Loss of power and excessive fuel consumption

My 91 chevy 4wd, 350cu pickup becan showing loss of power during acceleration this week. The strange thing is the amount of fuel that is being consumed is excessive. It burned through a half tank of gas just driving 70 miles. The carburator sounded like it was opening fully to get any acceleration at all. When accelerating up a slight incline the pedal is floored and the truck moves about 10 miles per hour. The transmission shifts normal and the transmission fluid smells normal. When my truck is in park is revs up normal. I'm confused.
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If stock, your '91 should have throttle body injection (TBI) not a carburetor.

You don't mention seeing any trouble lights on the dash, so I'll skip the talk about issues with emission components, but report back if there are any.

First, check that air filter. This is a no-brainer...no air, no go.

Next, check your throttle body and vacuum lines for leaks by spraying short directed shots of carb cleaning fluid around the TB gaskets, vacuum lines and their parts while the engine is idling. The engine should stumble when you hit a leak. TB gasket leaks and cracked vacuum hoses are common on older trucks.

Next I'd pull your spark plugs and "read" them. The deposits and color on the electrodes can tell you how each cylinder is running. There are many color photos online showing what each plug condition means, but the link below goes a bit further in explaining how to service them:

http://www.aa1car.com/library/sprkplg2.htm

For you, you're looking for sooty deposits, which is an indication of too-rich combustion...too much fuel for the air to react with. If you see the soot on all plugs for both cylinder banks, you may have a faulty fuel regulator or worn injector seals. Seeing the deposits on just one bank could indicate a clogged injector (those two, fat, bullet-looking pieces just above the throttle plates). Each injector feeds fuel to the respective cylinder bank.

Next, check your injectors. A trick if you have an inductive timing light: hook up your timing light and attach the pulse clamp to the #1 spark plug wire (forwardmost, driver's side bank). Idle the engine and point the strobe at the driver's side injector. You should be able to see the fuel spray pattern in stop motion. It should be a nice, conical spray with no big fuel drops falling into the throttle body. Look at it from different angles. Do the same for the passenger side using the #2 spark plug (forwardmost, passenger side). Any irregularity in pattern or pulse length means they're clogged and need to be cleaned. You can examine the spray pattern without a timing light, but it's much harder to see the details because it goes so fast.

Finally, check your positive crankcase ventilation (PCV) and exhaust gas recirculation (EGR) valves. Make sure they are operating smoothly and not gunked up or clogged.

Good luck!

My 91 chevy 4wd, 350cu pickup becan showing loss of power during acceleration this week. The strange thing is the amount of fuel that is being consumed is excessive. It burned through a half tank of gas just driving 70 miles. The carburator sounded like it was opening fully to get any acceleration at all. When accelerating up a slight incline the pedal is floored and the truck moves about 10 miles per hour. The transmission shifts normal and the transmission fluid smells normal. When my truck is in park is revs up normal. I'm confused.
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this could be the cat is plugged up..
REMOVE THE O2 SENSOR AND RUN THE VEHICLE FOR A SHORT TEST ...if the vehicle has improved power then you know whats up..

Just to add to what jdmccright has already said.

The amount of fuel delivered to the engine is based on Throttle Position, MAP Sensor value and Engine Coolant Temperature.

A leaky vacuum hose to the MAP (or bad MAP sensor) can cause too much fuel to be delivered. If the vacuum port is clogged, it will also cause the MAP to report an incorrect value. A bad ECT sensor can also affect mileage.

A bad O2 sensor will also affect mileage.

Loss of power can also be mechanical, such as a jumped timing chain or clogged exhaust.

What is the fuel pressure?

//2000CAYukon

Excellent point 2000CAYukon. Here's the check procedure for the MAP sensor:

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528008f0fd