311B running poorly

I need some advice from those who know how to tune a carb. My tractor has started running poorly. It will idle, but applying throttle will cause it to die. If I manually choke it, it runs smooth as I could hope. This seems to suggest the oxygen to fuel ratio is not quite right. There are two adjustment screws on the carb, but I need someone to explain the function of each. I am fairly good at understanding and following directions if someone could lend a hand. Thanks in advance.

First, if it's been running fine, don't change the the adjustment screws, you have a fuel supply
problem, and the level of fuel in the carb can't be maintained, lending itself to a lean mixture. The
screw on the base of the carb (usually the lowest one) is a main mixture adjustment. Something you'd
tweak a a bit in the winter (cuz' the mixture leans out with the greater oxygen content of a cubic ft of
air). Also if you're just pulling a rack around the yard, it should be a bit leaner than burying a plow
or disc all day. The other screw (usually smaller) is the idle (or low speed) mixture. Pull your fuel
line off at the carb and you should see a solid steady flow (catch it though). If you don't, upstream
is a filter or sediment bowl and usually in an older tractor, a fuel tank full of rust scale and scum of
all description. If your fuel stream if good, and watch it for 30 seconds at least, it's inside the
carb. If your gas is more than 6 months old you can get it gelling and plugging internal parts of the
carb. That's a start, but don't start twisting adj screws, w/o a reason

If there is a restriction in fuel before the carb, why does closing the choke make the fuel flow and run properly? It does not make sense to me. Please explain.

It will do that because you're closing off or limiting the air. The same fuel is still available. You
are also increasing the vacuum in the carb venturi which also richens the fuel mixture. Think of
atmospheric pressure on the fuel in the carb bowl, 14.7#/sq in at sea level, and a vacuum in the carb
throat giving us, say 10# of air pressure. The approx 5# of pressure difference pushes the fuel into
the carb throat. The main fuel pick up is located in the venturi of the carb, the point of greatest
restriction. Think of it as a drinking straw going from your throat to submerge in the fuel. Just like
a drinking straw...... You expand your lungs, creating a low pressure area in the straw (as long as
your lips are sealed around it), atmospheric pressure difference then forces the liquid into your mouth.
Remember there is no such thing as suction, it can do NO work, you are creating a pressure difference
and the higher pressure does the work. How about an air compressor: 125# of air in the tank, 15# of
air pressure in the shop, or 110# of difference. The low pressure in the room is not sucking the air
out of the tank, the higher side is doing the work. The vacuum in the carb throat comes from 2 places.
The piston going down, assuming the rings are good, allows the air in the cylinder to expand, thus
lowering the pressure. The cylinder is connected thru the manifold then the carb then the air cleaner to
the outside. The greater the pressure difference the more air flow in. The air, moving thru the carb
throat also helps to lower the pressure, think of how an air foil works on an aircraft wing. The
restriction of the throat speeds the air, lowering the pressure even more, and that's were our fuel pick
up is located. If you can grasp that concept of pressure drops and understand that a partial vacuum or
a suction, does nothing you can figure this little bugger out. If you know how it works, the diagnosis
is simple. What's not happening? So what part is not doing its job? And then why.... Kinda lengthy,
but rather than twisting things and seeing what sounds better, this type of thinking will solve
anything. Keep asking..............Find a schematic of an simple updraft carb and locate the low speed,
and the high speed circuits and it will help it make sense

As said, clean out the supply line from the carb to the tank. Remove the sediment bowel assy. from the tank and clean all the ports in it, and then reinstall it. That should cure your problems.
Loren

Simple fact if you need choke after the engine is warm the combustion mixture is too lean. The primary reason is the carb needs cleaned.

The 311B usual factory carb was Marvel-Schebler TSX635. If your carb is an MS TSX go to the link for a manual that covers TSX maintenance and adjustment. I'll add another pic because it is important to adjust the float level properly, note that the measurement is with gasket in place.

Joe


http://www.carburetor blog.com/manuals/dltx_tsc_manual.pdf

Today I pulled the carb off and opened it. Not shocking I found some sediment in the float bowl. I rinsed it out and used a syringe to push gas backwards through the jet. After reassembly she runs like she used to run.

I want to thank you all for the info about how the carb works in theory as well as the practical tips. Just caught a rain, but should be good to go at finishing shredding after it dries.