Advice

• Jetting at the Track
• Driving Basics

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.

Driving Basics

When it comes to driving, there is no subsutitution for experience. Expect a child to take at least 12 months to become competitive, and longer for younger children. It is very easy for adults to expect unrealistic progress (I was guilty of this). Let a child drive around following other drivers. A child can be expected to know that there is only one fast line on the circuit and it is worth pointing out to the driver if he is not taking this line, however even this is should be left until the driver has some experience and is confident handling the kart.

A driver should attempt to be a master of his craft. The following driving basics should become second nature.

Braking - In Progress

Brakes are the "enemy of speed". New drivers have to learn to brake HARD. They have a tendency to brake gently to avoid any chance of losing control, especially if they are not braking in a straight line. Many are uncertain about how deep they can brake. There is only one way to find out (until the driver gets enough experience to just "know"): keep braking later and later until it clearly too late - then use the last braking point that "worked".

Not that if a driver overshoots a corner, he should not try to force the kart back to hit the apex. Fighting the kart can lead to loss of control and will result in lost time. (Even F1 drivers sometimes make this mistake)

How hard is braking too hard? Ideally under brakes, the tyre should be about to or make chirping noises under brakes. If the kart begins to slid, not only the driver have limited control, the braking distance is increased. Accidents and lost time are the results.

Learning to brake is a frustrating time for drivers (and parents). An indicator that a driver is braking too gently or early is a broken rear bumpers, which can be very expensive. Fortunately this is usually a phase that disappears once the driver becomes more experienced.

Cornering

Right-angle bends

Tyres can generally only do one thing - brake, corner or accelarate.

Hairpings and U-bends

When does the driver put on the power?

The Racing Line

There is no single racing line - it varies with the type of kart. In particular, it varies with tyre stickyness and power. The diagram below shows the racing line for Mini-max and Junior Rotax over the bridge at PFI.

The Mini-max line is far over to the left so as to take the widest possible line through the right-hand exit over the bridge and get the maximum speed down the hill.

The Junior line is approximately 2/3 of a meter towards the centre of the track. Junior karts have sufficient power that they can reach maximum cornering speed through the right-hander over the bridge (especially since it has a nasty bump on the apex) without going all the way over to the left. The more central racing line allows the kart to maintain a higher entry speed into the corner before the bridge.

The line is very precise. Observation shows that if a kart is being followed and it drifts as much as 1/3 metre to far to the right going across the bridge, the probability of being overtaken at the end of the straight going down the hill is very high.

The exact racing line is not always obvious and it is for this reason that drivers should study the the line taken by other karts and attend any track walks. A good driver with knowledge of particular circuits can also be very helpful.

Overtaking

The idea when overtaking is (usually) not to get in front of the kart; it is to get along side the kart when braking into a corner. See below. The cart on the inside of the corner can then "claim" the corner on the exit, the front of the other kart naturally falling behind the back of the overtaking kart.

A kart can defend by taking an tighter line approaching the corner.

Wet Weather

Driving in the wet is totally different to driving in the wet.

Don't wear tinted visas on your helmet - it can make it difficult to see where the track is drying out and therefore where the drip is.