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The initial assumption is that the engine is running lean.
The first, and most frequent possibility is a restricted fuel supply path.
Pull out a quart jar, open up the carb drain, and let fuel flow from that drain into the quart jar. It should fill that jar in under a minute or so.
IF it doesn't... disconnect the fuel line from carb and see if it flows faster.
IF it does... remove the fitting between fuel line and carb casting- it's a catch screen. Blow it out... clean out the carb, drain the tank, blow out the fuel line, get it clean.
Check the float bowl level- get it right... float height... is more important to fuel mixture than the main jet... too low, the engine will NOT run right.
IF that doesn't do it, the second possibility is that the airflow in, or exhaust out of the combustion chamber is hindered.
It's a four-stroke. 1 Intake air/fuel drawn. 2 Compression 3 Ignition -> Power 4 Exhaust.
Pulling out the choke doesn't affect ignition. It can affect intake, compression, and exhaust, it also affects fuel mixture... but a weak spark circumstance frequently 'fools' us into thinking fuel problem... or as my friend Noel says: "Most carbeuration issues can be traced to the ignition system".
On an Allis D-series, there's several things that cause running issues, and they MAY be lurking... and while I won't suggest that they're your root cause, it's best you're aware:
Condenser. Condensers tend to fail. Brand new condensers from China suck... frequently, they're bad-out'a-da-box.
Contact points/spring... they're subject to wear, they run into seating problems, and the spring gets flaccid with age (and viagra don't fix it).
Distributor shaft bushing... when it wears, the shaft gets sloppy... and this causes the points to not open-close etc with accuracy. Rebuild the dizz. Dismantle, clean, and reassemble the mechanical advance, they wear, they get stuck, and they suck. You could get new points, or order an electronic ignition kit (PerTronix, etc), and make condenser and points problems gone for good.
Ignition switch "RUN" contact... they get worn over time, and eventually the switch contact gets so bad that the engine will crank, and fire when cranking, but stall as soon as you release the key. Prior to that failure mode, the ignition system just gets lousy. Wiring to the coil, battery grounds, etc., all degrade with age. If you bypass an old wire with a test jumper and things get better, it's because the wire you bypassed is crap.
When done with ignition rehab, you should have a really hot-mean-snappy spark... pee-your-pants-if-you-got-zapped hot sparky.
Got Champion plugs? are they newer than 40 years old? Yank and junk them- the crimp gaskets leak compression. Use the Autolite or NGK or AC flavors that others suggest.
Now mak absolutely certain that you don't have a plugged exhaust path... restricted exhaust is just like having a closed throttle. Next, check the exhaust an intake manifold-to-head mating surface for leaks. They'll generally warp and burn out the #1 cyl exhaust runner, and that WILL make it run like crap.
It IS possible for a crack to form on the casting between intake and exhaust sides of the manifold. Doesn't happen often, but once there's some rust or embrittlement, a pinhole or crack will cause exhaust leakage into the intake, and it'll run like crap.
After going through all of these, there's really only one other thing: Adjust the valves. Pop off the valve cover, take a feeler gauge and wrenches to the rockers, get the clearences right.
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Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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