What's the best way to make a modern two stroke faster? "Don't mess with it," says renowned tuner John Lassak, who prepared Chris D'Aluisio's AF1 Aprilla in 1994. "Really, these modern racers come almost-complete from the factory, and it's very easy to make them slower. For instance, I don't move port heights unless I get instructions from the factory. These days, power is being made with 16-bit digital ignitions and expansion chamber designs, not port heights, so don't go in there with a grinder."
So how do professional tuners find more horsepower? "It's all in the jetting, that's where the power is."
Fine-tuning a two-stroke's jetting begins with the full-throttle carburetor circuit. To find out if you've got the proper mainjet (the lowest-sitting jet you see after removing the float bowl), you need to do a 'plug chop,' which entails running your two-stroke wide open in top gear for as long as possible, then, with the throttle held fully open, hitting the kill switch, pulling in the clutch and coasting to a stop. This leaves the spark plugs colored from the high-speed run (if you shut the throttle before killing the motor, or idle around, you won't get a full-throttle mainjet reading).
"Plug color, read of the inside, center electrode, depends on your fuel," says Lassak, "but most bikes run best on top with a plug that is really light tan, near-white color. Darker than that, and you're probably too rich, and if the plug is 'death' white, you're too lean."
From that point, it's a matter of seat-of-the-pants tuning: "If you get up to 12,000 or 12,500 and it just stops, that's normally a sign that it is too rich. A lean sign is when the bike will really rev everywhere but not make any power."
Comparatively, tuning the midrange is much harder. "Guy's will come up to me and say 'boy, look at the bottom of the electrode, can't you see where the midrange is too rich?' I don't even know what they're talking about! I've never had any luck reading electrodes to tune the midrange. The rider has to be the one that tells you what the bike is doing in the midrange.
"The rider has to be the one who tells you what the bike is doing in the midrange. A symbol of being rich is, when coming off the corner, you crack the throttle and it hesitates for a second and then hits like a light switch. And a lot of times when it's lean, it will kind of kick and buck and not go anywhere.
"To adjust the midrange, I lean down the nozzle--commonly known as the needle jet, which the clipped needle sits in--instead of moving the needle's clip up and down. If you change that clip, you're changing a broader range of carburetion than you do if you change the nozzle jet, so I usually leave the clip on the suggested setting from the factory. If it is too lean or rich in the midrange, I change the nozzle, which doesn't effect the mainjet very much."
A word of caution: It's critical not to be too lean in the midrange because it is much easier to seize a modern two-stroke in the midrange than on the top end--especially if there is a corner where you're at half throttle for a long time.
And what about those nasty rumors that say the bikes will blow up if not blueprinted: "That's not really true anymore" says Lassak. "Sure, you hear stories of parts being left out in the engine, such as clips left out of the transmission, but that hasn't happened in a long time. They'll run right out of the box--but you do need to tear them down for inspection."
There are a few simple things that should be checked. For instance, the engine cases are frequently put together at the factories with too much silicon sealant, which can block oil holes. It's best to clean out the cases and use a light, thin sealant, not necessarily a silicon-based one. "Also," adds Lassak, "check the cylinders for chunks of Nikasil coating, which can break loose and ruin the motor. Usually this happens in the transfer ports, so gently knock it out and smooth the area.
"Now that the motor is apart, there are a few things you can do to gain power. First, I usually clean up the transfer ports to remove any high spots. Next, set the cylinders on the upper case half and look to see if there are any transfer port mismatches, which you can see and feel as lips that hang over the mating surfaces. If so, either fill the area in with Devcon F Epoxy or grind them down to match, or both.
"Also, you don't know how long the bike sat in Japan, so the crank seals might have gotten hard. Replace the outer seals to ensure that no air leaks into the motor--this will cause a seizure. After that, you're pretty much ready to go. The transmissions come ready to go, and should last the life of the motor."