Engine Stumbles or coughfs through carb

  • 1984 GMC SIERRA
  • 7.4L
  • V8
  • 2WD
  • 140,000 MILES
On acceleration, the engine will sometimes stumble or cough through carburetor and sometimes will almost die. For a 454 dually truck, this also seems to be pretty gutless. There seems to be no change I the problem as to if the engine is cold or warm. I live in interior Alaska so reliable (smart) mechanics are few.
Do you
have the same problem?
Saturday, April 25th, 2015 AT 8:43 PM

1 Reply

A stumble on acceleration is often caused by a worn or inoperative accelerator pump. You have to know what "normal" is to identify if this is the cause, but what you're looking for is two nice strong streams of fuel spraying into the carburetor under the choke plate when you work the throttle by hand. If those streams are weak or missing, the engine may also be hard to start, especially in cold weather.

Also be sure the preheater tube is in place. That's the flexible tube running from the bottom of the air filter housing's snorkel down to the sheet metal cover around the exhaust manifold. That warms the incoming air which makes the fuel vaporize better. Liquid fuel won't burn. It has to be a vapor. Check if the vacuum motor is pulling that door closed to block air flow when the engine is cold. That's what forces the incoming air to go past the hot exhaust manifold.

If you're also having trouble with black exhaust smoke and the choke isn't staying open, there's an exhaust passage under the carburetor than can become plugged with carbon. Your choke coil might also have an electric heater to insure the choke opens, but the second purpose of that passage is to warm the bottom of the carburetor to insure good fuel vaporization and to prevent carburetor icing.
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Saturday, April 25th, 2015 AT 9:31 PM

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