Hop-Ups for Dummies - Honda 50 Carb Tuning by Stanton High

First things first! Tuning any motor is a process of hit and miss. So, don't get discouraged! Although most carburetors use the same process, this guide is aimed at tuning the stock Honda XR/CRF 50. Some of this process can be applied to tuning an aftermarket carb. Use some common sense and you’ll figure it out for your setup too. Due to elevation changes, humidity, modifications done, and personal preference, all carb settings will be different. If your carb is setup wrong, you will know it. Fortunately, these motors are very forgiving and won't break easily.

The more you understand the way the carb works, the better you will be able to diagnose a problem and know how to change it. When you see the term "lean" it means that the gas to air ratio is lower than optimal. "Rich" is the opposite of lean; too much gas in the mix.

The parts:

There are three screws on the side of the carb, from top to bottom; the highest is the idle adjustment, next is the air or fuel screw, and the lowest is a drain to empty the carb's bowl. The carburetor's “slide” is the part that the needle and cable are connected to. The slide is also the main control of air into the motor. The needle and clip position are located on the slide inside the top. The air screw or fuel screw (two names that get mixed up) help to control the mixture no matter what the throttle position. The clip position on the needle helps to control the 1/4-3/4 mixture. The higher the clip the more fuel gets in. When adjusting the clip, be extra careful not to damage any of the parts. The main jet size changes helps 1/2 turn through wide open.

Note: One component will not tune a carb, but combine them and you can tune your engine exactly how you want it.

How to make adjustments:

The air screw is the easiest to adjust and used only to fine tune the other adjustments. Turning the screw in makes the mixture richer, out leaner. Do not turn the air screw out more than full 2 turns.

The idle adjustment is used to adjust how much air is coming in to the mixture. The easiest way is to set this after your other adjustments are made. Turning the screw in makes the idle speed faster. Don’t turn the screw out more than 3 full turns.

The needle's clip position is adjusted by taking off the cap of the carb and pulling the cable up and out of the carburetor. You must take the slide (the round piece that connects to the throttle cable) off of the cable. To do this, cover the top of the slide and gently push the needle up.

Notice how the spring holds it down and also notice the small clip and notches at the top of the needle.

To adjust the clip, you should use needle nose pliers as the clip is very fragile. The higher up the clip in the notches, the leaner the adjustment. To richen it, lower the clip's position.

When you are done adjusting the clip, reassemble the slide/cable and put it back in the carb. The side of the sleeve with the long notch goes in opposite of the adjustment screws. If reassembly is difficult, and it feels like the spring is really hard to compress, take out the slide and reposition it.

To change the size of the main jet, the carb needs to be drained of gas. Drain the bowl with the lowest screw on the carb. Don’t forget to turn off the gas on the tank ;) Disconnecting the fuel line and throttle makes things easier, but is not necessary. Remove the carb with the intake attached. This makes reassembly easier. Next, remove the bowl (the bottom of the carb, held on by 2 screws). Be sure to remove this slowly so you don't hit the float. Turn the carb over gently and watch for extra gas. Be careful that the bowl comes off and goes ON the right way. The main jet is in the largest post. It can be removed with a flathead screwdriver. Be careful not to damage it by using the wrong size screwdriver. Now install the new jet. Make sure it is threaded all the way in. Reassemble the carb carefully, being sure to keep everything clean.

For the higher elevations, the stock jetting (#58 main jet) will be good for just about any other mod you make. At sea level, a 62 main jet is a good match with an aftermarket (or modified) air filter and an aftermarket (or modified) exhaust system. The process of tuning is making the best mixture of air and fuel throughout the throttle positions. A 60 could be used for the middle elevations, or if a 62 is too rich. A 65 jet is the largest Honda makes but I don't recommend higher than 62 for stock (OR) without a bore kit as it would be too rich. The Honda part number for the #62 is “99101-1870620.” “99101-1870600” is for a #60.

Ok, now that you know what everything does and how to adjust it, you can figure out how to fix it.

Diagnosis Help

The easiest way to find out what is going on inside the power plant is by doing a plug chop. A plug chop is a technique that is used to figure out how you need to adjust your carb. First, get your bike warmed up. It doesn't need to be hot, just 30 seconds of running before you start the test. Ride the bike in first gear, at half throttle, for about 30-45 seconds. As quick as possible, shift into neutral and shut off the motor. Now, be careful and remove the plug. A white color on the top means a lean mixture. Going up on the main jet size will fix it. A light brown or brown color indicates a good tune, and no adjustments should be needed. A black sooty color plug shows that the mixture is rich. A smaller main jet, clip adjustments, and air screw adjustments are needed.

Examples of tuning problems:

The motor is bogging off the idle; the idle is too low, the engine is too rich or too lean from the air screw. Check your plug! To fix this, turn the air screw in. A a needle clip change may also be needed.

The motor is sputtering while running wide open, and the plug is sooty black; the engine is running to rich (higher number) of a main jet, or the air screw is to rich/or lean.

To fix it, you need to change to a smaller/leaner main jet, and then move the needle and adjust the air screw.
The motor components can compensate for each other, but only to a point. Don't try and lean out the air screw if the jet is too big. You’re going to have to make some internal adjustments. This problem requires special attention, but is worth the trouble once completed.

And finally...

Clean and oil your air filter the night before doing a tune up. After every adjustment, rev the bike and let it go back to idle. Don't be afraid to try it all, you can always set the carb back to the stock settings = clip centered on the needle, 58 main jet, 1/2 to 1 turn out from stop of the air screw.

A few things I need to mention, before closing this up. Don't get frustrated! Trial and error is how it’s done, so don't give up after ten minutes. There is no perfect adjustment that works for everyone, that’s why the options are there. Don't be scared, its all worth it!

Have fun,
Stanton High

About the author: Stanton High has ridden on wheels for as long as he can remember. In 2003, he discovered the xr50 and was instantly hooked. Recently, Stanton was in a bad accident and half his body was burned. "It was hell," Stanton recalls. But after four months and 13 surgeries later he got back on the xr50 and is now riding harder than ever. These days Stanton rides as much as he can and is looking for a job in a machine shop. You can check out some of Stanton's riding in the Rider Gallery.

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