My 8N carb won't adjust

First do a thorough cleaning of the carburetor and replace the gasket between the two halves. Be sure that the float does not have any gas in it to cause it not to float properly.

The reason the carburetor may not want to idle is that the gasket that goes between the two halves of the carburetor is not sealing good. The castings are not finished too good to begin and lots of people damage the mating surfaces at the outer edge of the carburetor by banging on the side of the carburetor to stop it from flooding when the float sticks or there is foreign material between the needle and seat. (this treatment does work) :O) This gasket must be tight at the middle where the idle orifice gets it’s gasoline.
Remove the float attaching hinge by carefully prying it up until you can get a small screwdriver under it. Don’t bend the two small blind rivets. They will have to be re used to re assemble the float hinge.

Use a new sheet of 80 grit wet or dry sandpaper that is laid on a hard flat surface and lap both the mating surfaces until it is perfectly flat on both mating surfaces.

Re assemble the carburetor and you should be able to adjust the idle.

Adjust the idle by lowering the RPM and then turning the idle mixture screw in or out until the engine runs at the highest RPM. The RPM is what you will be listening for and not the way the engine runs. MAXIMUM RPM!!! At idle.

First let the engine warm up a bit. With the engine at full throtle,turn the main needle in clockwise untill the engine stumbles (not out) Then turn the main needle out until the engine runs smoothly. Reduce the throttle to idle and then quickly advance the throttle and see if the engine falters. If it does then open the high speed needle a fraction more and repeat until the engine will accelerate without stumbling. Reduce the throttle to a low idle speed. Turn the idle needle in and out until the highest engine speed is found. This is where to leave it, regardless of how many turns in or out. HIGHEST SPEED! Not sound of engine. The highest speed will be the best idle. Reduce the RPMs to where you want them after the highest engine speed at idle is reached when adjusting the idle needle.

You must pay attention to engine RPM when tuning the carb. If you try to do it by ear like you would tune a carbureted car, you will be fooled by the different RPM ranges that this carb was designed to operate at.

The main mixture sytem begins to be effective at about 600 RPM. Most tractors tuned by ear are set to idle above 600 RPM. So you are tuning a mix of the main metering system and the idle jet, unless you use a tachometer and tweak up the throttle linkage and governor to actually allow the 400 RPM idle. Most carbureted cars run almost exclusively off the idle system up to 1000 RPM, so the tractor doesn't tune like you "expect".

It is a lot easier to separate out the various effects if you disconnect the link from the governor to the carburetor and substitute a spring to hold the throttle closed. Otherwise the governor can try to compensate for your tuning.

The idle screw works in reverse. As you unscrew it, you admit additional air to the fuel being supplied through the idle jet. The tuning range afforded by this design is rather limited, and a common lament is that the mixture starts out too lean already, even with the screw fully closed. Unscrewing the screw can only make the mixture leaner, which won't do anything for you if you were starting out lean. The reproduction Marvel Schebler carbs seem to work like this out of the box, by the way.

The idle system draws its fuel through the same flow restrictor as the main metering system. If you close the main metering screw all the way, it cuts off all flow to both systems.

I suspect you are running at a relatively fast idle, and that you are running mostly off the main metering system. You can unscrew the idle jet all the way without effect, because of the previously mentioned reverse idle system and limited tuning range, and because the main metering jet is taking up the slack anyway.

You can unscrew the main metering jet all the way, without seeming effect, and not run "sloppy rich", due to a combination of the flow restriction in the passage leading to the main nozzle and the limited effectiveness of the venturi at less than 1000 RPM.

You should find you have some tuning effect at a 700 RPM idle of turning the main metering nozzle in, prior to it contacting the seat. If you are running slow enough, you may find you can run the idle screw all the way in, and acheive some tuning effect as well.

The transition from idle to main jet is one of the weak points of the Marvel Schebler design, even in perfect condition, and I suspect you are trying to tune it right in that range.

If you have the idle set by ear it is probably so high that the idle mixture screw has little if any control. Kind of like the six volt system cranking slow. If you compare it to late model stuff your ears will fool you.

Your mileage may vary

One other thought, "rebuilt carb". Don't remember for certain, but I believe that the main jet and idle jet (not the needles) are interchangeable? (same screw thread size) Idle jet is 0.0250 and main jet is #49 drill (which is almost 1/16" or 0.0625) This size difference should be eazy to visualize. Even for us ol'foggies without cheaters (grin)