Technical Junior Max Evo Jetting

PB28

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Name
Paul
Hi, after some advice with Junior Max Evo carb setup / jetting / needle etc. I watched the power republic video on you tube but none the wiser. I am fine with how to adjust the carb / jetting / needle its the process of where to start in terms of main Jet and how to assess if that is optimal whilst at the track. I am mechanicing for my son so its what questions to ask him and what to look for in the data. If I turned up on a cold March day at PFI ie 6 Celsius what Main Jet should I start with, if its raining does that make things different? What Main jet should I start with if temp starts to pick up as we move into the spring and summer ie 15c or 25c. I look at the Rotax Jet App and the suggested main jet for Junior Evo seem higher than I am expecting so havent been using it but am I wrong to do so? Appreciate any help you can give cheers
 
Following as I'd be interested to know these sorts of things too
 
Hi.
The jet app is not the way to go imo. Just as you say it most often give you a way to rich jet.
Below is what I use as a start, this is my personal settings on our junior and may not be correct for everyone.
15c or more 122 25c or more 120/118
15c or below 125
If it rains there´s often a good idea to go on the more lean side because the engine does not need as much fuel. I tend to drop the needle down in rain to limit the fuel during cornering.
We´re going senior know, so probably the jetting will be a little richer for us this year.
 
Last edited:
Thanks for the feedback, appreciate it
 
Carb jet settings are individual and can change over time for a specific setup. The best way to know is to have it on a dyno. Or install a lambda sensor as it is permitted in some classes in the UK.
The Rotax jetting app is not accurate as mentioned above and it gives a richer set up. However it is a good way of knowing when to go richer or leaner, if you have established personal settings.
I used to play it by ear, listen to the engine reving and look at the spark plug colour. However the colour changes quickly during slow drive in the pits.
 
Thanks, like the point you make about using the app for a different reason.

Do people on here mess around with the clip position on the needle? When would you do that? Do you select the main jet first then adjust using the needle clip position?
 
Thanks, like the point you make about using the app for a different reason.

Do people on here mess around with the clip position on the needle? When would you do that? Do you select the main jet first then adjust using the needle clip position?
Yes, I normally start with the main jet, and from there adjust the needle position and air screw. Needle position does effect the acceleration after corner exit, if the engine starts to 4stroke after cornering you need to drop the needle for less fuel. Again, I´m not an expert, but this works for us.
 

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