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