Tuned On A Slant?


I wrote you before about a rebuild of my [Super] Slant-Six, which
didn’t go well. Troubleshooting improved
when I found a mechanic who seemed
to have more experience on Slant-Sixes.
He was familiar with adjusting valves on
the engine running, hot, to factory specs,
.010 intake and .020 exhaust. He set ignition
timing to 17° (+/?) BTDC. There was
convincing improvement, but he wasn’t
happy that he couldn’t get timing closer
to 12° BTDC. He thought the engine was
still running rough and lacked pep.
I had an NOS Mopar distributor which
the rebuild shop had removed while
restoring the original distributor, as they
tried to solve the rough running. I asked
the new guy to put the NOS unit back in.
I thought he’d probably fix any issues left
by the rebuild shop. He manually tested
top dead center in cylinder one, then
dropped in the new distributor. As he
retuned it, he found the engine ran better.
Not perfect. The timing marks didn’t line
up. He thought the damper might have
slipped, between the hub and rim.
It’s a new damper, from a vendor in
Australia. The bonding rubber in the original
damper was in pretty bad shape, but
I’m going to try to get it refurbished. The
machine shop did balance the new one.
The mechanic decided he
would set timing, “the oldfashioned
way, by ear.” There
was a big improvement. But
the timing light indicated the
advance at 37° BTDC. He
believes the light is misreading
advance, that it is really in the
area of 12° BTDC.
We decided to leave it there
for the time being. Ride is
smooth, more like before the
rebuild. I was keeping up just
fine with rush hour traffic on
the Garden State Parkway.
Is this a situation I should
live with? Maybe the new
damper is the problem?

There’s lots of variables here, and I
have published so many Q&As like this,
and loads of tuning articles, so I’ll keep
this short and sweet. My response
will assume this is a stock rebuild (not
cammed-up, etc.) and I’ll also assume
that the problem is related to ignition, not
valve, timing (such as possible cam/crank
sprocket mis-clocking).
• First step is to get a proven-accurate
TDC mark, regardless of the damper’s
brand or condition. Remove #1 spark
plug, insert either (a) TDC positive
stop tool, (b) hacked-up old plug with
3/8˝ screw, or (c) a length of 14mm
threaded rod. Rotate the crank CW
(manually) until the piston hits the stop.
Make a mark on the damper under the
“zero” hole or notch. Now rotate CCW,
repeat. Using flexible ruler and find the
exact center between these two marks.
Make an indelible mark on the damper
there—that’s true TDC.
• Start the engine, disconnect the vac
advance, and hold revs to ~3,500 RPM.
Set timing to 33–35° BTDC. Return
to idle. If spark is between 5 and 15°
BTDC at curb idle, you’re pretty much
home. If over 15°, I suspect stuck
advance weights in the distributor.
• Reconnect vacuum advance while
idling. The RPM should not change. If it
is prop-

erly connected to the ported-spark
fitting, I suspect either a cam timing or
carb problem.
Again, understand that this is greatly
simplified, but, for a stock rebuild, it
should get you very close.

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