1. Clean the air filter - Sounds stupid, but you can be sure there are a lot of bikes at the races with expensive racing parts and a dirty air filter. The difference in power between a dirty and clean filter is probably more than the difference in power between a stock and aftermarket pipe. The amount of air an engine breathes is directly proportional to the amount of horsepower it makes. Some bikes will benefit from enlarging the air passages in the air box, or even cutting more openings. Be careful not to cut openings that will allow mud to get splashed onto the filter.
2. Less Weight - A Titanium bolt kit costs about $800. and will knock off about 1 pound. Or you could get the same weight savings from running 20 ounces less gas in your tank. If your running scrambles or enduros you need a full tank to avoid making more pit stops, but for motocross there's no reason to fill up for a 15 minute moto. Time the moto's where you race then keep track of how much gas you use on practice day for that amount of time. The difference in weight between a full tank and enough to run one moto will probably be about 10 pounds. Reducing weight at the fuel tank makes the bike less top heavy also.
3. Cool Fuel - Keep your fuel can in the shade, better yet, pack your fuel can in ice. (You can use the empty cooler since you and your friends drank all the beer the night before). As fuel vaporizes in the carburetor it has a cooling effect on the incoming air. Cooler air is denser and has more oxygen, the cooler the fuel is when it vaporizes the cooler the air will be and the more power it will make, plus the engine will run cooler also. In drag racing it is common practice to run the fuel line through a canister full of ice. Also the fastest drag race times are run on cold days because of the denser air.
4. Race Coolant - Racing coolants are more efficient than the standard 50/50 mix of antifreeze and water. There is an alternative to expensive racing coolant. Straight water cools better than antifreeze, but it does boil at a lower temperature, and it doesn't protect the aluminum and magnesium from corrosion. Use a mixture of 80/20 distilled water to antifreeze (do not use tap water because the minerals in it cause corrosion) and add Nalco to prevent corrosion. If Nalco is not available at your local auto parts store then check at a truck parts dealer, it is used commonly in trucks to prevent corrosion (follow the instructions for how much to add per gallon). This mixture will cool better than 50/50 and still protect from boil over and corrosion. You can also add a "wetting agent" such as "Red Line". Wetting agents cause the coolant to flow closer to the metal, they have the opposite effect that wax has on water (beading).
   
5. Pipes An aftermarket pipe is one of the most popular performance modifications. When choosing a pipe be sure to keep in mind that although some pipes can increase power all through the powerband, in most cases the power is just moved from one rpm range to another. A top end pipe will usually sacrifice low or mid range power in order to get more top end, and a low end pipe will usually sacrifice top end power. Compare dyno charts for different pipes to determine which pipe will give you power where you need it.
   Check the inside of the pipe for carbon build up or rough weld's at the end where it attaches to the head. Remove rough welds with a die grinder then polish with a sanding wheel. Polishing this area of the pipe is like polishing the exhaust port. Polishing will increase power and help prevent carbon buildup.
   Make sure there are no leaks where the pipe attaches to the head. Leaks are annoying and they cause a lose of power. Replace the O-ring, if it still leaks apply silicone sealer around the pipe flange. For more about pipes click here--->. How expansion chambers work
6. Reeds Reed valves also are designed to improve power at certain RPM ranges. Stiffer reeds are for top-end power (less floating at high RPM's) and softer reeds are for low-end (open easier and breathe better at low end ). You can replace the reed pedals only or replace the entire reed cage. I recommend replacing the entire reed cage. Most stock and replacement reeds are made of carbon fiber. Older bikes had fiberglass or steel reeds. Carbon fiber reeds can increase power in low and top end because the lighter material can open and close faster without the high tension needed to prevent floating with steel or fiberglass. Make sure that the reed valves are closing completely, you will see light through the reed cage if the reeds are not closing completely
   
7. Ceramic Coated Piston - Thermal coating the piston dome and cylinder head helps keep the heat in the cylinder where horsepower is made. The thermal barrier also makes the piston run cooler for longer piston life and less chance of seizing or burning a piston. Ceramic coated pistons have been in use for years, in drag racing, snowmobile racing, circle track and even NASCAR. All motors benefit from Ceramic coating the piston, head and valves, but 2 stroke motors gain the most. The exhaust valve is the hottest part in a 4 stroke motor and the most likely part to fail. In a 2 stroke the piston and exhaust port do the job of the exhaust valve, that's why 2 strokes are so prone to piston burning and seizing where 4 strokes are prone to burning exhaust valves. The ceramic/metallic compound protects the piston in 2 ways. It has a higher melting point and since it does not conduct heat as well as aluminum it acts as a thermal barrier (insulator). More heat in the combustion chamber will generate more horsepower.
   
8. Compression - Increasing compression will gain power all through the powerband. The head can be resurfaced to remove material which will increase compression. Measurements must be taken to determine piston to head clearance before removing material. Higher compression will raise cylinder pressure and temperature. Too much compression will cause overheating, detonation and possible piston failure. C.C. the head before and after machining to determine the compression ratio. See C.C. ing the head.
   
9. Squishbands - The area of the head where the piston to head clearance is the closest is called the "squish band". As the piston approaches T.D.C. the fuel/air mixture is "squished" out of this area and toward the center of the combustion chamber. The turbulence caused by the squish band causes the fuel/air mixture to mix with the unburnt exhaust gases and burn more efficiently. By decreasing this clearance the squish band can be made more efficient. The minimum clearance for most bikes is .020" To check the clearance remove the head and place a 1/4" long piece of solder on the outer edge of the piston (above the wrist pin and on both sides, to keep even pressure and to avoid piston "rocking" in the cylinder) , apply some grease to keep it in place. Install the head and turn the engine over by hand using a wrench on the flywheel bolt. Remove the head then measure the solder with a micrometer. Surface the head to remove enough material to bring this clearance closer or to a minimum of .020" Be sure not to remove too much material or the compression will be too high and could cause engine damage. In most cases you will only remove .010" to .040". Do not remove more than .040" unless you C.C. the head before machining, then machine the combustion area of the head to bring the head volume back to stock.
   
10. "C.C. ing the head" is done by placing a piece of Plexiglas over the head (Drill a small hole in the Plexiglas). Put grease or vaseline on the head gasket surface to prevent leaks. Use a syringe (or any measuring device marked in c.c.'s) and fill the head chamber with water. (don't forget to put a spark plug in the head)! This will give you the stock volume of the head. Mill the head to the desired squish band thickness then open up the combustion chamber to bring the compression back down to the stock or preferred c.c.'s.
    
11. Power valves - Power valve to piston clearance can be reduced, which will increase the effect of the power valve, which increases low-end power. There are many different power valve designs and each has a slightly different area to modify to change the clearance. Check the factory recommended clearance. You can decrease this clearance from factory specs but it will require periodic checks to be sure wear in the linkage does not bring the valve in contact with the piston.
12. Porting-Back in the 70's "Porting" actually meant grinding the ports larger. This also changes the port timing. Making the ports wider does not change the port timing but grinding the top of the exhaust or transfer ports does. Altering the port timing changes the RPM were the powerband occurs. Intake ports were increased in diameter, combined with a larger carb. On piston port motors the intake timing was altered by removing material from the bottom of the piston skirt or grinding material from the bottom of the intake port. Exhaust ports were widened and exhaust timing altered by grinding on the top of the exhaust port. These changes improved top end power at the expense of low end power.
    On modern moto-cross bikes the factory has done a real good job of determining the size and location of the ports. They have spend a lot of time with high tech equipment: dynometers, flow benches etc. There's little chance of improving the stock port timing by experimenting, since altering the port timing usually requires altering many other specs to work properly with the new port timing such as: Pipe dimensions, ignition timing, carb jetting, compression ratio etc. Since the factory mass produces the motors, they don't have time to smooth out the port surfaces or to make sure the ports are located exactly where maximum performance is reached. Today most "cylinder porting" jobs consist of just cleaning up the casting flaws in the stock cylinder. Some other porting mods are: "Case matching the ports" which involves matching the transfer ports in the cylinder with the transfer port cutouts in the engine cases (This requires completely disassembling the bottom end). And "degreeing the ports"or "port mapping". "Port mapping" involves placing a degree wheel on the crankshaft, rotating the crank to the exact port timing (stock or altered timing) and marking the cylinder by spraying bluing dye and scribing a line on the cylinder at the top of the piston. Then the ports are ground to the scribe line. If the ports are already higher than the scribe marks then the base of the cylinder with have to be machined to lower the ports. An easy way to change port timing slightly is to use a thinner or thicker cylinder base gasket. A thicker gasket will raise the ports which increases exhaust port timing. This will give more top end and less low end. The same thickness should be removed from the cylinder head to keep the compression the same as stock. A thinner gasket will decrease exhaust timing which will give less top end and more low and midrange power. Piston to head clearance should be checked to be sure there is at least .020" inch clearance. For piston to head clearance see Squishbands.
    
    
13. Fuels- Some common misconceptions about race fuel are: 1. It burns hotter and can cause your motor to overheat 2. It will give your motor more power. 1. Race fuel is a high Octane fuel. Octane is a reference number that tells how much heat the fuel will withstand before detonating without a spark. Octane itself is a fuel similar to gasoline, it was given a rating of 100 to use a reference point to compare to gasoline. The amount of heat produced by a fuel is measured in B.T.U.'s. All gasoline's produce about the same amount of heat so race fuel will not cause your motor to overheat, in fact in most cases it will run cooler (race gas has additives that increase the cooling effect it has when it evaporates, like the cooling effect rubbing alcohol has on your skin). If high octane fuel was more likely to do engine damage they wouldn't use it in aircraft engines 2. Every engine requires a slightly different minimum octane to operate properly. If your motor requires 91 octane and your using 93 octane then it probably wont gain much from 108 or 114 octane race fuel. Most race fuels will require rejetting because the fuel carries more oxygen than pump gas, without rejetting it's possible that your engine will run leaner and produce less power. So why pay more for race gas? High octane race gas allows you to build an engine with higher compression, more spark advance or leaner fuel mixture, without detonating the fuel. The power advantages of race fuel come mostly from these changes. The higher oxygen content means you will run bigger carb jets to get the correct fuel mixture. Your engine will burn more fuel because it has more oxygen available to burn the fuel. Octane is just one of many specifications of race fuel. The specs of race fuel are measured and adjusted to the legal limits according to racing organizations. Every gallon is check and will be the same every time you buy fuel (if the fuel is fresh). Pump gas can vary greatly from one week to the next or from one station to another, and they only check samples of pump gas from thousands of gallons they produce. Different additives are added to pump gas according to the time of year and location where it will be used. In winter they add more alcohol to help remove moisture and they use additives to make the gas evaporate better at low temperatures ("vapor temperature"). Gas that will be used in very hot climates is given additives to lower "Vapor Pressure" to prevent "vapor lock". This is great for your car (especially fuel injected cars that measure and adjust fuel mixture electronically) but for a race bike it can alter your carb jetting from one tank of gas to another. The main thing to remember is to check carb jetting if you run race fuel, also remember if your going to go back to pump gas you may have to switch the jets back. In most cases stock 2 strokes run well on premium pump gas with the proper carb jetting and will gain little from race gas. Unless your building an all out "race only" motor I suggest using premium pump gas or mixing one gallon of race gas to 3 or 4 gallons of pump gas. Whichever you choose you should use it consistently Carb Jetting -Each thing that you change will effect the carb jetting. It is not possible to tell you exactly what jets to run. Carb jetting is time consuming but dollar for dollar will pay off in performance more than any part you can buy.
    Here's some instructions for jetting a carb:
    Start with the jetting already in the carb or to be safe on a rebuilt motor or big bore kit start with about 2 sizes bigger on the main jet.. Always warm up the motor by riding it, do not warm up the motor by revving the motor excessively in neutral. On 2-strokes use a quality synthetic 2-stroke racing oil and jet the carb with the same oil that you will be using. On a new motor- Break the motor in for at least an hour. The only thing you need to do for breaking is not over-heat or over-rev the motor, wide open is ok for short periods, just normal riding. Start out running it easy and progressively run it harder. A new piston will run slightly hotter because it does not have any carbon to slow heat transfer into the piston, also new rings will allow some blow-by which causes the piston to run hotter. Do not add extra oil to the gas or do anything different. Extra oil changes the fuel mixture. If you are going to use race fuel, pump fuel or a mixture of the two use that fuel while jetting the carb. Most race fuels contain more oxygen than pump gas and will cause the motor to run lean. Jet the carb with the fuel and oil you will normally run. Be sure you have a clean air filter, the condition of the filter greatly effects the carb jetting. Do not over-oil the filter excess oil will cause the motor to run too rich. Try to jet the carb on a day when the weather is closest to the same weather conditions when you are doing most of your riding or racing.
    Find an area where you can run the bike or quad safely on a long straightaway.
    #1. Start with a warm motor and install a clean spark plug (sometimes a new plug is harder to read than a plug with some carbon on it)or just remove the plug and take note of the color (black, gray, white). Start the motor and run it at wide open throttle, run through several gears at wide open, then pull in the clutch and hit the kill button. Remove the plug and check the color, white is too lean and black is to rich, (you want a light to dark shade of gray) The plug will usually be lighter in color toward the center electrode and darker toward the outer edges. This will give you a reading of the fuel mixture at wide open throttle (main Jet). The idea is to let the motor run at wide open and avoid part throttle or idling which will change the plug reading. If the plug is to white install a larger main jet, if its to dark install a smaller main jet. Go back to #1. If the plug color is a light shade of gray, its good. Dark at the outer edges is OK.
    #2 Run the motor at part throttle (1/4 to 1/2) as much as possible, try to avoid idling or wide open. Shut the motor off and pull the plug. Check the plug color as in step #1, If the plug is to dark, lower the needle (moving the clip up, lowers the needle) If the plug is to light, raise the needle (moving the clip down, raises the needle) If you end up with the clip at the top or bottom groove and still not jetted right you might have to go to a different needle(richer-thinner or leaner-fatter). Usually the stock will work
    #3 Run the motor at a normal idle speed. Turn the air mixture screw in until you can notice the RPM's going lower, back the screw out until the RPM,s are highest, if you go to far out the screw will have no effect, try to find the point where it idles the fastest but the screw is not out so far that there's no effect. When you have the right setting, a half turn in will slow the RPM but a half turn out will have little or no effect. Now shut the motor off and turn the air screw in until it stops (gently tighten so as not to damage the air screw seat) It should take about one and a half to 2 turns. If it takes less than 1 ½ turns the pilot jet (slow jet) is to small ( try a larger pilot jet). more than 2 turns and the pilot jet is to big ( try a smaller pilot jet).
    Some other indications of carb jetting are:
    1. If the motor takes an excessively long time to warm up (hesitates until it is hot) its a sign that its running lean. Also power may seam to fade when it gets hot, and popping sound at high RPM's
    2. If the motor runs good when its cold (very little hesitation at start up) , its a sign that its running rich, after warm up the engine will studder and run rough or load up and foul plugs often, when too rich on fuel.
14. check back later more tips will be added
    

1. Top-end Power - All pro's.
   "A" class riders in 125 MX or SX.
   some 250 "A" riders
   
2. Mid and top end power - Pro and "A" class enduro's
   "A" class riders in 250 MX, or SX.
   "B" class 125's MX or SX.
   some "B" class 250 riders
   some "C" class 125 riders
   
3. Mid-range power - "B" and "C" class enduro's
   "B" class riders in 250 MX, or SX.
   "C" class 125's MX or SX.
   
4. Low end Power - Trail riding, beginner enduro or MX
   

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How expansion chambers work

The drawings of pipes below ignore bends in the pipe to simplify the drawing and to make the differences in the pipes clearer. When the exhaust port first opens, the sudden release of pressure sends the exhaust gases out through the pipe. At the same time the exhaust pressure is released a sound wave also travels down through the pipe. When the expanding exhaust gases pass through the tapered cone (B1) the expanding diameter cone creates a vacuum which helps draw more gases out of the cylinder and also helps to pull more of the air/fuel mixture into the cylinder. This process is so efficient that it can draw some of the air/fuel mixture out into the exhaust pipe. The sound wave traveling down the pipe bounces off cone (B2) and returns to the exhaust port where it creates a "Wall" which holds the incoming air/fuel mixture in the cylinder until the piston closes off the exhaust port. The returning sonic wave only reaches the port at the correct time when the engine is operating at a certain RPM. The RPM that the wave will return at the correct time, is determined by the length of the pipe. Sound always travels at the same speed (about 1700 ft. Per second) so the length of the pipe must correspond to the RPM of the engine, that's why the pipe only helps over a narrow range of RPM'

Below are some descriptions of how the shape and length of different parts of the pipe effect the way the pipe works.

A. Shorter - moves the powerband to a higher RPM. Longer - moves the powerband to a lower RPM.

B1. Short, steep angle causes a narrow but hard hitting powerband. Long, gradual angle causes a wider but softer hitting powerband. This section determines the RPM where the pipe will help pull the exhaust gases out and draw the air/fuel mixture into the cylinder.

B2. Short, steep angle causes a narrow but hard hitting powerband. Long, gradual angle causes a wider but softer hitting powerband. This section determines the RPM where the sonic wave will return to the exhaust port. The lengths and angles of B1 and B2 will usually be very similar since they must work together to cause the pipe to work at the same RPM.

C. This length will be longer if B1 and B2 are long and short if B1 and B2 are short. The total length of B1, B2 and C equal length F. Long dimension F will give a wide soft powerband and short will give a hard hitting narrow powerband.

Dimension "E" determines the RPM where the pipe will be in the powerband. Shorter will put the powerband at a higher RPM and longer will put the powerband at a lower RPM. At high RPM's the exhaust port is open a shorter period of time therefor the distance the sonic wave travels must be shorter in order to return at the correct time.

The diameter of sections A & D (the silencer and short section of pipe in front of the silencer) are proportional to the size of the motor. If the diameter of section D is reduced or the length increased (more back pressure) top end power can increase, if the diameter is increased or the length reduced (less back pressure) low end power will increase. Increasing back pressure also increases cylinder temperature and can cause overheating. Just remember that rule if a little is good a lot is better, that will get you in trouble every time !!!!

The reason the 80 and 125 pipes are shorter is not due to the size of the engine, it is due to the RPM where the engine is designed to operate. The smaller motors need to make maximum power so they sacrifice low end power in trade for high RPM power. 500 cc motocrosser's are tuned milder so they make a lot more low-end and are more controllable on top end. A 500 tuned like a 125 would make about twice as much power as a stock 500 and be almost impossible to control unless you were road racing. A 125 tuned like a 500 would have a powerband suitable for a trail or street bike.
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