acceleration problems

[amikk97]

recently, my truck has been losing power. it starts fine and idles perfectly, and when I step on the gas in neutral/park it revs up but if I floor it it just dies out. when I put it in drive or reverse, its very hesitant to go, if I push on the pedal a little but it just sputters out. ive changed the fuel filter, and it worked for about a minute, then went back to sputtering out. I then took off the whole air cleaner assembly, and it seemed to run a little better now, but it still sputters out. I don't have bad gas, or at least I don't think I do, so im thinking my next culprit is the plugs may be fouled or the wires need to be replace, but if it starts fine and idles fine, could it still be the plugs? please help, im really stumped on this.

[Super_Dave]

Not sure what you are running, but if it's a carb, it could be a bad accelerator pump.

"To get an engine to accelerate smoothly, you need both an advance in the timing and an increase in fuel flow. The vacuum distributor senses when you open the throttle and provides the extra advance timing. It is the duty of the accelerator pump to provide the increase in fuel flow.

When you open the throttle, the airflow increases immediately but the fuel (being more dense than air) takes a moment to catch up with the increased airflow. This would result in a lean mixture for a few moments (the car would hesitate), so the carburetor has an accelerator pump built into the side of it which supplies a shot of fuel to ensure smooth acceleration. The accelerator pump connected by a linkage to the accelerator; when acceleration is required, the pump squirts a spray of fuel directly into the throat of the carburetor to momentarily increase the fuel-to-air concentration. You can see it operate if you pull the air cleaner off and look down the carburetor throat.

There is a small bent brass pipe right pointing right straight down the throat of the carburetor-- this it the delivery tube. Grab the throttle arm and pull it firmly (like you were pushing down on the accelerator). You should see a squirt of fuel from the delivery tube straight down the throat of the carburetor.

If you pull on the throttle arm very slowly you'll see that the accelerator pump does nothing -- no squirt. It's set that way because when you open the throttle slowly the fuel flow has time to keep up with the increasing airflow - no extra shot of fuel is necessary.

We want as little accelerator pump squirt as possible to remove the stumble from the engine. Too much gas will wash the oil off of the cylinder walls (wearing out the pistons and rings) and will reduce the miles per gallon. so be conservative in your adjustment of the accelerator pump."

Also...__________________________________________________

"An accelerator pump is a device found on a carburetor that sprays a controlled amount of gasoline every time the throttle linkage is fully depressed. The amount of fuel introduced into the intake when the accelerator pumps is sufficient to start the engine. The other purpose of the accelerator pump is to dispense additional fuel into the engine when the engine is revved up in certain situations, such as when passing another vehicle. This extra fuel prevents the engine from missing or stumbling until the fuel system can catch up with the needed fuel flow requirement.

In a fuel injection system, there is no accelerator pump. In this type of system, the vehicle's on-board computer senses the need for additional fuel and sends a signal to the fuel injectors to spray more fuel. Typically, carburetor-equipped vehicles do not incorporate a computer or fuel management system in the fuel system. The amount of fuel dispensed when the pump is activated is controlled by a plastic cam situated in the carburetor's linkage system. By manipulating the cam, the quantity of fuel dispensed by the pump can be adjusted from a small spritz to a large squirt.

The accelerator pump is located on the bottom of the fuel bowl of the carburetor. On a typical barrel system, there will be a single pump. On four-barrel systems, there can either be one or two pumps depending on the size and style of the carburetor. On a single-feed or single fuel line-equipped carburetor, there will typically be a single accelerator pump. On a duel-feed or a two-fuel supply hose-equipped double pumper carburetor, there will be a double pump consisting of a pump on both the front fuel bowl as well as the rear fuel bowl.

On a double pumper-equipped carburetor, the first or front pump is actuated when starting the engine as well as when revving the engine to half throttle. The second pump is used when the rear butterflies or throttle plates of the carburetor are opened rapidly. This is done to prevent a lean condition from occurring, as the flood of fresh air enters the carburetor from rapidly opening the rear throttle plates. On a single pump-equipped carburetor, the added fuel is dispensed through the front pump.

The leading cause of failure for an accelerator pump is a torn rubber diaphragm within the pump. Age, frequency of use and linkage misalignment can all lead to a torn pump diaphragm. Replacement of the pump is a minor repair and typically requires the removal of only four screws."

[Dead Parrot]

Direct answer to question, Yes the plugs can be mostly shot and it still start and idle fine. Idling puts very little stress on the plugs. Wouldn't hurt to pull them and see what they look like. Pull up one of the "how to read spark plugs" charts first. If they have over 15K miles, could be due for replacement anyway. Other ignition suspects are wires, cap, button inside of cap.

Any changes on where you get gas?

May just be time for a good fuel system clean out treatment.

[rich weyand]

Are you sure your distributor is clamped down tight? Check your base timing. If your distributor is loose, it will drag retarded and you will get the symptoms you are seeing.

[cadillac_al]

It sounds like lack of fuel pressure to me.

[77 Big 10]

I was thinking the same thing, check the fuel pressure coming out of the pump.

[rich weyand]

Originally Posted by cadillac_al

It sounds like lack of fuel pressure to me.

Which could either be the pump or old fuel lines. Rubber age hardens. Old rubber fuel lines, vacuum lines, and brake lines should all be replaced on something as old as our trucks.

[amikk97]

good news, I found out what was wrong. I dropped the fuel tank to check the fuel pump and it turns out the screen on the bottom of the pump had fallen off and the intake hole on the pump was all gunked up. after a good clean shes running strong now.

[rich weyand]

Excellent! And thanks for letting us know.