The Garage
More In-Depth Explanation on Carburetors.
Basically, there are usually three carburetor systems in one carburetor. The first, or primary, provides the proper gas/air mix when the bike is idling (or when it's at a very low rpm), the next provides gas/air as you open the throttle (the middle range), and the last provides the proper mixture as you scream by that geek in the BMW at full throttle who thinks he can out run you at 120 mph (the writer of this article admits to breaking no laws, just dreaming about it!). The three carburetor parts are called the pilot jet (or idle jet), the needle jet, and main jet. The pilot and main jet are just fixed nozzles, while the needle jet has a needle that slides up and down, depending on the amount of vacuum provided. The throttle, which more accurately is called the "butterfly valve" (because it has a central pivot point and two "wings" off of the center that essentially form a circular disk), is primarily responsible for letting the air flow, and hence the vacuum, increase and decrease in the carb. When the throttle is "closed," there still is air that comes in through the "bleed, or pilot screw" and the gas comes up through the pilot jet. So, even when you think the throttle is closed, there still are bypasses in the carb to allow the air and gas to flow to the engine (otherwise your engine would die).
As you open up the throttle, more gas is sucked up by vacuum through the needle jet. The rate at which the gas travels up through the needle jet is determined by the position (and condition) of the jet needle (different than the needle jet!), which travels up and down inside the needle jet as you open (up) and close (down) the throttle, or more accurately, as the vacuum in the carb venturi increases and decreases. The main jet takes over usually around 80-90% throttle open. It is the largest of the jets (and has the largest hole), and therefore lets the largest amount of gas pass through. Think of the jets in a carb like a nozzle on an aerosol can, just in reverse. Instead of pressurizing the can and forcing the liquid through the nozzle and atomizing into the air, a carb provides vacuum to suck liquid through a finely tuned nozzle and atomizes it. The diameter of the holes in the jet nozzles varies from less than a millimeter to maybe a couple or few millimeters. You can see why getting gunk in there really messes things up! Take a 1 millimeter chunk of goop. That’s all it would take to fully clog one of your pilot jets on your bike. Now the bike might not stop running, but you may notice that it is idling very poorly. Ok, so you basically understand the innards of the carb. So how does the gas get into the carb and up the jets? The gas is sucked up through all of the jets from the float bowl, or float chamber. Think of it as a mini gas tank for each carb. When the fuel level in the float chamber drops to a certain level, the floats will fall down by gravity and pull on the bottom of the needle valve (again, different than the needle jet and the jet needle…don’t blame me, I didn’t name them!). The needle valve is a small (usually less than a half inch long and a quarter inch in diameter) metal needle that pushes up and seals on a rubber "seat". The rubber "seat" is pretty much just a small rubber O-ring. Anyhow, the floats pull down on the needle valve to let more (liquid) fuel into the float chamber. The fuel will then flow by gravity from the fuel tank (as long as your fuel valve is on PRIme or the vacuum of the engine is holding the fuel valve open). You can see why dirty carb parts make for a poorly running carb – there are a lot of little pieces that operate on nothing but gravity and air to work! The choke. The choke used to always be literally a choke. Originally, it was a smaller butterfly valve that sat at the inlet of the carb. On the small engines for a lot of yard equipment, and for many motorcycles, this is still how the carbs are choked. By closing the choke, you "choke" off some of the air coming into the carb, thereby forcing the cylinder to pull a higher ratio of gas into the engine, making it easier for a cold engine to start and run while it was warming up (but gives poorer performance at higher rpms). Some newer (actually, they’ve been doing it for years) bikes will use a small needle valve (separate from the one that lets fuel into the bowl) to allow more fuel to flow through the carb using small passage-ways that bypass the regular method of metering the gas. Regardless of the method of choking, they are still usually controlled by a single cable that is attached to a cam at the carbs via a set screw (to set the amount of slack in the cable – and there should be some slack – usually less than 1/8") and run up to your handlebars somewhere where there is a lever.
A simplified Carb Diagram (click for a larger view)
The Three Things it Takes to Make an Engine Run.
The internal combustion, reciprocating engine needs only three things to run.
Spark, Fuel and Compression! Yep, that’s it. Nothing more, nothing less.
Is there spark?
The spark needs to occur at somewhere near the correct time. This is a function of the spark plugs, the wires, the distributor, the timing, lots of things come into play here. We can assume that if you are getting a spark at the plug then you’re probably ok. This is unless the distributor is 180 degrees out. Not that above I used the term "at somewhere near the correct time".
Let’s see if you have spark. Remove one of the spark plugs and unhook the wires to the others. Put the spark plug in the cap and lay the plug body (so it’s grounded) on something like the exhaust manifold and have your assistant turn the engine over as though you were trying to start it. The reason we unhooked the other plug wires is that we don’t want the engine to start while you’re under there. A four cylinder engine will start on only 3 cylinders. You have spark at the plug? Good! No spark? We probably need to look at the distributor, coil, points, etc., then.
Is there fuel?
Are the plugs wet with fuel? If you can smell raw gasoline on them then you’re probably getting fuel to the cylinders. No fuel on the plugs? Hmmm. Is there fuel in the tank? Don’t laugh. It’s happened!
Are the float bowls for the carbs full? If not check the pump. Don’t assume anything. Look things over. Try squirting a little fuel into the throat of the carbs. Does the engine fire and then die when you do this? Perhaps we are not getting fuel to the cylinders. Look the carbs over. If the car has been sitting for a while then you’ve perhaps found a stuck needle in the seat and there is no fuel getting to one of the carbs.
Is there Compression?
Here’s where it can get expensive. Unless you’ve lost compression on several cylinders (2 out of 4 or so) you should be able to make the engine run. If more than one cylinder is low on compression then we’ve got a problem. If two cylinders out of four are low this could be a bad sign. It could also be a good sign. Two cylinders side by side being low on compression can be as simple as a bad head gasket. It can also be a bad sign as this problem can be caused by a cracked head. If all the cylinders are above 75 pounds then the engine has enough compression to run. If only one cylinder is low on compression then it could be a head gasket in this case too. Remove all the plugs. Unhook the coil wire from the coil. Have your assitant spin the engine with the starter for 6 revolutions with a compression gauge in the plug hole. Record the compression. Do the same for the other cylinders. Now, if you have good compression in all the cylinders, somewhere in the neighborhood of 125 pounds or better, we’re good to go.
If you have a low cylinder(s) we need to do one more test. With the cylinder that is low, squirt in some motor oil. Four squirts will do. Now check the compression again. If the compression RISES noticeably then you have bad rings. If the compression stays the same then you can bet it’s either a valve problem, a blown head gasket, cracked combustion chamber or something in that area.
Again, remember that these are relatively simple engines in our little cars. They don’t require a great deal of equipment to work on but they do require some tlc. Get back to basics when there is a problem. Don’t assume anything. Start at the beginning with the THREE things our engines need to run.
Tire Pressure
Although many riders spend a great deal of time discussing the virtues of various tire brands and rubber compounds, rarely does the subject of tire pressure enter the debate. Besides performing the essential function of holding a tire on a wheel, tire pressure affects a variety of factors. While most riders know that the wrong pressure can reduce a tire's life expectancy, the amount of air you carry can also dramatically affect handling and stability.
For optimum performance while heeled over on the track, tire manufacturers usually recommend running pressures lower than those reserved for the street. Why? The lower pressure allows for a larger contact patch and therefore more grip in the corners. How low should the pressures be? They shouldn't be so low that they allow the carcass to deform and cause handling problems. Also, if the pressure is too low, the tires could overheat and the extra traction being sought will slip away. In the days of bias-ply tires, setting tire pressure was critical for keeping the tread in the correct operating temperature. Although radial tires have minimized some of the temperature fluctuations caused by pressure sensitivity, setting a tire to the proper pressure will pay off dividends measured in lap times.
While the most scientific means of determining if a particular pressure is working for a tire is the use of a pyrometer to assess whether the rubber has reached the manufacturer's recommended temperature, charting the pressure increase of a tire after track sessions will give a good impression of how hard a tire is working. Dennis Smith of Dunlop's Sport Tire Services recommends an increase of two to four pounds in front tires and six to eight in the rear. But he adds, like most of the tire representatives we contacted, that club racers should talk to their tire vendor at the track since their knowledge base will negate the need for a lot of trial and error.
Since maximum grip is a good thing, why not just run race pressures on the street? First, according to Michelin's Claude Leroux, increased cornering grip comes at the expense of stability and feel. Lower pressures decrease straight-line stability, and regardless of how talented the rider is, most street bikes spend a high percentage of their time straight up. A side benefit of using the proper pressure is that the front tire will feel more precise and turn in quicker-a good thing during point-and-shoot sessions. Second, you can easily overheat your tires simply riding in a straight line by running the pressure too low. Remember, the lower pressure is to ensure a large contact patch, which is created by the carcass flex. The same process happens when you're not cornering. Take your sport bike out on an extended interstate ride with too little air, and all that flexing of the tire's carcass can cook the life right out of it.
Tire manufacturers spend a lot of time determining what pressures will provide the best compromise of performance and tire wear on the street. While some manufacturers recommend running the same pressures listed in the owner's manual for the bike's OE tires, a significant number-such as Metzeler and Pirelli-have proprietary pressures that should be run on particular tire/bike combinations. Be sure to ask your dealer or check the tire manufacturer's product literature for specific numbers. And check the pressure before every ride! Cory Johnson from Metzeler/Pirelli says between 75 and 80 percent of the tire warranty claims he reviews are caused by underinflation. When the cost of today's premium rubber is considered, investing in a good tire gauge-and using it religiously-is cheap insurance.