Expert: Ben Deutschman - 11/21/2010

Question
i have a d-150 with a slant six with a 1 barrel carb. since
ive had it, it stumbles at higher speeds. now its doin it a
little worse, i cant get past 35mph without it stumbling and
seeming like its going to die. any ideas why, or how i can fix
it?

Answer
I'd check the Points, making sure they are set between .017-.020, producing a Dwell of 42 Degrees, at Idle, with the Vacuum Advance Line disconnected and temporarily plugged.  Also check the initial Timing, which depending upon the year of your Truck, could be anywhere from 2 Degrees Before Top Dead Center (BTDC), or even Zero. If there is an Emissions Decal on one of the Inner Fenders, or on the Radiator Support, the specs are usually found there.

If the Truck does NOT idle smoothly, then I'd  check the Valve Lash (.010 Intake, .020 Exhaust). The Lash is both checked and set on a fully warmed up Engine, with the Tappet under no load, determined by cranking the Engine over until the Tappet you are checking has moved the Valve to the open position (down), and fully returned to the rest position (Up). The Pushrod should not have any tension on it, nor should the Tappet. A flat Feeler Gauge is inserted between the Tappet and top of the Valve Stem to check the Air Gap. There should be a slight drag on the Feeler Gauge if the adjustment is within the specified limit. If the Gauge slides in freely, then the Locking Nut has to be carefully turned clockwise to decrease the Air Gap. Recheck the clearance again, and if there is a slight drag on the Feeler Gauge, proceed to the next Valve Tappet. The Intake, or Exhaust Runner directly in front of the Tappet you are adjusting, is how you know whether or not you are adjusting an Intake, or Exhaust Tappet. You can use a Remote (Wired) Starter Switch to crank the Engine over in order to do the adjustments. The Remote Starter Switch can be found in most major Auto Parts Stores (i.e., Autozone Part # PL-SW19)

Have you checked, or replaced the Fuel Pump? I'd also check to make sure the Float and Needle and Seat in the Carburetor are working freely. In order to check the Needle and Seat Assembly and the Float, you will need to disassemble the Carburetor, which means having a rebuild kit handy, as you will have to replace any gaskets you remove. You will need a Carburetor "Tag Number", or code from the Body of the Carburetor in order to get the correct Rebuild Kit for your Truck's Carburetor.