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on this picture you see a daxengine, with a regular type 20mm carburetor and a powerfilter.

a downstream carburetor

a mikuni 24mm flatside carburetor

a 20mm manifold for the regular type carburetor

adjusting the jet needle to get the best mixture in mid revs
click for a larger version

The Fuelsystem

If you want to make your bike more powerfull you'll need to give him enough to drink. The drinks distributor of your bike is your fuel system, the carb.

There are a lot of different sizes in carbs, from 9 to 28mm and more. A standard 50cc engine has a small carb. most probably a 12 or 14mm (even 10mm on economy heads) Some old types of honda's have more, like the ST50 before 1972 has a stock 17mm (o.t.)

The standard carburetor is a bit small for most bikes, the bigger the carburetor, the bigger your power.
If you have a 50cc you can also buy a 20mm and put it on but the effect won't be what you've wanted, ofcourse you'll notice some difference but if you want to effectively use your fuel system you'll have to tune up the cilinder too.

If you have a 70cc cilinder with a stock 16mm carbuteror (ST70) it will not run at it's maximum performance, if you buy a 20mm or maybe even bigger, your bike will have much more acceleration power and you'll notice a increase in top speed.

If you want to place even a bigger carburetor you'll need more cc's, for instance, if you have 100cc's on your bike you'll need a BIG carb, like a 24mm flatside or a 26mm, there are even bigger carbs but they cost loads of money (like the Keihin Fcr 28-downstream).

There are 2 different types of carburetors:
1) the "regular" type, found on most honda mopeds.
2) the downstream type, mostly found on C types.

The downstream type is more effective because the fuel gets trough more easily, a 17mm downstream should be compared with a 19mm normal carburetor. The downside of the downstream carb is that it's hard to tune it the right way, and it is placed straight to the cilinderhead, it's not a regular choice, and it is mostly found on the C types.

If you buy a bigger carb it's a wise thing to also buy a filter that lets your engine breathe a little bit easier, like a power of a paper filter. (see: how to start)

The carbs and the filters can be bought at your honda-shop (see: links)

As we already told you: don't go out and buy a 24mm for your 50cc moped because it does not make sense to buy such a big carburetor for such a small cilinder, it's impossible for a small cilinder to burn so much fuel, so first think about the size of your cilinder and then about the size of your carburetor if you don't know what size you need for your cilinder you can always ask in the honda-shop where you want to buy your stuff, the people there will answer all your questions (as long as they can sell you things) I've figured out a little schedule that can help you figuring out the size of carb. you need, this is just a pointer, variations are possible.

50cc>max. - 20mm
70cc>max. - 22/24mm
85/88cc>max. - 24/26mm
100+cc>max. - 24/26/26+mm

if you choose the regular type of carburetor, you'll also need a manifold. This piece connects the carburetor to the cilinderhead and provides a nice flow of the fuel before it gets into the head.
Be sure the manifold has the same size as your carburetor to prevent false flows... if it's too small the bigger carburetor has no use at all. so don't use your stock manifold when upgrading. There are lots of different shapes and sizes available, so get yourself informed and check if the combination of manifold, carburetor and airfilter will fit in the narrow space between your frame and engine.

since the carburetor is one of the most important but also the most difficult part to adjust for top performance I have included an Adjustment guideline.

Carburetor Adjustments and Re-jetting your Carburetor

After modifications to an existing carburetor or the installation of a new carburetor, final adjustments and jetting changes are required to get the most power from the modifications. Without a dynamometer and air fuel mixture test equipment, making jetting changes to your carburetor can be very difficult.

the most important parts in adjusting are:

part 1) IDLE
Set idle speed to proper r.p.m. by adjusting the IDLE SPEED SCREW. Turn the AIR SCREW to achieve the highest speed and best response. After adjustment has been made reset the IDLE SPEED SCREW to the proper r.p.m.

part 2) OFF IDLE to 1/4 THROTTLE
The SLOW JET and AIR SCREW are most effective in this range When you want a richer mixture use a larger SLOW JET or turn the AIR SCREW in. The opposite holds true for a leaner mixture

part 3) 1/4 to 3/4 THROTTLE
The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the chip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.

Changing the MAIN JET affects this range. Select the size which offers the best wide open throttle. performance, then install one size larger MAIN JET for ideal engine durability.

OK let's start!!:

Step 1) Check your work:
Whether you are tuning a carburetor on a stock street bike or a modified race bike, the procedures are essentially the same. Start by making sure the carburetor's jets, adjustable settings and float levels are at a reasonable starting point. These would be the recommended setting that came with the installation instructions for the carburetor or re-jetting kit. While checking the carburetor look for fuel leaks, signs of air leaks, make sure the throttle works smoothly and opens to the proper full throttle position. Often overlooked by a Dax is fuel tank ventilation, fuel line and shut-off valve operations. All these items are required to ensure maximum fuel flow from the tank to the carburetor.

Step 2) Starting the Engine:
Assuming everything is OK, you are now ready to test your handy work. Warm the bike up to full operating temperature. You may have a little trouble keeping her idling, but one thing to remember is you now have a carburetor which is not running as lean out as the factory setup, so DO NOT follow the owner's manual directions for cold starting. Here is a new starting procedure to use: If the temperature is below 70, pull the choke out all the way, above 70 pull the choke out half way. If the bike has been run in the past two hours and the engine is still warm, the engine should start without using the choke. After starting the bike, quickly adjust the choke to reduce fast idle to a reasonable level. After 30 seconds, push the choke in all the way, use the throttle to keep the engine idling.

Warm the engine up for 8-10 minutes by riding a few miles at a slow pace. The engine must be up to operating temperature before setting the idle and idle mixture. A word of advice, find a small screwdriver that can be used to adjust the idle speed and mixture screws BEFORE the engine is hot. Fumbling around trying to determine the correct screwdrivers will probably result in burned knuckles if you are not careful.

step 3) Preliminary Idle Mixture Adjustment:
With the engine at proper operating temperature and at idle speed, turn the idle mixture screw inward (clockwise) SLOWLY until the motor starts to falter. If the engine will not idle on its own when you begin this procedure, turn the idle speed adjustment screw until it does. Throughout this procedure try to keep the RPMs at 900-1000 RPMs. Having turned the idle mixture screw inward until the bike falters, now back it out slowly, keeping count of the number of turns outwards until the motor begins to run smoothly. Re-adjust the idle speed set screw as necessary to maintain the RPMs about 900-1000 RPMs. Blip the throttle a time or two, and observe the results. If the motor responds with a gratifying blast without backfiring through the carburetor, the idle mixture is correct. If it backfires through the carburetor, back the idle mixture screw out another 1/4 turn. Do not go too far, as too rich an idle mixture can cause problems and poor gas mileage. This sets the idle mixture and idle speed for initial testing.

step 4) Adjusting Full Throttle Mixture:
When the main jetting is to rich, the engine will feel sluggish and acceleration will 'feel' slow.
The best track side method to determine the size of the main jet is to fully load the engine on a long straightaway or hill. At the end of the stretch, chop the throttle and hit the kill button simultaneously. Now pull the spark plug. The parts of the plug you should be looking at are the positive electrode and last 1/4 of the ceramic insulator. Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode. The electrode itself should have fairly sharp edges. For example, if the ceramic insulator has a nice tan coloring but the electrode has a white ring around the tip and the plug is of the correct heat range, then you can easily run a size larger on the main jet. Start with bigger difference in sizes. Like if you have a main jet size 65, then first try a 75. then you can finetune in smaller steps later on.. like say a 75 is to big, then try a 70.. and then regarding the results a 72 or 68.

basic spark plug reading:
dark or black plug - to rich mixture
brown or cappucino plug - good mixture
light brown or white plug - very lean mixture

Plug reading is as much an art as a science for most people. Lots of experience is needed to REALLY do it up right. For most street riders, making sure your plugs are a nice tan color is usually good enough. Don't bother reading the plugs until you have done preliminary fine tuning. You will need several sets of new plugs, gapped and ready to use. If additional tuning runs are required to adjust the carburetor, make sure you install a fresh set of plugs before each run. You can not read the condition of the plugs unless the plug is fresh.
Please keep in mind that the different brands of gasoline can give different readings.

When jetting your main jet, try to remember to jet for the best power in a specific situation. As you gain experience and knowledge, you will be able to use other methods to determine your jetting. A good tuner can "feel" most of the circuits by slowly reving a parked bike, or just by looking at the color of the unpainted pipe and silencer.

You can also check the main jet by quickly closing the throttle from wide open to 7/8 position when the engine's RPM is greater than 4500. If the engine accelerates slightly, the main jet is to lean. A larger main jet is needed. If the engine hesitates or misses slightly, the main jet is to rich. A smaller main jet is needed. If the engine just slows a slight amount, the jetting is very close to correct.
Another test is to accelerate through the gears at full throttle. If the engine backfires through the carburetor, misses, cuts out or quits running, the main jet is lean. Increase the size of the main jet. If the engine acceleration seems sluggish, does not react to the throttle or sounds flat, the main jet is rich. Decrease the size of the main jet.

step 5) Adjusting 1/8 - 1/4 Throttle Mixture:
Note - keep in mind that the idle adjust screw (air/fuel screw) gives a good indication of a properly sized slow jet. The slow jet calibrates the mixture from both the idle bypass and the idle orifice in the jet block. If the idle screw is properly adjusted, but the engine does not have good response when the throttle is wicked open, it is usually a sign of a lean mixture and the slow jet will need to be replaced with one size larger (richer) and the air/fuel screw re-adjusted. Consequently, if the throttle is only partially opened, such as in a trailing throttle situation, and the bike tends to load up, emitting a deep tone when the throttle is returned to full open, it is usually a sign of a rich slow jet. If the slow jet does not clean up this part of the circuit, the slide can be substituted for one with a different cutaway. The higher the number, the larger the cutaway will be, allowing more air to the jet block/nozzle screen leaning the mixture and, conversely, a smaller cutaway will richen the mixture with a greater effect up to 1/4 throttle.

Note: Normally you don't have to change the slow jet.. this is only for big differences in CC en carb diameter.. let's say if you use a 22mm on a 50cc engine, the slow jet may be too big.

step 6) Adjusting 1/4 - 3/4 Throttle Mixture:
for this part, the mid revs, you have to adjust the jet needle. just like adjusting the Full throttle mixture, you need to check the spark plug everytime. In this case you will only use a half opened throttle for a few minutes and then stop your engine quickly. check your sparkplug. It's adviced to start with the clip placed mid position. If the plug turns out dark, then place the clip one place higher, so the mixture will be leaner. If the plug seems to be light, or you hear little backfire in your muffler than you could try a richer mixture by moving the clip one step lower.

Step 7) Adjusting the idle Mixture
When you've gone through all previous steps and it all looks good then you'll have to refine the adjustment by running through step 3 again. Now you're done.

If you are in doubt as to the jetting, make sure you jet on the rich side (tan>brown>black). Because of potential engine damage, you are better off slightly rich rather than lean. A lean engine can detonate, causing permanent damage.

If you try these and your bike still runs very bad you may have some other problem (clogged fuel screen/filter, bad petcock, ignition trouble/electrical malfunction, timing wrong, fouled plugs, bad gas, etc. etc.).

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