Larry,
It's sounding like maybe the accelerator pump is putting too much
of a shot into the carb when you step on it. If it runs fine above
that RPM range and you think the main jetting is fine, then see if
you can reduce the amount the acc pump puts out. I don't have one of
those carbs in front of me, so I can't offer any more useful
information on how to change it.
Shawn
On Sep 11, 2007, at 10:18 PM, Larry Shouse wrote:
Thanks for your reply Brad. Yes, a half ton.
The carb is an Auto Zone rebuilt that I recently installed. I
replaced the points, plugs, wires, cap, rotor, condenser and coil.
Of course new doesn't automatically equal good, but all items
listed seems to be functional. Can an old and possibly degraded
distributor cause this type of symptom? The dwell is running at 31
degrees and steady, which should be close enough. When I rev it up
with the timing light on, the mark climbs up and off the gauge so
the advance curve at least starts off correctly... The plugs are
running gaps of 40.
As a youth, I was taught black smoke means to much fuel, which
means not enough air or too much fuel at those RPMs.... Is there an
adjustment for the fuel/air mixture off idle on these carbs?
Larry Shouse
----- Original Message ----
From: Brad Olson <[EMAIL PROTECTED]>
To: The Chevelle Mailing List <[email protected]>
Sent: Tuesday, September 11, 2007 6:31:31 PM
Subject: Re: [Chevelle-list] OT - GMC TRUCK HELP
-------------- Original message ----------------------
From: Larry Shouse <[EMAIL PROTECTED]>
> I'm hoping some of you my be able to help with a couple of questions
> I have concerning my 1970 GMC truck. It has a straight 6 250 CID
> engine with a 3 speed manual tranny and a 1 barrel MonoJet carb.
Half ton, right? My dad had that very same truck from '72 to '85.
The shift linkage went bad in the column and he converted it to a
Hurst "Indy" floor shifter. I used to take it out on the back roads,
wind it up, pop the clutch, and the back end would bounce around
like crazy. ;^)
> First question is on timing. My service manual says to refer to
the sticker
> under the hood for timimg, but my sticker is long gone. I did
find one of my old
> tune-up guides from the 70's that says on L6 Chevelles, it should
be 0 degrees
> on automatics and 4 degrees BTDC on manual tranny vehicles...
Should I go with 4
> degrees BTDC? Also, each mark on the timing guide by the flywheel
on these
> things is 2 degrees right?
Yes, the timing tabs are marked in 2-degree increments.
I checked my Haynes book and it too defers to the underhood
sticker. 4 BTDC
is as good a place as any to start, and if it runs fine then try
adding a
little more advance until performance degrades. If you use it more
as a
driver and less as a hauler the original factory specs may be
allowing for
some engine loading that your truck doesn't see.
> Second question is on my carb. While accelerating normally, it
starts to spit
> and sputter at around 2000 rpms, then clears up at around 2500
rpms or so. While
> it's doing this, I see black smoke coming out of the exhaust.
Before and after
> this rpm range, my truck doesnt smoke. Well, it does at red line
but that's gray
> smoke and I thing an entirely different issue with this tired old
engine. Do you
> think in the 2000 to 2500 rpm range, it's getting too much fuel?
If so, how do I
> fix it?
Sounds like a carb issue, especially if it's been years since the
Monojet's
been gone through. If possible, put a vacuum gauge on the engine and
observe what it reads in the RPM range in question. Is the rest of
the ignition
system up to snuff?
Brad
