Jetting at the Track

This discussion is focused on Rotax-Max karting, but is also be relevant to other kart classes and some motorcycles.

How to jet your carburettor at the track

The process of selecting the best jetting consists of the following steps:

1. The float level is set.
2. The idle is adjusted.
3. The main jet is selected.
4. The pin position is chosen.

Settings the float level

The best way to select the float height is to use Prometheus. It may take considerable testing to find a configuration that optimises both main jet, clip position and float height. If you do not have Prometheus and cannot perform the required testing, set the float at the height recommended by your engine builder. Experiment with the float height after performing the other steps.

Adjusting the idle

The idle adjustment is described in detail in the Dellorto documentation [s3.4 Dellorto Motorcycle Tuning Manual] as follows.

In the UK, noise restrictions means that it is not possible to run a kart at a track for more than 10 seconds - not enough to warm up the kart and adjust the idle screws. On race day it makes sense to run the kart in practice and adjust the idle immediately afterwards. It also makes sense to mark the air and fuel screws so these can be adjusted without needing to actually run the engine.

Selecting the main jet.

The main jet is selected to optimise AFR at high RPM. This can be done a number of ways:

1. Listening for popping on the straight and going richer by one jet.
   
   
2. GPS
   

   GPS is great if you can afford it. Some of the tools display the calculated engine power at different RPMs. You can then select the best jet by finding the one that creates the best top-end power.

3. Experience
   

   If you have been to the same track literally hundreds of times, which is the case for some team managers, then you should be able to rely on your past experience to select jets. Saying that, even karting professionals do not always get it right.

   You should always record as much information as you can from your track session, including at least the temperature, pressure, humidity, main jet, pin position, idle jet (and float weight), float level and notes on the engine performance (e.g. too lean - pops at end of straight)

   You can record session information into Prometheus.

4. Reading the plug.

   This method takes some skill. The plug only reflects the engine conditions over the last lap or so. If the drivers slows too much on the slow down lap, the plug will be misleading. The plug appearance is also affected by the choice of fuel; some fuels make plug reading very difficult.

Selecting the pin configuration

Again there are a number of methods of establishing the correct pin position.

1. Driver feel.
   

   Selecting the pin position by driver feel is more difficult than selecting the main jet. There is no clear indicator, such as engine popping, that signals the AFR is right or wrong. In general, if the AFR is not right then the kart will simply be slow out of corners, or the engine will hesitates in some corners. If your driver is not experienced or sure of himself, then any slowness can be put down to other factors.

   Be confident and trust in the driver.

2. GPS

   Select the pin position that generates the best low and mid-range power.

3. Experience

   Most teams rely on past experience when choosing the pin configuration, in combination with driver feel and observation. Saying that, there a lots of "rules of thumb" in use out there that simply don't work. E.g. "Add a bit more fuel" to make an engine go faster.

Jetting you carburettor using Prometheus

Prometheus removes the trial and error associated with popping or GPS. You can predict in advance what the jetting should be. If the weather conditions change between heats, you can determine exactly how you should change your jetting.

No Royal Road

There is no royal road to jetting.

Prometheus, and similar software, cannot predict jetting with accuracy without calibrating the software to your engine. That means that initially you still have to go through the trial and error process of selecting the jetting without Prometheus. However by recording your results in Prometheus you are able to use your experience quickly and efficiently to move to the next step: accurate prediction.

Other things to you should do

To maximise the accuracy of the Prometheus jetting recommendations, you should:

• Always use the same fuel.
• Always use the same oil.
• Always keep the exhaust matting and air cleaner gauze fresh.

The effect of idle jets.

The 30/30 idle jets provide a wide cover of atmospheric conditions, however under some circumstances it is possible that the configuration cannot provide the necessary fuel flow. In this case, the 60/60 idle jets provides an alternative and Prometheus will issue a warning.

Copyright (c) Ventimore Ltd (UK), 2011, 2012.