1984 GMC 1500 Pickup Truck 350 CI

When the truck sets for an extended period of 24 hours or longer, it takes many engine crank overs before the fuel is pumped back up to the carburetor for the engine to start. Thus far have changed out the sending unit in the gas tank, the fuel pump, and sent the Q-Jet carb off for rebuild (twice) to fix the problem but no luck so far problem still exists. Any ideas? Thanks.


The carburetor has a float bowl and will have fuel in it at all times. So thinking the carburetor is out of gas would be incorrect. It would be easy to test by simply working the throttle on the side of the carburetor and visually seeing fuel squirt.If the float bowl is empty, then you have a leak. Either an external leak or an internal fuel pump leak. An external leak should be easy enough to find by putting a piece of cardboard under it and leave it set after you park it for the night. An internal fuel pump leak you will not be able to see but the fuel pressure will bleed off from an internal leak.

I am thinking more along the lines that the choke is not functional or is not being set properly before initial start up.

The proper way to start a non-fuel injection engine would be to press the accelerator pedal to the floor and then let off completely. With your foot still off the pedal turn the ignition key to the “START” position and hold for no longer than 10 seconds.  By pressing the pedal to the floor before cranking it sets the choke and squirts a bit of gas into the intake manifold.

Carburetor Adjustments

8-350, 8-400, and 8-454 Engines


1.All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
2.Set the parking brake and block the rear wheels.
3.Automatics should be place in Drive, manuals in Neutral, except as noted in the Tune-Up Specifications chart.
4.Check the underhood emissions sticker to determine which hoses, if any, must be disconnected.
5.For two barrel carburetors, make sure that the idle speed screw is on the low( L) step of the fast idle cam.
6.For vehicles without air conditioning, do the following: A.Turn the idle speed screw to adjust idle speed to the specification found in the Tune-Up Specifications chart or on the underhood emissions sticker.
B.Open the throttle slightly to allow the solenoid plunger to extend.
C.Turn the solenoid screw to adjust the curb idle to specification, as given in the chart or on the emission control sticker in the engine compartment.
D.Then disconnect the electrical connector from the solenoid. The idle speed will drop.
E.Turn the idle speed screw to set the slow engine idle to the figure given on the emission control sticker.
F.Reconnect the solenoid and shut off the engine.

7.For vehicles equipped with air conditioning, do the following: A.Turn the idle speed screw to set the idle to specifications.
B.Disconnect the air conditioner compressor electrical lead at the compressor, and turn the air conditioner on.
C.Open the throttle slightly to allow the solenoid plunger to fully extend.
D.Turn the solenoid screw and adjust to the specification on the emission control label and the Tune-Up Specifications chart.
E.Reconnect the air conditioner compressor lead.

8.Reconnect any hoses that were disconnected.


Mixture is adjustable on heavy duty emissions V8s with the four barrel M4MC carburetor. This procedure will not work on light duty emissions trucks.
1.The engine must be at normal operating temperature, choke open, parking brake applied, and the transmission in Park or Neutral. Block the rear wheels and do not stand in front of the truck when making adjustments.
2.Remove the air cleaner. Connect a tachometer and a vacuum gauge to the engine.
3.Turn the idle mixture screws in lightly until they seat, then back them out two turns. Be careful not to tighten the mixture screw against its seat, or damage may result.
4.Adjust the idle speed screw to obtain the engine rpm figure specified on the emission control label.
5.Adjust the idle mixture screws equally to obtain the highest engine speed.
6.Repeat Steps 4 and 5 until the best idle is obtained.
7.Shut off the engine, remove the tachometer and vacuum gauge, and install the air cleaner.