What needle do you use in a rotax max?

Answer 1

Usually, within a standard Dellorto carburettor (With a 12.5 or 8.5 Venturi) , for a FR125 Rotax Max engine, a K98 or K27 needle will be used.

The needles can be determined from each other due to the amount of tapering on it.

A K98 has very little tapering, which creates a leaner fuel/air mixture, which improves acceleration, however using a K98 needle has a negative effect on the higher rev range (around 12,000+ rpm), as often not enough fuel reaches the cylinder, and you get a "coughing" or "popping" effect.

On the other hand, using a K27 means that the needle has a much higher taper to it, causing more fuel to pass, which creates a richer fuel/air ratio, reducing acceleration, but improving high rev performance.

Therefore we can conclude that the K27 needle is preferable at higher speed tracks, whilst lower speed tracks will be best suited by a K98 needle.

Answer 2

Assuming you have a standard Dellorto carburettor (8.5 or 12.5 Venturi) a K27 or K98 needle is used, and the difference between these is the taper at the end.

Due to the K27 being smaller at the bottom, due to it tapering more, it allows more fuel through, therefore leaving you with a richer fuel/air mixture, however this does affect acceleration, whilst improving higher engine rev performance.

The K98 is the opposite, it tapers very little, therefore making the fuel/air mixture leaner, increasing acceleration, this does affect top end speed though, and could potentially be a cause of "coughing" or "popping" at high revs, in which the engine looses power.

Therefore, for higher speed tracks, a K27 needle is adviseable, whilst a K98 needle is preferable on tighter tracks.