51 B - What are causes of running rich?

The tractor is up and running now, but is not running super
smoothly and seems to be running rich. The exhaust contains
puffs of black smoke. I believe this to be a rich condition, it
doesn't appear to be burning oil.

I just rebuilt the cap end of the mag (new cap, rotor, points,
condenser) and timed and installed per the service manual
procedure. From the electrical end timing should be good.

I verified that all the valves are opening and closing, and
adjusted tappet clearance per the manual.

I did take off the carb and clean the best I could. I did not drill
the passages, I don't have the bits. Could a carb contribute to
rich running conditions? I typically think of carb problems
generating no fuel/lean conditions. I have the needles set to
one full turn out. The float level seems okay, if anything a little
bit low. What else can I look for that would be causing a rich
running condition???

I'd start by adjusting the carb. you should be able to lean it down to where it will die. Load needle is closest to the fanshaft . Idle needle is towards the LH outside. You will have to apply a load to it to final adjust the load needle. These mainly run on the idle circuit until a load is applied.

Partially plugged air cleaner. I've even found wasps nest in air intake pipe.

If the float level is to high the tractor will run rich and it will fill the crankcase with gas while sitting if you don't shut the fuel off. The 1/8" nipple screwed in the bowel drain with a little piece of clear fuel line is the best way to check fuel level in the bowel.

One problem I have had is the float would not float. It leaked and had gas in it. Check for this problem.

You can buy the drill bits from Robert's in Spencer Ia. They aren't expensive, also they have a video on carb repair, and any or all parts you need.
www.robertscarbrepair.com or phone 712-262-5311. The video that will help you is "How to Rebuild DLTX Series Carburetors Part II"

I will check this. It looked like it was okay, but I will see if it floats when I put gas in the bowl.

I thought about this, so I disconnected the air inlet to the carb and ran it that way. Same result.

Wouldn't happen to have plug wires interchanged would you?

If you only have them one turn out and it smokes and float level set right, then three things : The float can still float and still be problem from reduced bouyancy (sp) Take it out and submerse it in hot water and watch for bubbles. If that isn't it then it either isn't sealing correctly at inlet needle/seat or....and this is a problem in only late A and late B carbs...they have two little brass vent baffles in top roof of float bowl / round base where float bowl seals against , and they keep fuel from slopping up into vent passage because these carbs only , have a higher float setting/fuel level than others ( 5/8" as apposed to 3/4") What happens is they can get crap caught in above them and seal off or limit the venting action of the bowl. This pressure from incoming fuel will hold float down so needle won't seat and it will run fuel right out the front of carb if bad enough or cause a rich condition even if partially plugged. There is no cleaning them, you have to destroy then getting them out and then clean passage above them. It takes two, you can get them at Roberts Carb or maybe others. My e-mail is open if more questions. Good luck,let us know what you find.

Just removing the Oil Cup from the air cleaner will by-pass the Inlet Pipe..!!

You may well be able to lean those settings a good bit..

You will get to a point where just ONE "Click" makes a definite difference when you drive it..

Always set the Idle as lean as you can reasonably set it and still get good throttle response..

Autolite # 3116 are the best I have found ( Autozone, etc.)

Sounds as if you are kind of new to carbs and their settings. But if you have it running and got occasional puffs of black smoke, I would start with the simple things first. So make sure that the choke is fully open first. If the choke is fine, then I would turn the idle needle in a couple of clicks at a time with allowing the tractor to run for a bit between adjustments. If this the issue, you should hear the tractor start to pick up rpm as it starts clear up the rich condition and then lose rpm as you go to a lean condition. Once it starts to lean out and lose rpm, stop and open up the adjustment until rpm picks up again and leave it there. I would try this before messing with the float.

One other easy thing to check is to unscrew the idle adjustment screw altogether and see if it is worn. If it is the tip will not seat properly into the seat and will allow excess gas to pass through which could explain a rich setting at one turn out.

If the engine collant is cold the air fuel mixture will have to be either adjusted roo rich via both the idle jet & main jet . Or the choke will have to be applied to prevent stalling.
Odds are iif the coolant temperature is at 195 F the idle and main metering can be leaned out and the plugs won't foul.

I almost forgot one thing I have found on two throat and single throat carbs with the spring loaded choke like your should have if it is original and has correct # 67 carb on it. Either people don't get the spring oriented correctly on shaft or spring is broken or rotted away. Basically what happens is you really have no control over where choke is at any time, no matter where the rod is. I have seen duplex (two throat) where blades were out on shaft wrong and so when throttled up would suck choke shut and stall engine out. In the case of yours, it could very easily just be "floating" in the bore randomly not really hooked to the control lever on outside. This would cause just what you are experiencing. When it's assembled and working correctly, when you pull choke rod on full choke, blade should be shut and but you should be able to push on lower half of blade and against spring pressure, push it open and it should slam back shut when you remove pressure. When choke rod is in to open choke wide open, you should not be able to close blade or wobble it by hand pushing on blade. If that spring is broken or gone or not hooked right you have no control over where choke blade is. Pretty easy to check. Someone below said check simple things first....good place to start.

I agree check that the choke butterfly is operating correctly and is forced wide open when the choke is off.