

[mattfranklin]

Now that the go-kart project is done I finally pulled the motor. The first sign of trouble was that the clutch pressure plate had 4 of the 6 attaching BOLTS MISSING. And another one was loose. Also, many of the fingers of the diaphragm spring seemed to be warped and/or bent.

Have not yet had a chance to pull the pan and check rod bearing clearances.