Can somebody explain to me how the float level affects air fuel ratio?

Josh Slade

Club Member
I've heard a lot of people say that if you float height is set incorrectly that your engine will run rich or lean, surely the air fuel is dictated by the mixture screw and what needle you have etc? and that as long as there is fuel in the bowl the engine will just draw as much fuel as it needs? does the float height have to be millimetre perfect?
 

jonbills

Membership Secretary
Site Administrator
the amount of fuel added to the air in an SU carburettor is determined by the shape of the needle, how far out of the jet the needle is and how high up the jet the fuel sits.
The fuel level in the jet is the fuel level in the float chamber. That's why it affects the air fuel ratio.
 
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Josh Slade

Club Member
the amount of fuel added to the air in an SU carburettor is determined by the shape of the needle, how far out of the jet the needle is and how high up the jet the fuel sits.
The fuel level in the jet is the fuel level in the float chamber. that's why it affects the air fuel ratio.

Thank you that makes perfect sense now!
 

johnymd

Club Member
Just to add to what Jon has said. There is an optimal height for the fuel to sit in the jet so that as the needle rises it will suck the required amount of fuel. If the level is too high then at very low needle heights it will suck big globs of fuel and run rich. If the fuel height is too low it will be harder to suck the fuel and probable run a bit lean initially. So, although the float level will affect the AFR as Jon says, you should not use this to adjust the fuelling. If you can get the float level just right then the car will generally run way better and start much easier as the carbs will be working as designed. Float level is the most critical part of these carbs operating well.
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
I stripped, cleaned and adjusted my carbs (including choke mechanism) last year and it improved starting.

re Float level - when dismantled I thought my carbs were a mix of two different models because the 'fulcrum posts' that the floats hinge from were different heights but on investigation this is normal. I have no idea why but it can be misleading when setting the float height.

Another thing that 'threw' me when having to strip a carb down in a layby once is that the float chamber lid will not clear the body of the carb until a hidden 10mm nut is removed to allow the float bowl to be slid sideways.
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