The Library of All Knowledge

Wrenchmonkey Series - Carburetor (Setting up the Big Bang.)

Carbureation is simple. It's just the mixing of air and fuel to make an explosive mixture. It's all the variables thrown in that makes things complex. Variables include the amount of fuel and air in the mix. (I'll also talk about the relationship between the fuel and oil in the mix too.)

A basic carburetor (herein shortened to carb.) works on the principle that a rapidly moving column of air flowing over a body of liquid will draw some of the liquid into the air because of the lower pressure. (Known as the Venturi Effect) In a motorcycle carb the fuel is kept in the float bowl and the suction of the engine provides the air current. The airflow sucks fuel from the float bowl and mixes it with the air. The amount of fuel in the air is regulated by the proximity of fuel to the top of the bowl and the size of the needle, which regulates the size of the port through which the fuel is drawn. The slide also regulates the amount of air drawn through the carb. The idle air screw also regulates the amount of fuel and air drawn into the engine when the slide is closed and the engine is at idle.

Fuel in a two stroke is concerned with the amounts of three ingredients; fuel, air and premix two-stroke oil. Too much air and the bearings in the engine don't get enough lubrication, overheat and the top end seizes. Too much fuel and the engine will flood and hamper combustion. If this sounds silly, consider that gas itself does not burn. Instead it's a combination of fuel and air that goes "kablooie". Too much oil and the combustion is hampered again. If the fuel/oil ratio favors the fuel then the bike runs very fast, but it also runs hotter and the top end receives less lubrication. The premix oil not only serves to lubricate the top end but it also insulates the top end components from the heat of combustion. If the ratio favors oil, then the bike runs somewhat slower. The top end lasts forever but there is a problem of carbon build up and fouling of spark plugs, because the engine doesn't run hot enough to burn off the excess oil. If your mind is reeling then consider that I haven't gotten to the effects of jetting yet.

A carb has five parts that can be adjusted; float bowl level, main jet, pilot jet, needle, airscrew. The float bowl level is adjusted by bending the tab on the float arms. The objective here is to raise or lower the fuel level in the bowl. This in turn regulates the amount of fuel entering the air stream. The size of the main jet also regulates the amount of fuel that enters the airstream when the throttle is open. Otherwise the needle blocks the hole in the main jet. The needle also affects the fuel flow. The taper on the needle determines at what point of the throttle opening the fuel begins to flow and how much fuel enters the airstream. The more radical the taper, the faster the fuel/air ratio increases. Also raise the needle in the slide and the fuel/air ratio begins to change sooner. This provides more of a hit when the throttle is opened. The fuel/air mix at idle is determined by the pilot jet and the airscrew. The slide also regulates the rate of airflow within the carb. Other factors that influence carbureation are how well the air filter is maintained, how well the airbox breathes and the type of reeds used on the intake port. Also since an engine uses the seal on the rings to provide suction to draw in air, a properly working top end also has an influence.

The effects of an improperly set up bike is evident in the way it handles. Blackened deposits, excessive oil, carbon build up and fouling are all indicative of too much oil or not enough air. Worn down leads on the spark plug, bits of melted metal are usually indicative of a bike that's running too hot. This is turn is due to an excessively lean air/fuel mix or a lean fuel/oil mix. The premix oil actually serves to cool down the components in the combustion chamber. A proper sparkplug for a two stroke should have a chocolate brown deposit on it. A properly tuned four stroke should have covering of a light tan ash. It should also shred the knobbies off a tire and roost all over the competition.

Do you need to rejet?

If your bike is acting lean then it could be several problems:

air leaks, in the airboot, top end gasket, base gasket, crankcase seals,
clogged fuel line, clogged fuel strainer,
shut off petcock,
overheating,
too much ignition advance,
or jetting,

If the bike is running rich, then it could be:

clogged air filter,
dirt in the engine,
clogged muffler or spark arrester,
leaky main seal on the wet/clutch side,
not enough ignition advance,
or jetting again.

Too rich vs. Too lean.

Too rich

Engine misses
Spark plugs foul
Sluggish performance

Too lean

Blow itself to bits
Ping, detonate and preignite
Run hot or run on (diesel)
Sluggish performance

On a modern carb there are basically four parts that can be adjusted. They are the; main jet, needle, pilot, and airscrew. At any given throttle opening the fuel is flowing through at least two of these jets simultaneously. The exception is a full throttle where the main jet is used exclusively.

Other parts you need to know:

Airscrew: It's located above the float bowl. Consider it a fine tuning adjustment for the pilot jet, since they affect the same range for the throttle. (closed to 1/8th.) Turn it all the way clockwise to richen the mix. If it needs to be turn out more than 2 1/2 turns then the pilot jet needs to be replaced.

Pilot Jet: It's the one buried in the carb right next to the main jet. The numbers denote the flow rate. the bigger the number, the greater the flow rate. It works over the same throttle range as the airscrew.

Needle: This one causes grown men to cry considering all the factors involved.

Changing jetting:

More confusion here: But hey at least there's an excuse to go riding. Ride the bike with the throttle at the wide-open position. This means that the only jet being used is the main jet. Any problems with the jet will show up here. Then play with the needle clip position. Dropping the clip will raise the needle and richen the mix. Raising the needle will have the opposite effect. Make sure your new needle isn't too short since at full throttle it may come out of the jet and jam the throttle slide open. If you mess with the pilot make sure that the idle is set properly first. (Unless you like messing about with the carb instead of riding.) You can check the pilot jetting by holding the throttle a 1/8th way open then twisting it wide open. If the main and the needle are OK then anything funny can be blamed in the pilot. (Call it "pilot error".) Before you play with the pilot jet, fool with the airscrew first since it serves to fine-tune the effects of the jet.

When you go up in altitude, consider that the air gets thinner. This means that there will be more fuel in the system. You should decrease the size of the main and pilot jets 1% for every 1000ft in elevation gain. Since temperature also affects air density. Lower temperatures increase the air density. So for a 10 degree increase in air temperature there should be a decrease of 1% in the jet size.