This is my "daily driver," and currently my ONLY transportation, so being down is particularly crappy.
1968 520, J13, garage kept. It's been running fine for a very long time (I don't recall any issues in at least the last year or more). Last time I drove it was Tuesday. Today (Thursday), I started it up and backed it out, then drove down the fairly steep hill of my long driveway, so no accelerator needed. When I got to the street, I started off in 1st and immediately noticed the truck felt very "heavy" under the accelerator. I stabbed the clutch and revved the engine, which _seemed_ to rev fine. I got out, popped the hood, and checked the fuel filter and bowl. Both were full (I had replaced the fuel filter about a month ago during routine maintenance... so figured I'd start there), and the fuel level through the glass face on the carb bowls was fine. It's 100° outside, so I decided just to turn around and go home rather than get stranded somewhere. Getting back up my driveway was a significant challenge. I had to fiddle the throttle and slide the clutch the whole way up. In other words, it simply wasn't making power.
I'm old and tired, and don't have much stamina, so after a rest, I went out and pulled off the top of the air cleaner, and reached through the driver window and started it with key only. It started flawlessly. So, my first though was that the issue was just under load (so, possibly a vacuum leak that suddenly sprung. Nope. The vac line to the distributor is metal, and fitted at both ends with factory threaded/compression fittings. While running, I sprayed carb cleaner at both ends, as well as at the base of the carb, and didn't get a reaction from the engine from the extra fuel. So, I feel like vac leak isn't it.
While running in the garage, I played with the throttle... it definitely will die if I hold the throttle steady, high, low, or otherwise. But it will idle. I checked down the throats and only see ONE spray from the accelerator pump on the inboard venturi. Should both sides be spraying? Could it be that something (suddenly) clogged one side and it won't hold throttle because it's running out of fuel? Any ideas?
I'm without additional transportation, and have very limited stamina, so I want to try to avoid "try this, try that" ideas that aren't well reasoned.
Thanks in advance for any assistance you can provide,