I had been having trouble with keeping my truck running. Discovered that the carb needed to be rebuilt (it was extremely filthy). Once I put the carb back on the truck, it sounded like it ran fine. Idles at a decent rpm when first started, 600-1000 depending on temperature. However, when the accelerator is pushed it boggs down really bad, to the point that it almost stalls, but only for a few seconds. Once the rpms reach over 2500 the pedal can be pushed all the way to the floor with no hesitation whatsoever. I am guessing the secondaries (sp?) are opening too soon but not sure. All the vaccum lines are routed correctly. Any ideas on what the problem could be? more importantly does anyone know where I can find out how the linkage is supposed to be, maybe i put it back on wrong. Any help would be greatly appreciated.

 

I would guess that the acceleration pump is not working properly. Open the throttle while looking down the carb and see if fuel squirts in it. If it does not, then your acceleration pump is bad.

 

I thought the acceleration pump was the problem as well, already changed the diaphram twice. I will double check to make sure it is working.

 

If you feather in the throttle, as opposed to mashing it down firmly, does it still bog?

 

unless the pedal is pushed really slow, the motor stumbles/boggs when accelerated. the faster the pedal is pushed the worse the problem is

 

I checked to see if gas squirted when the throttle was opened, could hear it but not see it. didnt look any farther yet because it is snowing here.

 

It would be a dream to know this issue was resolved. I know this is 10 years old but damn if the description isnt perfect. I'm not sure where to look. But my carb is rebuilt as was my aap.

 

Can you see fuel squirt when opening the throttle? Even though it is rebuilt, you might have a problem with the new diaphragm/spring, or the check valves in the carb might still be stuck. The accelerator pump linkage could also be worn, so it is not squirting the right amount at the right time. If you are sure the accelerator pump is working properly, re-check all the vacuum lines. There are a few things that start receiving vacuum at partial throttle, and if one of those lines is going to the wrong place it might cause some hesitation. You might even try plugging the vacuum ports temporarily and see if the problem stops. That would rule out vacuum line routing problems.

 

See I initially thought it might be a vacuum line leak somewhere but I'm not confident that the lines are even properly linked right now although all vacuum ports have a hose or cap on them. AND looking at it gives me a headache. I can check the AAP again. If it's working properly would I be able to see gas shoot into the barrel when I pull the throttle cable? A while back I tried making the vacuum line setup match my single cab 87 that runs well but between the custom setups on each of em I got discouraged because it looked harder than I imagined lol.

 

Sounds to me like a bad vacuum advance on the distributor. I don’t know how to test one but a mechanic I had diagnose my newly rebuilt engine told me mine was not working at all, and it was stumbling on acceleration too. I got a new distributor from Summit Auto. I had just put a new Weber carb on it which has been running great.

 

Pull the distributor cap off, so you can see the little arm the advance operates. Put a length of vacuum line onto the advance's fitting, and give it a gentle suck, or use a vacuum pump. Harbour Freight carries a hand-held pump that works perfectly for this application. Just enough to cause the little arm to move. Don't over do it, as you can damage the diaphragm.
Once you've gotten the arm to move, block the vacuum line off, and see how long it holds the little arm in it's "moved" position. The Book doesn't specify a length of time, so just: "will it hold vacuum" is the requirement. If not, you will either need to replace the diaphragm, maybe from a junkyard, or the entire distributor, including the vacuum advance unit, which is still available. Dealerships, and so on, carry it.

Does that help any?
Pat☺

 

You are correct. When you open the throttle, fuel will squirt into the throat of the carb. It should squirt as soon as the throttle is open. If it squirts, but not until the throttle is partially open, then the linkage might be worn. Also make sure the diaphragm returns properly. I rebuilt a carburetor once and after a few pumps the accelerator pump got stuck and caused the exact hesitation you are describing. I should also clarify this is the for main accelerator pump. The AAP is an Auxiliary Accelerator Pump that is on the opposite side of the carburetor and controlled by vacuum only when the engine is cold.

If I remember correctly, the factory manual specifies various amounts of vacuum to advance the timing a specific number of degrees. Each diaphragm in the vacuum advance is different, and both can vary depending on your smog designation. I basically just make sure both diaphragms hold vacuum and advance the timing smoothly. If you rule out the accelerator pump, then it is worth looking at the distributor for any loose parts or shorts in the pickup coil wiring, and check the air gap.