Dispelling Two-Stroke Tuning Myths - Frank Wrathall

Let's start out by dispelling common two-stroke tuning myths, using the Honda RS125 as an example.

Carburetion. There is no magic jet. There is no magic number. Do not listen to others. You must carburete your own bike.

Trick Parts. Many aftermarket parts may not work well with your set-up. There is no quick fix or bolt-on horsepower. Settings may need to be adjusted to make the parts work.

Fuel, aka pouring power into the tank. Without optimizing settings, no fuel that you pour in the tank will give you more power, despite what the manufacturers say. All the settings will have to be optimized to make the fuel work. The carburetor is the key to power. No rider uses the same jetting. In a two-man team, running identical bikes, the carburetor settings may differ from one rider to another, simply because of the way each rides. One may ride with relatively high corner speed, one may ride with relatively low corner speed. One rider may open up the throttle early on in the turn, another may open the throttle later in the turn, but open up the throttle harder or quicker, so these would require different carburetor settings, different needles, different main jets, etc. Also the bikes may run slightly different.

Basic Carburetion:

If the bike is too lean, it will rev but will have no grunt, and it will run good on the first lap then gradually get worse.

If the bike is too rich, it won't rev, will have good mid-range, and be stronger later in the race rather than earlier in the race.

How do you tell if it's rich or lean? Look at the following:
Plug readings. How do you read a plug? If you look at the ground electrode, it should burn white to within two-thirds to three-quarters of the distance from the tip towards the body of the plug. You can also look at the steel body of the plug around the outer diameter, and that needs to run biscuit brown to light brown, depending on the oil that is used.

You can look at the piston, and read the top of the piston. You should see a nice deposit of carbon on the top; the color will depend on the type of oil and fuel you are using. The piston should be dry with two small areas of bare aluminum exposed at either side of the exhaust port at the front, with slight traces of dampness. You don't want to see any detonation-this is erosion of the piston through pre-ignition caused by being too lean or too advanced or running too high a compression.

You need to see a burned oil deposit the size of a dime on the underside of the piston. The size of this deposit will depend on the temperature at which the oil burns, i.e., it depends upon the type of oil that you use. You can also measure the exhaust temperature, and you can also log the maximum rpm, using data acquisition.

You can look at the exhaust gas temperature, the cylinder pressure and such like. You can also use a detonation counter. This counts the detonations per kilometer. That, basically is how you read the carburetor.

Generally, the back-of-the-piston readings denote the carburetor on the needle. The airscrew and the pilot system work right through carburetion from no throttle to full throttle. Most people use standard pilot jets and airscrew settings, but if they have problems with the performance of the bike, and their riding style, this may have to be changed.

Some riders may spend lots of time off the throttle, which will load the engine up with unburned fuel, and to compensate should run either a smaller pilot jet, or the airscrew will have to be wound out.

Likewise, some riders may find that they need a richer pilot setting for sharp response off the bottom of the rev range. The needle can affect the mid-range performance of the engine and different riders prefer different needles, again it depends on what kind of power delivery the rider needs from the bike, and entirely depends on their riding style. Some people run lean needles, some people run rich needles, quite often, riders tend to run too lean a needle, because they will carburet the bike on the throttle on the stand, and when the engine sounds sharp and feels crisp, they will go with that needle. But in effect, the needle will be too lean and under load won't deliver enough fuel to give good drive out of the turns. But again, a lot depends on the riding style of the rider.

The main jet needs to be the right size to make the engine rev, and over-rev. Over-rev being between 12,000 and 13,000 rpm. The power jet switches off on the standard bike at 12,200 rpm, and is important for the mid-range. Most riders do not change the power jet, and they would only change the main jet, leaving the power jet as a constant.

What affects the carburetor? Several things. The weather: The air pressure, the humidity, and the temperature all affect the relative air density (RAD). The denser the air, the bigger jet is needed. The less the air density, the smaller the jet. Basically when the air pressure is up you need to jet up, when the temperature is down, you need to jet up, when the humidity is down you need to jet up.

How do you monitor the weather conditions? Many people use RAD gauges or will measure the air pressure, the humidity and the temperature, and work out the relative air density (RAD), and then will formulate some kind of a pattern as to what their bike needs. The length of the straightaway also makes a difference. On a short straightaway you will need a smaller jet to reach the most efficient combustion chamber temperature and exhaust temperature in such a short time. On a longer straight the temperature needs to be kept down, and you would need a bigger jet so that you still reach optimum temperature at the end of the straight. If you run too small a jet, you have problems with seizures, etc.

Cylinders:

Cylinders have changed on the RS125 from year to year. The port shape has changed over the years, and also the port timing. On the later bikes, from 1997 onwards, Honda has tried to make a torquier engine and the shape of the ports is responsible for this. The size of the ports has increased over the years and staggering of the transfer ports and the various port timings have changed. Probably the new bikes will come out with more peaky cylinders to work with unleaded fuel. The cylinders are very prone to cracking at the back and inside the exhaust port. There are some aftermarket sandcast cylinders available that work quite well, but again are quite top-end biased cylinder because they have been developed for two years for use with unleaded fuel.

Pistons:

There are two types of pistons generally available, the standard and A Kit. The A Kit piston is slightly different in that it uses a thin piston ring and has the piston-ring locating pin in the rear of the piston, running up and down the back boost port and not on the bridge between the secondary transfer and the back bridge. The reason for this is that a thinner ring gives more power, and also you can make the secondary transfer ports much bigger. The standard ring is a keystone ring, which is tapered. This was introduced in 1993, to try and eliminate piston ring sticking. This is a problem using standard or plain, parallel rings.

Cylinder Heads:

The cylinder head is split into two separate parts. One is the combustion chamber shape; the other is the squish area shape. The squish area is the area at the side of the cylinder head and should be machined to run parallel with the top of the piston. The combustion chamber is the rest of the cylinder head. The most important factors are:

Squish clearance. This is the difference between the top of the piston and the cylinder head at top dead center.

Width of the squish.

Volume of the cylinder head.

Unleaded cylinder heads now come with a bigger squish area, i.e., the width of the squish is wider, and will run with a lot more volume in the cylinder head to lower the compression. We would run a squish clearance, i.e., the distance from the cylinder head squish area to the piston at top dead center, of around 0.6mm, which is 0.024-inch. The compression will vary, i.e., the volume of the cylinder head on the engine, at TDC, to the top of the plug hole will vary according to the fuel used, anywhere from 9.8cc to 10.4cc to 10.5cc on leaded fuel. On unleaded fuel it may be over 11cc. On leaded fuel we run less squish area than we would with unleaded fuel. We run a standard combustion chamber shape, and adjust the volume to suit. Mostly it depends on the kind of fuel that is used, and the octane, and the way that it burns as to the volume, but the squish clearance can normally be kept around the 0.024-inch range.

When people speak about squish clearances, and what squish they run, the volume is more important than the squish, so if someone runs 0.028-inch squish it is not as important as the difference in the volume.

There are aftermarket cylinder heads available with interchangeable inserts. These are a good idea, but offer no extra horsepower. It is just easier to alter the volume of the squish, and cheaper to do so, but they do not make any more power than the standard cylinder head.

The concept of the interchangeable cylinder heads has been brought into relevance with the introduction of unleaded fuel, where compressions need to be changed to suit the varying conditions to combat detonation when using unleaded fuel.

Crankcases:

The standard crankcases are mass-produced, and aren't the best in the world. We alter the volume of the crankcases slightly and gas flow them but don't carry out too much extensive modifications.
Sometimes the bearing housings can give problems, and not grip the bearings correctly, in which case we fit bronze inserts. Some crankcases can be quite porous, and after approximately 1000 miles tend to produce less power than a good set. This is because porous metal tends to make the crankcases not rigid enough and they tend to vibrate and lose power. The only answer is to replace the crankcases.

Reed Blocks:

There are many kinds of reed blocks available that fit the RS125. The standard block is quite adequate. It is not recommended to do reed block testing on the track, it is better to use a dyno for this. The types of reed block available vary in style, but very few work better than the standard one, and the same can be said for reed petals. Some reed blocks and petal types and thicknesses will work better on some bikes than others.

Ignition:

The ignitions on the later bikes are fairly sophisticated digital ignitions. The main factor is the ignition timing. We set the ignition timing to where the bike produces most horsepower. This would depend on the year of the ignition. The bikes are set up on the dyno individually. It is impossible to say "if your bike is a 1998 you need a certain amount of advance." It depends on factors such as how the bike has been tuned. It will require an ignition timing to suit that particular engine tune.

There are kit ignitions for sale on the market, most of which are now predominantly unleaded. We have not yet found that any of these B-Kit type CDI boxes work that well. They only alter the timing, which can be done manually to a standard ignition to get the same result. There are now, however, ignition wiring looms that enable you to switch the power jet off at slightly different times, and also now you can use them for switching the power jet off at different times in different gears.

This is related to the rate of acceleration-if you are accelerating quickly you need to switch the power jet off earlier than you would if you were accelerating slowly, so in the lower gears it would switch off early, in the higher gears it would switch off later. The reason for this is because when accelerating quickly and the power jet switches off it will take a considerable time for the mechanical power jet to actually switch off and then drain the tube that holds the fuel, so you would switch it off earlier to enable it to rev on. The reason we have an electronic power jet now is so that we can run quite a lot of ignition advance in the mid-range to burn a lot of fuel, and then as it retards at peak horsepower (which will make the engine run rich), we will switch off the power jet to lean the engine off, which enables it to rev.

At some tracks you would run more ignition advance than others. Quite often you may need less ignition advance at tracks with a long straightaway because you need the engine to rev freely at high rpm. Advancing an engine has a similar effect to raising the compression on an engine, and the relationship with the compression and the exhaust pipe and the port timing of the bike must be correct to the ignition setting on the bike.

Exhaust Pipes:

Exhaust pipes are a very complex subject. The standard exhaust pipe from 1997 onwards works very well. Several aftermarket exhaust pipes are available that work a little bit better. Again, it doesn't mean to say they will work with your bike, and the tuning that has been carried out on your bike. It is possible, with exhaust pipes, to alter the power delivery, alter the over-rev, alter the drive. Some people like to run a revvy pipe with power up top, and not a lot of drive so they can run high corner speed and high rpm, other people need a gruntier pipe so that they can drive out of the corners on the lower rpm. We manufacture exhaust pipes from mild steel, stainless steel, and titanium. It is possible to modify a standard pipe to alter the power characteristics. The shorter pipe gives more rpm, the longer pipe gives more mid-range power and more torque.