Rotax carburetor tips

Without a doubt the biggest source of headache for most Rotax owners is the carburetor. If you ask most of the Rotax racers, the Dellorto VHSB QD is shrouded in mystery. While the Dellorto can be finicky most Rotax owners are not familiar with the basics of the carburetor, which make tuning much easier.
The first thing we look at with every carburetor we look at is to check the float levels. This is a simple procedure that requires removing the carb from the motor. Once removed from the motor turn the carb upside down and remove the float bowl. Make note of how the plastic floats are positioned, so as to facilitate easy reassembly. Continuing with the carb upside down and level, observe the silver float prongs. These prongs should be absolutely parallel to the horizontal surface of the carburetor. If the prongs are not level, DO NOT BEND THEM! Simply bend the small tab that rests on the needle seat. Once you have adjusted the prongs, check the needle seat to be sure that it moves freely and springs very nicely.

Bogging problems that occur at lower RPM's and less than 5/8 throttle are typically resolved by adjusting the needle & clip to be leaner. This is done by relocating the clip to a higher position on the needle. Essentially, you have just dropped the needle further into the carburetor, which allows for less fuel and more air. Make only one or two position adjustments at a time or you may run the risk of causing a low RPM seizure.

Failure to reach high RPM's when the motor is under full throttle usually indicates a rich condition, provided that the issue is not electrical and assuming that the fuel system is clean and operating normally. This can be remedied by installing a leaner jet. Each jet has a number stamped into it, which correlates to its size. Larger # jets have a bigger orifice and therefore provide more fuel. Before installing a leaner jet, it is always a good idea to get a good plug reading and interpret that information first. If a loud popping noise occurs during the higher RPM's you may want to consider going with a richer jet.

Jetting is almost completely dependant on temperature, humidity, barometric pressure, and altitude. Taking good notes and creating a baseline of weather conditions can save you a ton of time when deciding on a jet.

Carb Cleaning

The Dell'Orto carburetor requires cleaning on a regular basis and failure to do so will certainly result in loss of power and possibly even motor damage. Special attention should be paid to the fuel filter, the atomizer or needle jet, the needle seat, and the other jets. The fuel filter is easy to get at and should be cleaned at every opportunity.

The main jet is also extremely easy to inspect and clean. Simply remove the float bowl nut and this allows access to the main jet. Be careful when removing/changing main jets so as not to lose the jet cup. If the jet cup is omitted, or is installed improperly you will typically experience an extreme abundance of fuel, and the motor will not run.

The atomizer can also cause carburetion problems if not cleaned periodically. There are several machined holes along the shaft of the atomizer that can be blocked or plugged with only one grain of sand. The same goes with the idle jet, idle jet insert, and the starting jet. We recommend cleaning all of the above parts with regular carburetor cleaner. The needle seat has a small rubber bushing, and as a result, we recommend using a mild soap solution to clean it because the carburetor cleaner can break down the rubber composition. Unless of course, you don't mind having to purchase new needle seats more often. Be sure to have clean work surface that is easy to find small parts if you drop them while working on the carburetor. One lost part is guaranteed to end your day at the track if you don't have a spare.

Rotax Fuel Pump

The fuel pump is essential in the proper supply of fuel to the carburetor. The Mikuni pump has (2) gaskets and a plastic membrane inside that should be cleaned regularly and inspected for irregularities. When in doubt about performance of the fuel pump, it is always a good decision to rebuild the pump using new gaskets and membrane. When connecting the fuel pump to the pulse tube on the motor, be sure to use the shortest piece of hose possible. Also be sure to use the stiffest, smallest diameter hose available as this will aid in creating the best vacuum/pulse line possible.