I had been having two issues with the driveability of my 1968 Ford F-100.
First, for several years, I've had hesitation / stumbling when flooring from idle. This is the classic symptom of accelerator pump failure. When you floor a carbed engine, the amount of air entering the cylinders overwhelms the amount of gas the carb can provide and you go lean, therefore causing the hesitation. To prevent this, there is a mechanical linkage from the throttle to a rubber diaphragm which pumps an extra shot of fuel into the carb and therefore prevents the lean situation. Over time, and particularly with ethanol fuel, the rubber diaphragm dries out and is unable to effectively pump the extra fuel.
Even though I've known this was the problem for a long time, I get lazy / other things prioritized, and I just lived with it.
Recently, however, I started to have a stalling issue where the engine would just die at random times and I wouldn't be able to start it. Secondly, when put in park, the engine would surge between proper idle speed and a few extra hundred rpms. This was more difficult to diagnose because I couldn't replicate the stalling with any consistency. I couldn't find any vacuum leaks which might have been an easy answer.
I went ahead and decided to rebuild the carb to see if that would fix the problem. The carburetor is an Autolite 2100, otherwise known as the best 2 barrel carburetor in the world.
Here is a picture of the carb off the truck. You can see the linkage to the accelerator pump, which is behind the cover with 4 screws. There is a lot of staining from fuel leaking past the gasket.
The carb side of the diaphragm.
The old diaphragm on the left, new diaphragm on the right. Most of the rubber is hidden behind the metal plate. To the touch, the old diaphragm was stiff.
Here is the top and bottom of the carb all stripped down of parts, ready to soak in carb cleaner. You can see there is a lot of dirt and build up throughout. These went into carb cleaner for 4-5 hours. Carb cleaner, by the way, is amazing stuff!
After cleaning and putting everything back together, a somewhat cleaner carb.
I did have one "oops" along the way. When I put the carb back on the engine and went to start it, it started right away, but then stalled. After I cranked it a bit, fuel started pouring out of the top off the carb. After scratching my head I thought maybe a stuck float pin? I took the top of the carb off and figured out that I had forgotten to attached a clip that holds the float to the float pin seat. You can see this clip below my finger in this picture. Without this clip attached, the float can't hold down the float pin and fuel will just keep filling up the float bowl until it floods.
Once I fixed that, the engine started right up and idled smoothly. I've driven it a few times over the last couple weeks and my stalling issue is gone and so is the hesitation. I know what fixed the hesitation but I can only assume there was some gunk or clogged passages that once cleaned, fixed the intermittent stalling issue and surging.
I'm real happy with this cheap repair. Next I need to due some tuning since I think it's running too rich which hurts both my power and mpg.