Motorcycle Carburetor Problems
Only in theory are carburetors are simple devices. They are designed to mix a small amount of gasoline with the right amount of air so that an engine runs properly. How complicated can mixing two things be? Well, factor in cold starting, idling, and wide-open-throttle, and delivering the correct mixture all the time gets a bit more complicated.
Common Types of Motorcycle Carburetors
Unlike most classic American cars, most motorcycles have side-draft carburetors, which mount horizontally to the engine. Many classic European cars also used side-draft carbs - that was mainly for hood clearance. Some of the British-made SU carburetors found on older European cars can be adapted for motorcycle use.
Tube-type vs Float-type Carb
In a tube-type carburetor, air supply is taken through a single unobstructed channel of fixed air through the jet. There are no air valves or metering valves, so it is a simple design. The float-type carb has a float-operated needle valve that keeps the fuel level a constant distance below the edge of the discharge nozzle.
Carb Fuel And Air
To get your classic bike running just right, the carb needs the proper mix of fuel and air. Before you start adjusting your carburetor, remember that no matter how well-tuned it is, it will not make up for bad ignition, wrong spark plugs or incorrectly adjusted engine timing. Make sure all of these are in good shape before changing carburetor jets and settings.
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As with all mechanical devices, carburetors wear over years of use. They also require periodic cleaning and adjustment. The two most common problems are running too lean or too rich.
- A lean mixture gives an erratic idle, with tendency to stall when not fully warm.
- Too rich a mixture gives an even, rhythmic misfire and a bit black smoke in the exhaust.
- A correct idle mixture gives an even idle and colorless exhaust.
A lean condition exists when the carburetor is delivering too much air. If there is not enough fuel mixed with the air, an engine will run lean, which will cause it not to run it's best, or potentially damage the engine. Typical symptoms of a lean mixture include some or all of the following:
- Lurching acceleration
- Misfires and/or backfires
- Engine stalls easily
- Backfires when throttle is closed (primarily during coast-downs)
- Blueing on chrome exhaust pipes
- White or light grey spark plugs
- White or light grey on end of exhaust pipes
- Poor fuel economy
Fixing A Lean Mixture
Most carburetors have an adjusting screw that regulates the fuel/air mixture in the lower rpm range. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will richen the mixture (refer to your shop manual for correct settings).
Sometimes adding after-market accessories such as exhaust systems, air filter systems or replacement carburetors will contribute to a lean mixture. If no changes have been made to the bike, and it previously ran well, a lean mixture can sometimes be a leaking intake or exhaust gasket.
If there is too much fuel mixed with the air, the engine will have a rich condition. This may cause flooding, bogging, and stalling. It also wastes fuel.
Fixing A Rich Mixture
If the fuel level is set too high in the float chamber, a rich mixture will result. A rich condition may be caused by a dirty or clogged air filter(s). Also keep in mind that both altitude and humidity effect your engine's performance.
Carburetor adjustments should be done with engine at normal operating temperature.
Vintage Motorcycle Carburetors
The Linkert was a popular carburetor for both Harleys and Indians. From 1933 until 1956, Harley Flatheads, Knuckleheads and Panheads had Linkert model "M" carbs, which were simple, bottom-float units. The side-float Linkert "DC" carb was found on other Harley models, including the 1957-1966 Servi-car, 1957-1965 Sportster, and one-year-only (1966) Shovelhead.
read Linkert DC Carburetor Rebuild
A switch to Tillotson was seen with the 1966 Harley Sportster and 1967 Shovelhead. Unlike the previous Linkert, it had an accelerator pump and featured a hydraulically operated automatic "economizer" circuit.
Complex and having many parts, the Tillotson carb was not appreciated in its day. Lack of knowledge and poor parts availability tarnished their reputation, but when set up correctly, they are an excellent carburetor. The Tillotson Model HD was original equipment on Harleys until 1969/1970, replaced by the simpler Bendix unit.
read Bendix Carburetor Rebuild
S&S Super B Carburetor
When introduced in 1975, the S&S Super B was the carb to run. That was until the Super E came out in 1990. Although the Super E has gone on to become the best selling performance carburetor in the V-twin aftermarket, many riders still prefer the B over the newer E and G models, claiming it has better flow and more low end punch.
read S&S Super B Tuning and Rebuild
There's not too much more impressive than a dual-throat Weber sticking off your V-twin engine, but exotic carburetors are very time-consuming to dial in, simply because there's so many variable factors, and one adjustment usually affects others. The Mikuni-Solex pictured here is a car-type carburetor, although it is gravity-fed like most motorcycle carbs.
Adjustable items on dual-throat carbs include main venturi size, main jet, idle jet, air corrector jet, accelerator pump jet, emulsion tube, fuel inlet valve, and air horns. And if you care, a dual-throat carb on a Harley will cause interference with the rider's right knee.
Back when I was riding my 1972 XLCH Sportster, other Harley riders used to mock my 40mm Mikuni (Japanese) carb, but it worked well. To my delight and their surprise, the bike started consistently on the 1st or 2nd kick.
read Kick-Starting An Old Motorcycle
Octane Requirements (rule of thumb)
- 80 octane for 8:1 or less compression motors
- 90 octane for 9:1 compression motors
- 100 octane for 10:1 compression motors
Fuel System Maintenance
If you store your bike over the winter, untreated fuel may go bad in as little as six months. Longer than that, a fuel system cleaning is recommended.