Bike Carbs

[richard mackay]

I've just fitted a set of cbr 600 carbs on my 2.0 black top.My.question is,

what size of jets will be fitted as they are right now and what size do i need

for a standard engine.

[Guest nippynick]

To be certain of what size you already have you will need to strip the jets out and read off the numbers. As to what size you need will depend on how the engine performs and that can only be found out by testing it. There maybe some standard settings that others may have found "suitable" and this would be a good place to start, but you are only going to get it perfect by checking it out with tuning equipment to ascertain if its too rich or weak as the revs increase and the various jets take control of the mixture. Other factors will be choice of air filter or not, size of bell mouths or not.

 

It may help your enquiry if you could give some serial numbers of the carbs and also the exact model CBR they came from..

 

When you need to purchase your jets Allens are pretty good and seem to stock most types and sizes without breaking the bank.

 

www.allensperformance.co.uk/

Edited July 27, 2014 by nippynick

[knights_templar]

jet size now will as much depend on the year of the cbr600 as the type of bike, most accurate way is to strip carb and make a note of the numbers on each jet, also needle size and position, if adjustable, and the compensator tube.

 

As to what jets you need? opening main to 1.6 will get you running but the ONLY way to get it right is to take it to a rolling road with experience of bike carbs.

[richard mackay]

Cheers guys,i'll give danst a phone cause thats where i got carbs manifold etc

 

[Guest nippynick]

Oh and one more very important adjustment make sure all the float levels are set correctly this is best done with petrol flowing through the needle and seat whilst you adjust the float height. messy but very worthwhile as an overfull carb can give the effect of an over rich mixture.

[Guest davidimurray]

Don't forget - it is not just about the main jet. The main jet only sets fuelling at wide open throttle - the needle is just as, if not more important.

 

It is worth bearing in mind the different circuits :-

idle

Pilot

Needle

Main

 

Pilot gives fuel all the time and the idle screw basically adds/removes fuel as a trim.

With the throttle shut, the diaphragm sits at the bottom with the needle down into the nozzle in the carb body. Fuel sits in the carb upto the float level and when there is sufficient vacuum is sucked through the main jet, and through the gap between the needle and the hole.

When you accelerate, the diaphragm rises lifts the needle (which tapers) and the hole gets bigger so more fuel comes out.

When the diapragm is fully at the top the hole the needle creates is larger than the main jet, so the main jet size now becomes the limiting factor.