[Chevelle-list] OT - GMC TRUCK HELP

[Larry Shouse]

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 <bdo_chevy at att.net>
To: The Chevelle Mailing List <chevelle-list at chevelles.net>
Sent: Tuesday, September 11, 2007 6:31:31 PM
Subject: Re: [Chevelle-list] OT - GMC TRUCK HELP


-------------- Original message ----------------------
From: Larry Shouse <lpshouse44 at yahoo.com>
> 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
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