My Carburetor is Leaking at the Throttle Shaft

Carburetor Throttle ShaftI hear this quite often. Customers are usually looking for some kind of kit to seal the shaft, but the shaft isn’t the problem. What is most likely happening is that there is too much fuel entering the venturi, running down the inside of the carburetor, hitting the throttle valve, then leaking out of the shaft. Now the throttle shaft cannot be totally sealed, or it wouldn’t move. What is causing too much fuel to enter the carburetor?

  • The float could be absorbing fuel becoming too heavy. This would cause the needle and seat to stay open.
  • The float is adjusted wrong.
  • The needle and seat is dirty.
  • There is too much fuel pressure. This can sometimes occur when you replace the fuel pump and the new pump is putting out too much pressure. This happens more often than it should. Test your fuel pump. You can also add a fuel pressure regulator.
  • One other possibility is that the fuel is splashing over the top perhaps from a rough road. Made sure you didn’t leave out any baffles.

The bottom line is that the fuel leaking out of the throttle shaft is probably due to the carburetor flooding. The solution is to get a carburetor kit and rebuild the carburetor and test the float. Now, don’t discount the throttle shaft being bad. It could be worn enough to require bushings. You can test this by moving the shaft back and forth to see if there is any slack. Chances are that if you notice slack, then there is probably too much.

Motorcraft Carburetor Chokes

The electric choke system, shown, is an aid in reducing vehicle exhaust emissions by matching choke operation to engine requirements throughout a wider range of conditions. The electric choke consists of an ambient temperature sensing control switch connected in series with a positive temperature coef­ficient (PTC) ceramic heater. For vehicle operation above 60° F., electric current is supplied to the PTC heater. This warms the bimetal choke control thermo­ stat providing a faster choke come-off. For vehicle operation below 60° F., the electric choke system is inactive, and the choke provides enrichment until the engine warms to operating temperature.

Motorcraft Electric Choke

The automatic choke assembly is mounted on the carburetor body. It has a bimetal thermostatic coil which winds up when cold, and unwinds when hot. A vacuum diaphragm and spring controls the initial operation of the choke. Engine coolant, flowing through a choke water cover, heats the bimetal coil and controls the final choke opening. To start the engine, the accelerator pedal is depressed, closing the choke valves. This permits fuel to flow through the main metering system as well as the idle system. When the engine starts, air flows past the off-set choke valves, and manifold vacuum, acting on the choke vacuum diaphragm, opens the choke valves to a predetermined position, As the engine coolant warms up, it circulates through the choke housing, heating the bimetallic choke coil. The coil unwinds, permit­ting full opening of the choke valves. If the cold en­gine is suddenly accelerated, the resulting drop in manifold vacuum on the vacuum diaphragm allows
the choke valves to momentarily close.

The fast idle cam, actuated by the choke rod, con­trols idle speed during engine warm-up. When the choke valves are fully opened. the fast idle cam ro­tates free of the fast idle screw. An unloader tang on the throttle lever partially opens the choke valves when the accelerator is fully depressed. This permits unloading or breathing of a flooded engine. During deceleration there is a high vacuum condition, causing fuel from the bowl to pass through a restriction
where it is mixed with air from another calibrated restriction. This mixture passes through a horizontal passage where it is mixed with a larger quantity of air, and then flows through a vacuum-operated deceler­ation valve into the intake manifold.
Automatic choke

In order to start and operate a cold engine, a richer air-fuel mixture must be introduced into the cold in­take manifold. The need for a richer air-fuel mixture is due to condensation of the atomized fuel inside the cold manifold and resulting poor mixture distribution to the cylinder. The choke system permits a richer air-fuel mixture which is required for starting and operating a cold engine. The choke late is operated either manually or by some automatic means. It may
be closed during the cranking period and partially opened during the warm-up, confining manifold vac­uum below the choke plate. The greater vacuum caus­es both main metering system and idle system to dis­charge fuel into the cylinders.

The manual choke system shown is cable-operated, and the choke plate positions are con­trolled by the driver. The cable, which is pulled out for starting, closes the choke plate. When the engine starts, the driver pushes the cable in halfway. This places the choke plate in the mid-position. When the engine warms up sufficiently, the driver pushes the cable in completely, thus placing the choke plate in the wide-open position.

Manual Choke
When the engine is cold, the torque, exerted by the bimetallic spring, forces the choke linkage to close the choke plate. As the engine starts, a pressure dif­ference, created by intake manifold and atmospheric pressures acting against the offset choke plate, causes
the choke plate to open slightly.

Manifold vacuum,  applied to the choke piston or diaphragm, pulls the piston or dia­phragm downward and further assists in opening the choke plate. The choke plate opens to a position where the tension from the thermostatic spring is bal­anced by the pressure differential force acting on the choke plate and the pull of vacuum on the piston or diaphragm. Enough air is drawn around the choke
plate to prevent engine flooding and to enable the engine to operate.

Manifold vacuum
The piston initial pulldown travel is limited by vac­uum bypass slots in the piston cylinder wall and by a slot in the piston. When the slot in the piston is ex­posed to the slots in the cylinder wall, vacuum travels through the channel, bypassing the piston, reducing the pulldown force, and restricting further downward movement of the piston. The choke plate then is opened a predetermined distance, but no further.

When the engine reaches its normal operating temper­ature, the bimetallic spring exerts torque to hold the choke plate open. If the engine is accelerated during the engine warm-up period, the drop in manifold vac­uum, applied to the choke piston or diaphragm, al­
lows the thermostatic spring to partially close the choke plate for a brief moment. This action provides a richer air-fuel mixture to prevent engine stumble or stalls.

If the engine should flood (over-rich mixture) during the starting period, the choke plate may be opened manually to lean out the excessively rich air-fuel mix­ture in the intake manifold. This is accomplished by fully depressing the accelerator pedal and engaging the starter. A projection on the throttle lever contacts the unloader face on the fast idle cam and, in turn, partially opens the choke plate.

During the engine warm-up period, it is necessary to provide a faster idle speed to prevent engine stalling. A fast idle cam is rotated into position by the automatic choke lever. A fast idle adjusting screw on the throttle lever contacts the cam face to provide the increased engine speed that is required.

Buying a Carburetor Kit

When finding the right carburetor kit for your carburetor, it is important not to rely on the vehicle application, but to use the carburetor number. It is easy for someone to sell you any kit that is close, but they aren’t paying the shipping, you are, and who needs the hassle of shipping kits back and forth until the correct kit is found.

Carburetors are 30 or more years old and the chances of the carburetor having been swapped out is high. Even if you think your vehicle is using the original carburetor you should still be using the carburetor number. It would be easy to forget that 40 years ago, the carburetor was replaced because of some kind of problem. Replacements were often different carburetor numbers than the original. I often hear “I know it is the original carburetor”, just to find out later that “the kit doesn’t work”. Guess what? The carburetor turns out to be non original. Take a little time and do your homework before you buy a carburetor kit.

Where is the carburetor number?

Many carburetors had a tag placed on the top of the carburetor, usually held on with a float bowl screw. Some manufacturers use a tag and also stamped the carburetor somewhere, and still others only stamped the carburetor. Want to know where your carburetor may be stamped? Carburetor Identification

So, your carburetor doesn’t have the tag anymore and there is no number stamped anywhere on the carburetor. What do you do?

  • Determine what type of carburetor it is. Not just the manufacture, but the series also. Don’t call me up and say you have a Holley 1 barrel that needs a kit. I can’t help you with that kind of information. Just to get started and at a minimum you need to know if it is a 1904, 94, 1940, or some other series. It wouldn’t take very long to poke around on the internet and find a carburetor that looks like yours. Still not sure, email me some pictures of the overall carburetor. I can probably identify it. This still won’t get you a kit number because a carburetor type could use four different kits.
  • Visit my YouTube channel and watch some of the videos. you may discover your carburetor type there.
  • Once you know the series you may need to take your carburetor apart. We want to get some hints from your carburetor parts.  I say may because in some cases, just knowing the carburetor type will be enough. For example: My Carter BB kit comes in two numbers. One fits all without a dashpot and the other fits only the dashpot type. My listing for this kit explains how to determine which one you have. In this case you could easily come up with the correct kit.
  • Measure the accelerator pump. You need to know the overall length and the cup size. I try to add this information for each carburetor kit under the technical info tab.
  • Measure the orifice size of the float seat. This is the hole where the gas comes through (the round brass thingy). You are going to have to do this with some kind of tool that will closely fit the hole. A set of small reamers are good for this. Small drill bits can also be used but you will need to convert any fractional measurement to decimal.
  • Put the carburetor type in the search box on my parts site. Mike’s Carburetor Parts
  • carburetor partsNow visit each carburetor kit product listing and compare your information with the information in the technical tab.
  • Compare your gaskets with the gaskets in the product picture, or video.
  • Another possibility would be to find a part number stamped on your carburetor and then match it up in a carburetor parts manual. Carter was one company that was good about stamping part numbers onto each part of the carburetor.

So, do your homework and get the correct carburetor kit the 1st time around. Don’t settle for a kit that might fit.