The truck starts ok, same as always, and remains drivable. The only symptom is a noticeable, but brief, bogging during accelerations from a complete stop, which it didn’t do until just a few weeks ago. If the above posters are correct that a completely non-functional accelerator pump prevents starting and makes the vehicle un-driveable, I have to conclude it must still be working, somewhat at least.
The engine should start with a non-operative accelerator pump circuit if the choke is working.
However, even with the truck sitting there and no load applied the engine will fall flat on its face once the pedal is mashed even lightly.
If you look down the carb and have pump the gas, you won’t see any gas squirt out if the accelerator pump is shot 100%. If gas squirts out, it’s not totally bad. A slight bog could be a number of things, make sure the points and timing are correct, and that the idle mixture and rpm is correct.
“If [gas] squirts out, it’s not totally bad”
+1 It’s best to operate the throttle by hand while you’re looking down the throat. Gas should start squirting into the throat immediately with the motion of the throttle.
Hesitation is often caused by incorrectly connecting vacuum lines. The distributor spark advance must be connected to ported vacuum. If manifold vacuum is connected to the distributor timing will immediately retard when the accelerator is depressed. That will result in severe hesitation.
The way that throttle shaft mechanism works is that the accelerator pump lever is held against a tang on the throttle actuating lever by the spring. When the throttle is opened quickly the acceleration pump lever lags the throttle openning and is advanced by the spring so the fuel enrichment is spread out over the possible bog spot. Once the acceleration pump lever catches up with the throttle lever the squirt ceases and the accleration pump is ready for any subsequent advance of the throttle.
I appears that what has happened is that the acceleration pump lever has gotten past the throttle lever tang and the spring has now driven the acceration diaphragm to full travel. A youtube video on rebuilding the 2150 showed the shaft out but did not show how to disassemble and/or rebuild the shaft. All the demonstrator mentioned was that he had cleaned the mechanism thoroughly (probably in carburator dip). If you look closely you might see how the accelrator pump lever is retained on the throttle shaft or where the throttle shaft lever is missing the tang.
Alternately, you might be able to see how to get the acceleration pump lever back over the throttle shaft lever tang.
Hope this helps.
Thanks for your assessment @researcher ; I concur that’s the most likely explanation. I’ll report back later after I have time to take the workings apart and make the final determination. It may be take a while though, as the truck is a special purpose vehicle and isn’t a daily driver and still works ok. It’s likely I’ll have to remove the carb and take the throttle plate stuff off on the bench to see what’s wrong. I’ll order up a re-build kit so I’ll have it on hand.