Serving the Gainesville and Hawthorne Florida Area
Carburetor Repair, Rebulding, and Diagnosis
When an engine is cold it needs a richer (more fuel to air mix than usual) mixture to run correctly. A carburetor has a device called a CHOKE to accomplish this. The choke is like the throttle, except it is on the top rather than the bottom of the carb, and it closes when cold and opens when warm. When the choke is closed, it restricts the air entering the carburetor at the top, while the throttle still allows the same amount of air to go through the bottom. This means the same amount of fuel is sucked, but less air: so the mix is richer. Most carburetors have a vacuum "motor" (that is, a diaphragm moved by vacuum) that is connected to a rod that pulls the choke open a bit after the engine starts. This is called a "CHOKE PULLOFF". This enables the choke to be fully closed and provide a super rich mixture to start the engine. Once the engine starts, the choke pulloff opens the choke a bit to let the engine "get some air". If your car runs great when warmed up, but smokes black and hardly runs when you first start it, you may have a bad choke pulloff.
A float can also stick shut, especially on an engine that has not been started for awhile. If you have gas going to the carb but there doesn't appear to be gas available in the carb, you may have a float stuck shut. In this case you can sometimes tap LIGHTLY (Carbs are made of soft metal, so don't beat the thing up!) on the top of the float bowl and the float (actually the needle valve operated by the float) will break loose and the carburetor will operate OK. Tapping on the top of the carb can also fix a float stuck open SOMETIMES, although not as often as one stuck closed.
When accelerator pumps go bad a vehicle will hesitate on hard acceleration. It also will be hard to start, especially when cold. It's easy to check an accelerator pump: Remove the air filter and look down inside the carburetor. Have an assistant pump the gas pedal, or open and close the throttle yourself under the hood. You should see a squirt of gas going down the inside of the carburetor, or see a "cloud" of fuel vapor building up inside the carburetor throat. If the choke plate is closed you may need to hold it open with your finger or a screwdriver to see down inside the carb.
On electric chokes an electric heating coil in inside the choke coil housing and it heats the coil up, opening the choke.
A third type of choke uses engine coolant via 2 small hoses to heat the choke. Sometimes these chokes have a paraffin pellet which heats up and expands, opening the choke.
If the choke doesn't open all the way, it might not be getting heated enough. Check the metal tube to the exhaust manifold and make sure it's intact and not clogged. If it's an electric choke, check the wire hooked to it with a test light or volt meter to make sure it has power. The heating coil on an electric choke can burn out. Disconnect the wire to the choke and test with an ohmmeter between the terminal on the choke housing and ground. There should be some resistance, but not INFINITE resistance.
On a water heated choke, feel the side of the choke housing when the engine is warm: the housing should be warm also. If not, disconnect one of the small hoses going to the choke housing: coolaqnt should flow out of it when the engine is running. If there is no flow, the coolant supply to the small hoses may be clogged or blocked. Look for kinked or collapsed hoses. If the smalll hoses are OK, you may be able to clear out the clog by blowing compressed air through the small hoses leading ton the choke housing.
The "curb idle screw" adjusts the idle speed when the engine is fully warm. It holds the throttle open a bit to set the idle at the desired RPM.
If the choke never opens up all the way the carb may idle too fast, because the fast idle cam never rotates all the way to its lowest level.
Click here for some images of different carburetors
(you'd think carburetor would be spelled carburator, wouldn't you? In fact, carburator spelling gets some hits, because a lot of folks think carburator is right. Carburator is WRONG though, but if you want to spell it carburator I don't mind: or carburettor, for that matter!)
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