RS125 Race Tips: Fear Of Jetting

By John Ulrich

The most common question we've been asked at the racetrack since starting this series on RS125 tuning is, are those the real jetting numbers? The answer is, yes. But even after being told that, most people are reluctant to jet their bikes down for fear that the bike will seize. Most people run their bikes very rich.

At the April WSMC race at Willow Springs, I spoke to a rider who knew that correct jetting produced more horsepower and yet was reluctant to put in the recommended jet even though she could walk over and read Vivier's gauges and see that atmospheric conditions were virtually identical to those listed in the Willow Springs jetting example published in the May issue. Her bike was four main jet sizes too fat, and the most she could make herself do was split the difference between what she had and what we had recommended.

What will happen if a bike is jetted too lean? At one main jet too lean from perfect jetting, the bike will just make slightly less power. At two main jets too lean, the bike will detonate, pitting the piston and head. At three main jet sizes too lean, it will detonate enough to seize. Which means that it takes more than being a jet or two off to blow the thing up.

It’s possible to feel detonation through the handlebars; the bike will feel like it's stuttering slightly at steady throttle mid-corner and accelerating off the apex instead of running smoothly.

A tuner can check jetting by pulling the cylinder head after a practice session and looking at the top of the piston, a light-brown color being good, a black color being too rich, and a very light gray being lean. The inside of the tip of the exhaust pipe can show the same basic information--a very light tan, with just a wisp of gray, is ideal.

Jetting an RS125 would be considerably easier if air density gauges all read alike and the numbers shown by the gauges were absolute. Each 2.8 percent change in air density equals one main jet change. We've pictured two Kinsler air density gauges that give wildly different readings for the same conditions, the one used for our specs reading 100 percent when one owned by Joel Hermoso reads 89 percent. That's about four main jet sizes, and using readings off Hermoso's gauge with our jetting specs could easily lead to seizure.

It seems like there's a need for an official HRC gauge set built with typical Honda quality and accuracy. Baring that, an electrical engineer I recently spoke to says he could build accurate, all-read-the-same electronic gauge sets to measure air temperature, humidity and air density, and sell them for around $100. If he decides to develop a prototype and then go into production, we'll compare the readings to the gauges used for this series and publish the information.

In the meantime, if you're at a race we're at, compare your gauge readings with our gauges, and use the information to figure out your baseline jetting. Having said that, a word of warning: Don't come over and expect a friendly, extended conversation when we're in the middle of working on the bike or just before a race when we're getting our race face on. We take our racing seriously, and don't just go to the track for social interaction.

If you're not at a race we're at, start about four sizes fat on the main jet and see how the tip of your exhaust pipe looks after practice, then jet down accordingly. Keep track of air temperature, humidity and density, and log your air readings, jet changes and pipe or piston color readings. Before long you’ll be able to figure it out yourself and lose the fear of jetting!