2013 23.1 Ask Albert

Dear Albert,
The other day my tractor started running hot, there was a loss of power under
load, and it started popping back through the carburetor. Can you help me
figure out what’s going on and what to do about it?

Dear Popping,
Sounds like the engine is running lean. It’s possible that a piece of dirt is plugged in the main jet of the carburetor. When it idles down does it run smooth and the temperature return to normal? If so, it’s more than likely a partly plugged jet or fueling circuit in the carburetor.

Air-Fuel System Components

Let’s analyze the different components of the air-fuel system. Fuel and air should be considered at the same time, as a successful operation of the engine depends on the correct proportion of the mixture. Air and fuel proportions are controlled by needle valve adjustments in the carburetor. When the mixture is correct the engine runs smoothly, delivering maximum power. Too much fuel in the mixture is called a “rich mixture”, and causes black, smoky exhaust and irregular running of the engine. Too little fuel is called a “lean mixture”, and causes a popping back through the carburetor, misfiring of the engine, or a high-pitched ping, referred to as “pre-ignition knock”. The fuel system of a gasoline-burning tractor consists of the carburetor, fuel tank, and manifold.

How the Carburetor Works

Gasoline is drawn into the carburetor, which has 3 functions:
1) vaporize or atomize the fuel, 2) mix the vaporized fuel with the correct proportion of air to produce a highly combustible gas, and 3) supply the engine with the proper quantity of air-fuel mixture, depending on the load, speed, and temperature. The gas is drawn into the combustion chamber, placed under pressure by the piston on its compression stroke, and ignited or burned by a spark which is timed to fire at the proper instant to deliver full power of the burning fuel to the piston. Fuel must stay liquid while in the carburetor until discharged through the nozzle as a fine spray. The carburetor has its own local fuel supply, called the “float bowl”, that is maintained at a constant level by means of a float. The nozzle, or jet, releases the liquid fuel from this bowl into the air stream. Nozzles are located centrally in what is called a “venturi tube” or “barrel”, which is a constricted section of the intake manifold designed to increase greatly the velocity of air at this point of the process.

Increasing the air velocity increases the ability of the air stream to pick up the fuel from the nozzles and atomize it. The throttle, or speed control, which meters the amount of fuel to reach the engine, is located beyond the jet nozzles and venturi tube. In most tractors, the throttle valve is controlled by a governor mechanism. Carburetors vary in type and construction. Because of this, the tractor’s operating manual should be consulted for proper maintenance and adjustment.

Basic Trouble Shooting Procedure

Fuel Tank and Fuel Filter
Begin a check for carburetor pop back causes at the fuel tank and fuel filter. First be sure the fuel filter is clean. If there is a sediment bowl, turn the fuel tank valve off, remove and clean sediment bowl, hold a container underneath sediment bowl, and open valve to check for volume of fuel. Make sure there is a steady and even stream of fuel. From there, check the filters. Check and clean or replace filters based on the type of filter – replacement element or wash-out cleanable filter. Most tractors are gravity feed fuel because the tanks are higher than the carburetor, so gravity flow feeds the carburetor. To drain this type of system, with fuel turned off, remove the inlet line to the carburetor. With a container to catch fuel, open the shut-off valve (because it was closed to clean the sediment bowl – otherwise your elbows will be dripping with gas). Doing this will refill your fuel filters and you can check fuel delivery at the carburetor.

For tractors with fuel pumps, most of the pumps will be mechanical on the engine. At this point in the check, with the transmission in neutral and brake set, turn the engine over until fuel comes out. The fuel will come out in spurts (remember, we’ve disconnected the fuel line at the carburetor), but the fuel line should be full of fuel and the spray should emit for several inches out of the hose with about 5-7 pounds of pressure. If the fuel does not come out in this way, the problem could be in the fuel pump, as it is not delivering a sufficient amount of fuel to keep up with the engine’s demand. If the fuel system checks out okay, the problem is probably in the carburetor.

Float, Needle, and Seat in the Carburetor
The next step in the process is to check the float, needle, and seat in the carburetor. First, remove the carburetor bowl. The bowl is usually attached with either one bolt in the middle or several screws around the perimeter. When removing the float bowl bevery careful not to tear the gasket or it will need to be replaced. Once the bowl has been removed, look to make sure the float is not crushed or filled with holes. Move the float up and down to check for freeness of action. Watch also to see if the needle moves up and down with the float. Most floats have a small pin on which it pivots and that attaches it to the carburetor. Gently pull the pin out with needle nose pliers. Warning: these parts are small and delicate. Be careful not to loose them. Check the point of the needle, make sure that there is no corrosion or particles on it and that the needle isn’t stuck in the seat. With the fuel line still off the carburetor, blow air back through the seat from the needle side. Observe if any

Main Jets
Reconnect the fuel line with the container under carburetor. Turn the fuel back on and observe the flow. If flow is good then turn back off the fuel. On some carburetors, the main jets are inside the float bowl and are non-adjustable. On other carburetors, the main jets are outside the float bowl and adjustable. Carefully remove the main jets and observe them for obstruction. If there is obstruction clean out and replace the jets. Additionally, replace the needle, float, and pivot pin. Gently lift the float to close the needle and seat. Be sure the float is parallel to the body of the carburetor. There are specific measurements for the float setting. See the operator’s manual for specifics. The basic rule of thumb is that if the float is parallel with the body of the carburetor, it is generally pretty close to good.

Reattach the float bowl to the carburetor. Reassemble the fuel lines and turn the fuel on to check for fuel leaks. If there is an adjustable outside main jet, gently turn it in until seated. Do not particles or junk are expelled. over tighten – just tighten until it stops, then back out 3/4 of a turn. This will be close enough to start the engine. Once the motor is running and warmed up, give it full throttle and adjust the main out until black smoke starts to appear, then turn the adjustment in just enough to make the black smoke disappear.

Good Luck

Thanks for writing in, Popping. That covers the basic procedure for a carburetor trouble shooting check and to determine if the carburetor is clean and the fuel system is functioning correctly. If problems persist, talk to your favorite local mechanic or dealer for more in-depth assistance.

Want More?

Tilth Members, if you like this column or are interested in a mechanic’s workshop in 2013, please contact Carey Hunter at or the Tilth Producers office with your suggestions. It was a pleasure to see so many of you at the Spring 2012 workshop at Grant Gibbs’ place (thanks, Grant – terrific teamwork!) and at the Friday workshop at the Tilth Conference (love those eager hands and faces).

Happy Holidays,

Tags: Air-Fuel system, Carburetor, Float, Fuel Filter, Needle, Seat