UK Karting - Notice Board (2-Stroke Karting) : Re: Rotax 12.5 carb for minimax . Which floats

Notice Board (2-Stroke Karting)

Re: Rotax 12.5 carb for minimax . Which floats
Posted by 'rotaxjettingcom' on 25 Oct 2012 @ 15:01

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rotaxjettingcomJoined: October 2006
Total Posts: 5
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It helps to get a better understanding of what you are trying to achieve.
Any change in fuel level in your carburettors float chamber will change the static head of fuel on all of your carburettors fuel jetting circuits and therefore the higher the fuel level, then the richer will be your overall jetting and conversely the lower your fuel level, then the weaker will be your overall jetting.
So the heavier the float, then the richer will be the overall jetting and conversely the lighter the float then the weaker will be the overall jetting.

As you know, you can use either the 12.5 or 8.5 insert carburettor in the Rotax classes. The 12.5 or 8.5 actually refers to the size of two small holes in the venturi insert and has nothing to do with the venturi choke size (which is 34mm). The "12" in 12.5 means that the "progression hole" is of size "120" or more specifically 1.2mm and the "5" means that the idle hole is of size "50" or 0.5mm. So the only difference between the two types of Venturi insert is the size of one small hole, the progression port.
What is the difference between the two with regards to performance? The reason Rotax introduced the 8.5 insert was because the engine was getting a little too much fuel on initial throttle application, which caused a poor response. The 8.5 gives a bit less fuel on the initial throttle opening and this should give slightly better initial response. The size of the progression port has no effect on wide open throttle. In fact after about Ľ throttle is has no effect at all.

Don’t be misled in thinking that you will need to change the main jet purely because you are using a 12.5 or 8.5 insert. You change your carburettor setup to enable the correct fuel/air ratio to be maintained across the throttle range. Whilst you have a limited number of options in the Rotax classes, the correct setup is still entirely dependent on the air density (amount of oxygen in the air), the fuel/oil mixture you are using and how that is delivered by the carburettor. So it will be the insert type, the idle jet combination, the float weight/height and needle type selected that will dictate the main jet size and needle clip position required, to maintain the correct fuel/air ratio.

Message Thread:

Rotax 12.5 carb for minimax . Which floats by 'newminidriver' (23 Oct 2012 @ 22:43)

Re: Rotax 12.5 carb for minimax . Which floats by 'raceinstructor' (24 Oct 2012 @ 10:48)

Re: Rotax 12.5 carb for minimax . Which floats by 'newminidriver' (24 Oct 2012 @ 20:48)

Re: Rotax 12.5 carb for minimax . Which floats by 'Ditch' (24 Oct 2012 @ 21:09)

5.2's & 30/30's for sure BUT by 'ScoobyScene' (25 Oct 2012 @ 8:11)

Re: Rotax 12.5 carb for minimax . Which floats by 'rotaxjettingcom' << You are here!

Re: Rotax 12.5 carb for minimax . Which floats by 'newminidriver' (25 Oct 2012 @ 23:25)

Re: Rotax 12.5 carb for minimax . Which floats by 'PoppaAl' (26 Oct 2012 @ 8:17)

Re: jet sizes etc. by 'rotaxjettingcom' (26 Oct 2012 @ 9:25)

Re: jet sizes etc. by 'davidmc' (26 Oct 2012 @ 11:41)

Measure them silly ;-) by 'ScoobyScene' (26 Oct 2012 @ 11:56)

Re: jet sizes etc. by 'rotaxjettingcom' (26 Oct 2012 @ 14:58)

Colin - Jet size gauge? by 'rjbcar27' (26 Oct 2012 @ 13:25)

Re: Colin - Jet size gauge? by 'rotaxjettingcom' (26 Oct 2012 @ 15:16)

Re: Colin - Jet size gauge? by 'PoppaAl' (26 Oct 2012 @ 17:56)

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