Tech Tuesday: Rotax Max Jetting and carb setup

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If you have an EVO, you need to read our Rotax Max EVO article.

There are several different brands of software available like the Jet-Tech Max program that can really help save a lot of time when you go to the track. However the software needs to be calibrated to your engine and carb. That is when you need to have a strategy and become very familiar with the signs that indicate the correct rotax max jetting. These signs can be the plug colour, the dryness and colour of the piston crown and the exhaust header. If the main jet is correct, the plug will be light brown in colour and the piston crown will be dry with a dark brown hue. The engine should also be reaching maximum revs at the end of the longest straight. Using the base settings on a software such as the Jet-Tech Max program will get you close to the correct jet. It then takes several practice sessions to find the correct jet and calibrate the software to the engine.

To choose the correct setting for the needle takes a little effort from the driver who really needs to give good, consise feedback to the mechanic mechanic and needs to be objective and honest about how the engine is performing. It also requires  disicipline not to waste a valuable carb development session just to chase someone else around the track in the heat of battle, try and get yourself some space. As the needle has most influence between 1/4 and 3/4 throttle, the driver needs to focus on how the engine is performing out of the corners. It’s best to keep it simple, pick one corner on the track and get a feeling for how the engine pulls out of that corner. Getting the fastest exit on the best line is not the point of this test, it is to try and optimise the needle setting so the engine pulls strongly and cleanly down the straight.

Jetting affects the amount of fuel mixing with the air

It is a good idea to start rich by lifting the needle up to its highest position. Because the two stroke engine is not very efficient lower down in the revs due to the design of the pipe, the cylinder can be insufficiently scavenged which means a fresh charge of air and fuel is then mixed with exhaust gases. To help overcome this problem the engines typically run a lot of ignition advance to help get the air/fuel mixture burning a little earlier in this dirty environment. If you lift your needle up so you can richen the mixture and make best use of all that ignition advance you may well feel quite good gains in bottom end torque.

By repeating the procedure of going out and feeling how the engine pulls from 1/4 to 3/4 throttle, and then coming in and changing the needle position, the driver can get a good feel for what setting is best. The driver could also try accelerating from different throttle positions on the straight, say from 1/2 to 3/4 throttle, to help evaluate how each needle position effects the performance. To confirm what works the driver could then focus on his line through a portion of track and use split times to see what setting is quickest.

A short note on needles: the K27 and K98 needles have slightly different profiles. The K98 has a steeper taper which will have the effect of richening the mixture ever so slightly above 1/4 throttle. It would take a good driver to notice but the difference would become apparent on a dyno.

Clip Positions on the K98 Needle

You may ask why does anyone need jetting software if they can just go out and feel how the kart is performing? After all that is what every karter did before laptops were common at the track. The truth is that setting up a carb correctly can often be very time consuming and sometimes frustrating for everybody, not just the beginners. What jetting software does is act as a time saving instrument that once you have put the hard work into calibrating the software to your engine and carb allows you to go to the track, plug in the figures from a weather station to the computer and be very very close if not dead on the correct jetting immediately.

The beauty of a program like Jet-Tech Max is that it will take into account things like your choice of floats and idle jets and has features that allow you to easily fine tune to a high degree how close the software will be to the actual needs of the engine. One of these features is called Flow Bench which is a quite advanced adjustment that, according to the developers of the software, is best left alone until the driver and mechanic are quite experienced and confident with how well matched the software is to the engine, as it is a very fine adjustment. It is also possible to set the program up so that you have database settings for several different engines and carburettors, even if they are spread among the range of Rotax classes.

One other feature of the software that adds value is the sheer amount of supporting information included in the package, such as a wide range of chassis setup guides from the manufacturers, as well as driving tips, carb preparation guides written by the developers of the software and official Dellorto and Rotax guides to the engine and carburettor. If you can absorb all that valuable information and put it in to practice then you are well on the way to be a winner!

Alternatively, You can also use this chart to help with your jetting


Like this article? Then read more Tech Tuesday here:

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