Basic Rotax Max Tuning Tips

When dealing with the Rotax FR125, there are three basic areas that need to be understood and paid special attention to. Those that take the extra time to perform the duties listed in these areas will be rewarded with a lot less downtime and fewer hours of frustration at the track.

Cleaning
The cleaner you keep everything on your Rotax, the easier it will be to diagnose problems and rectify them. A clean Rotax (or any kart for that matter), is a happy Rotax. Keep in mind you want to use appropriate cleaners for certain parts though. For instance, the power valve bellows is made of a pliable rubber. The condition of the rubber itself plays an important part in the proper operation of the valve. Cleaning the bellow with an aggressive cleaner, such as Simple Green, could weaken the rubber and cause it to "blow off" the piston a lot easier. Thus, we recommend using only mild soap and water for cleaning the bellows. When cleaning the carburetor parts, i.e., jets, floats, filters, etc., you should use a carburetor cleaner. To clean any brake component you should use a good quality, fast evaporating brake cleaner. All of these items are really common sense, but most of the time, problems could have been prevented with proper cleaning to begin with.

Carburetor
With the Rotax package the biggest issue people have is normally with the carburetor and the 'mystery' involved with adjusting it. Because the system uses a "fixed" jet system, jet changes are necessary on a daily basis (sometimes hourly) and in a lot of cases those changes are critical to the proper operation of the motor. We will address that in a little more detail further on, but the big thing to remember here is still, keep it clean. Many of my customers complain of a bog during turns, or after letting off the throttle. The float will operate the needle & seat and if incorrectly adjusted, can cause a seemingly untraceable bog. To correctly set your float levels do the following:

You must remove the carburetor from the motor to gain access to the floats.
- Turn the carburetor upside down and remove the float bowl. Make a note of how the floats (white plastic pieces) are positioned in the float bowl cavity.
-Continuing to hold the carburetor upside down, examine the float lever.
- The prongs of the float lever should be parallel with the base of the carburetor. If the prongs are not level, DO NOT BEND THEM!!! You adjust the prongs by bending the small flap that sits on top of the needle and seat. Bend this flap until the float prongs are absolutely level.

Now that you have adjusted the floats to their proper position, we need to set the clip appropriately. To begin, set the slide needle at its richest point. This is done by removing the small 'e' clip and setting it at the lowest groove on the slide needle.
NOTE: Visualize this: As you move the clip lower on the slide needle, it raises the height that the needle sits at in its resting position. This allows more fuel to enter the carburetor, thus it is making the system 'richer'. Conversely, if you were to raise the clip to a higher groove, this would allow less fuel to enter and thereby create a 'leaner' condition.
Next you will need to install the proper Main Jet for the current conditions. Remember that the proper main jet can only be determined by knowing all of the following conditions:
- Temperature
- Humidity
- Barometric Pressure
-Altitude

Once the Main Jet is installed and the carburetor reassembled, you will be ready to install it back on the motor. Be sure to tighten the hose clamps and that all of the bolts are tight as well.
Take the kart out on the track and observe proper 'warm-up' procedures. Once the engine is warmed up, begin pushing it to its limits.

LOW RPM TUNING - If the kart runs rough at lower RPM's (5,500 - 8,500), we need to adjust the needle setting to be 'leaner'. How 'rough' it is running should determine this adjustment. Never adjust more than (2) grooves in any direction, this can cause a low RPM seizure.

HIGH RPM TUNING - If the motor will not rev cleanly from 10,000 RPM through to 12,500 then change the main jet to ONE jet lower. For example, if you are running a 168 jet and the motor pulls well through 11,500 and begins to break up or bog at 12,000 you should change to a 165 main jet. If the leaner jet seems to help, but still does not feel 100% right, consult your plug reading to determine if a leaner jet is suitable. BE VERY CAREFUL. At this point, going too lean could cause an engine seizure. Once you have found the appropriate main jet, it is possible you may have to change the clip setting to eliminate any lower RPM bog.

If the 'leaner' jet does not remedy the problem, you should install a jet that is one size bigger (richer).
The indications of a jet that is too lean can include a loud 'popping,' or a sudden drop off in performance at high RPM. Any time the jet seems too lean you should consider immediately coming in to the pits, getting a good 'cut' on the plug and reading your plug indicators.

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