[Chevelle-list] OT - GMC TRUCK HELP

[Brad Olson]

-------------- 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