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Rotax Max Jetting

Correct carburetor jetting on the Rotax max engines is very important, using the wrong jetting will lead to poor engine performance and can also cause your engine to seize.

Carburetor Types

The Dellorto VHSB 34 carburettor is available in two models "VHSB 34 QD" and "VHSB 34 QS" fitted with either a 12.5 or 8.5 venturi insert.

Venturi Size

The general consensus is that the 12.5 venturi type should be used on tracks with long straights and the 8.5 venturi for tighter tracks with more corners. The 12.5 venturi gives more power at the mid to high RPM range, the 8.5 venturi provides more power at lower RPM helping you pull out of the corners.

Changing the venturi insert on your carburettor is quite a simple job but to save time at the track it's recommended to have two carburettors each one fitted with the different size venturi.

Jets & Floats

The rules of the rotax max class here in the UAE limit the size of main jet, needle jet, idle jet, choke jet and float weights that can be used with the Dellorto VHSB 34 carburettor.

Main Jetdellorto-main-jet

The minium size main jet allowed is:

158 - Rotax Max Junior & Senior

175 - Rotax Max DD2

Needle Jet

Rotax Max Junior & Senior - K98

Rotax Max DD2 - K98

Main Jet Selection Chart

Rotax max jetting chart

Using the above chart you can select the correct main jet using the current air pressure and temperature. A portable weather station can be used to get the exact tempreture and pressure at the track. We've also found a few Iphone and Android smartphone applications that can provide you with the current weather conditions.

Needle Jet Position

The clip on the K98 needle jet can be set in 5 different positions. Clip position 1 is the leanest position and clip position 5 the richest.

rotax k98 needle positions

Float Weight and Jet Combinations

The rules restrict the type of float that can be used with each size of carburettor venturi, the following float, jet venturi combinations must be used:

Rotax Max Junior, Senior & DD2
Rotax Max Junior, Senior & DD2
Venturi: 12.5
Floats: 5.2 gr
Idle Jet: 30
Idle Jet Insert: 30
Choke Jet: 60
Venturi: 8.5
Floats: 3.6 gr
Idle Jet: 60
Idle Jet Insert: 60
Choke Jet: 60

Float Height

Dellorto manufacture a range of different weight floats that are used to obtain the optimal carburetor jetting. Rotax rules only allow the 5.2 gram or 3.6 gram floats to used in the carburetor. Adjusting the float heights in the carburetor is a way of adjusting the fuel/air mixture in the carburettor while still using the 5.2 gram or 3.6 gram floats.

rotax float height toolAdjusting the float height involves bending the float arms to higher or lower the floats in the fuel bowl. A lower float level will weaken the fuel/air mixture making the engine run leaner, a higher float level will richen the fuel/air mixture. 

A specialist float height measuring tool is available and makes setting the float heights very easy. Getting the float heights right for your engine will improve the performance. Standard Rotax carburetors usually come from the dealer with a 6mm float height. A good starting point is 3mm float height.

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