" Question is would this carb benefit from a rebuild and if so, the basic or comprehensive? "
Before you go there, try adjusting it correctly.
It probably does need rebuilding, & you will know for sure if you can't adjust it.
First, clean 2 of the 3 the fuel screens. (carb elbow & sediment bowl)
Be careful w/ the elbow. Don't honk down on it; it's brass. First, use a flare wrench to remove the fitting. (as in the picture) Next, if you do not have another fitting to screw into the elbow (as in the picture) then use a crescent wrench on the top & bottom (NOT the sides) of the elbow to remove it. Otherwise, you will distort it & you will never get the fuel line fitting back into it.
After you get the fuel screens clean, remove the bolt in the bottom of the carb bowl & make sure you have a good fuel flow. (the gas should run in a steady stream as long as the sediment bowl knob is turned on).
Next, adjust the carb.
Make sure the tractor is at operating temp; that usually takes 10 – 15 minutes at idle depending on ambient temp.
Both Ford and Marvel/Schebler (assuming you have a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.
Then adjust the side-pointing idlemix for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed set-screw for very slow 400-rpms idle. Do that idlemix adjust for maximum idle at least 3-times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time & wait a second or two for the engine to catch up. Take your time!
Do it like this:
1. Adjust idle mix jet until RPM increases
2. Adjust idle-mix set screw until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)
Repeat steps 1 - 3 three times.
Remember the side-pointing idlemix is out for lean, in for rich.
If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpms.
Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than ½ turn for max power (remember, you already had it 1 ½ turns out) then stop right there because you have a dirty carb or a fuel problem.
As to rebuilding the carb, I've found it to be cheaper & easier in the long run to have them rebuilt. That's because the kit will run $50 or so, plus the cleaner. And most kits do not contain throttle bushing which you usually need.
GB in MT on this board does them; if he's booked up, these guys do 'mail order' work; give them a call to get a quote & shipping info.
And, they also do brakes, starters, generators, water pumps, clutches, etc.
Automotive Manufacturers Incorporated
2400 N Lombardy St
Richmond, VA 23220