Basic 125GP carburetor setup
The most argued about topic in 2 stroke racing has got to be carburetor setup. The complexity of it, combined with the numerous contradictions which float around pit lane, and the real risk of seizing an engine there's no wonder why so many shy away from 2 stroke racing. It really doesn't have to be a scary or difficult process though and you can have your bike run well without taking any unnecessary risks.
For this you need to know how your bike is set currently up, what to look for and some way to collect key data. I'm not going to touch on a number of really useful topics (which you should research further) like Relative Air Density (RAD), exhaust temperature or plug reading in detail but I will share some advice on how to get started and feel confident in what you're doing. This is all in the context of a Honda NX4 RS125 so please keep that in mind.
It concerns me how many 2 stroke racers don't have accurate data to make jetting decisions. Racers consistently breaking 2 strokes with poor decisions and complaining about them gives 2 strokes a bad name and does nothing for the sport! By no means is using a weather station and the jetting charts below the best way to set up a carburetor but it's a massive leap from taking a guess and will no doubt improve your chances of success.
Gather information to make a decision
On the right is a portable weather station. This one's got lots of information on it but you really only need to reliably and consistently measure ambient temperature and humidity. Use the same station every time and make sure the batteries are always properly charged to ensure consistent results. We need to know this information because it impacts how much fuel the engine needs for the oxygen it's consuming and therefore what size jets to use. There is more oxygen to burn when it's cold and again when it's less humid and vice versa. You can see how this plays out in the example chart below.
As part of your routine at the track, you should record temperature and humidity in your racing log before you go out into a session. It's important, especially when you've made changes to your bike setup and when you're getting to know your bike, that you establish a solid baseline to refer back to. You should also record as much data as you can along the way in case you need to diagnose a problem, or to use as reference when the conditions are similar.
Using a jetting chart to select a main jet
The links to the Fatbaq jetting chart below (link 2) is a simple matrix to get you started. As they're in Fahrenheit, I've done a rounded conversion to Celcius in link 1. There are different charts for different tracks because they're at different altitudes and altitude effects air density. Link 1 has all the New Zealand tracks already saved. If you go to the Fatbaq website you can create and save your own charts for the altitude of your local tracks.
When you begin racing your 125, you need to establish a baseline to work from then compare this with the jetting chart. I've found that the suggested main jet on these charts are accurate and safe to use as a starting point for my stock 1998 Honda NX4 RS125 (stock head insert, stock ignition, stock pipe, stock reeds, 100LL avgas, etc plus an airbox).
So put in the suggested main jet, then go out and ride a session. Pay attention to how the bike behaves through the RPM range, coming onto the throttle and the reading on your detention counter (if you have one). At the end of the session, do a plug chop so you can take a reading. Make any jetting changes based on how the bike felt to ride, det counts, and how the plug looks. Repeat this process while recording environmental data until you've got enough data to compare against the chart. For example, I've found that running 1 to 2 jet sizes up on what the chart suggests works for my bike at New Zealand tracks. This will be different for all bikes and likely be different in other parts of the world. Once you have that baseline, you can use the jetting chart and your knowledge of how to correct from the suggested jet to make changes or an educated estimate at the right main jet to use at the start of a day.
More than just a main jet
It's important to understand that you can end up chasing your tail if you never pay any attention to pilot jets, needles and power jets. There is an overlap between each circuit across the throttle and RPM range so they do influence each other. Don't get caught chasing the perfect main jet when you actually have a needle problem. My biggest suggestion here is to experiment, continuously check your data, your indicators (plug, piston crown and underside, EGT, det count etc) and develop a conscious awareness of how the engine responds to throttle input.
I hope this helps you get out there and enjoy 125GP life. It's the best.
1 - Click here for a set of printable jetting charts for New Zealand tracks
2 - Click here for link to the Fatbaq website and jetting calculator
Below is an example of the jetting chart for Ruapuna. See link above for a full set: