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AFR + Fuel Tuning

Two Stroke Snowmobile Characteristics and Setting the AFR+ for the PRO RMK

Tuning a two-stroke snowmobile is a challenging and rewarding process. The lack of two-stroke SAE documentation specifying AFR values for optimal horsepower and torque means that determining the best settings for an AFR+ required more effort from the end user versus a four-stroke. This being said, tuning a snowmobile for optimum performance is undeniably satisfying.

General Characteristics of Two-Strokes

When tuning two-stroke snowmobiles it is important to keep some general characteristics in mind. These items fall into two categories.
  1. Fueling Needs: The fueling needs of two-stroke motors typically vary significantly from four-strokes. In general, two strokes run more lean (an AFR between 13 and 14.5 should be targeted) vs. four-strokes. There is also less variance in optimal targeted AFR values through-out the power curve. For instance, a targeted AFR of 13 may work perfectly fine for both acceleration and wide open throttle.

  2. Temperament: Two-stroke snowmobiles are stereotypically temperamental. Each snowmobile should be treated as a special case and may require specific knowledge and development of testing and tuning procedures. Fueling needs can vary greatly from sled-to-sled based on numerous factors.

Tuning a Polaris PRO RMK

This guide focuses on setting the AFR+ for the Polaris PRO RMK. The general guidelines associated with tuning a PRO RMK can apply to all snowmobile applications. However, as stated above, every snowmobile is different and requires special attention.

Before attempting to set the AFR+ for a PRO RMK it is important to have a good understanding of AFR values and the functionality of the AFR+. To brush up on your knowledge review the following videos.

Understanding AFR Values
AFR+ Adjustment Modes

The steps and resources provided below will ensure you are on the right track towards achieving optimal fueling with an AFR+. It should be noted that these recommended AFR+ settings only apply to a perfectly functioning snowmobile.

To begin setting the AFR+ start with these settings:
  • Green Fueling Mode:
       (1) Warm the sled to proper operating temperature.
       (2) Adjust the green fueling mode to where the sled sounds the best just before clutch engagement
  • Yellow Fueling Mode: Set at an AFR of 14
  • Red Fueling Mode: Set at an AFR of 13
  • Green/Blue Mode: The setting of this mode should be determined by the user based on riding style
  • Yellow/Blue Mode: Set between 5,000 and 6,000 RPM
  • Red/Blue Mode: Set between 6500 and 7200 RPM
It is useful to understand that the settings for the AFR+ switch point modes (yellow/blue and red/blue) should follow the exhaust valve positions of the PRO RMK. This rule holds true for other sled models as well. Information on exhaust valve position can be found in the service manual for your snowmobile.

The above settings SHOULD have a PRO RMK running close to optimal. After these settings have been entered ride the sled and take note of any potential areas in the power curve that could be improved. If the sled is running poorly it can be assumed that there is a factor affecting the optimal functioning of the AFR+ in relation to the specific sled. If the sled is running well it is time to do some testing to fine tune your AFR+.

Tests Procedures for Fine Tuning the AFR+

  1. Warm sled to standard operating temperature
  2. Based on the initial test ride decide which part of the power curve you want to analyze
  3. Accelerate the snowmobile until the area of the power curve you are testing is reached then turn the sled off
  4. Proceed with one or more of the following tests

Spark Plug Test: For two stroke motors a spark plug test is a quick way to determine how the PRO RMK is generally running. While a spark plug test for a four stroke motor will not give you much information the existence of residue from oil bi-products that remain on a two-stroke spark plug will provide some indication of how the snowmobile is running. Do your research if you are unfamiliar with how to conduct a spark plug test.

Piston Wash Test: A piston wash test should be initiated if you do not trust the accuracy of the spark plug test. A bore scope and knowledge of piston wash testing is required. Again, be sure to familiarize yourself with piston wash indicators before testing.

EGT Test: Exhaust temperature is another reliable test that should be performed if the proper equipment is available.

If you are happy with the data you have collected change the settings on the AFR+ accordingly. Repeat the above procedure until you are satisfied with the performance of your PRO RMK through-out the entire power curve.

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