<VV> Mystery engine knock at low rpm only

[Brian Goldin]

I've been using my '63 as a daily driver for about a year with relatively
few problems until last week when it started to develop a knocking sound.
The sound is a pretty loud knocking or clacking sound and seems to be coming
from up front near the number 6 or 5 cylinder.  The sound is loudest when
the engine is cold and becomes quieter but still very much present when the
engine warms up.  Interestingly the sound only exists at round 1,200 RPM.
When the car idles in neutral around 900 rpm the sound is barley heard but
if you blip the throttle a little it gets loud and when you get it up to
about 1500 rpm it goes away and the engine sound great, almost like a sewing
machine.  If I take her on the road she drives normally with the exception
of the low rpm knock.

 

I've been trying to figure this out for about a week through the following
steps with no success.  Any help would be greatly appreciated.

 

1)     I checked the timing and it's where I usually keep it at about 13
degrees

2)     Vacuum advance seems fine. Just to make sure I pulled the hose and
the knock still persists.

3)     Pulling the fan belt didn't make a difference

4)     Changed oil and fuel filter (was almost time to do this anyway -
didn't see any debris in oil) 

5)     I pulled the top cover (blower housing) and checked out the
connecting rods and did a general inspection.  Everything looked great (no
noticeable wear anywhere; connecting rods had a slight bit of sideways play
nothing vertical; I put the torque wrench on the connecting rods bolts and
they were all fine) - put it back together

6)     Checked firing order (put new points on about a month ago - just
making sure that I didn't do something silly)

7)     Pulled one spark plug wire at a time (did not remove spark plugs);
sound persists regardless of dead cylinders.

8)     Checked compression - all numbers are within about 5-10 pounds of
where they were about a year ago (#1-145;#2-125;#3-140;#4-145;#5-145;#6-140)

9)     Jacked the car way up and took a hose to the engine again (one end to
my ear) and tried to find the problem.  It's hard to tell exactly where it's
coming from but it almost sounded like it was coming from the torque
converter / flex plate area.

10) Pulled drive train - replaced diff / powerglide with another one that is
known to work; flex plate was solid on crank as was the connection to the
torque converter - put everything back together - same problem, no worse, no
better

11)  Pull hair out.

 

I'm starting to think about going over the lifters carefully; but I doubt
that this is the problem since it's not a tapping sound.  Also thinking
about looking at the carbs; can a fuel mixture problem cause this (too
lean)?  My next move might be to pull the whole engine apart but obviously
I'd rather not dig into it that deep if at all possible.

 

Best Regards,

Brian

 

'63 Monza 80hp PG Convert. 

Travalon #163