Mikuni VM38 Air Jet Size

[Stevo17]

Hi, I have a new VM38 carb on my 84 KX250 (with removable air jet) to replace the OEM VM38 carb (which has a fixed air jet). Does anyone know what size air jet I should be running in the new carb as I cannot find a size for the OEM carbs air jet. The standard VM38 air jet is 0.5 and the idle mixture screw makes bugger all difference.   The bike runs sweet and crisp in mid and top end. Any advice would be most welcome. thanks steve

[Lozza]

Steve do you mean the air corrector jet located at the bottom of the front/airbox/bellmouth side of the carby?

[Stevo17]

Hi Lozza, I am no carby expert so please bear with me - the jet that I need a size on is located at the bottom of the bellmouth and mixes air with the needle jet. The Mikuni manual just calls it an air jet ??

[Lozza]

Yeah that's Japlish for air corrector. As air velocity rises in the throat more fuel is dragged up through the needle jet, if there was no air corrector, the mixture would go ever richer with rpm(the downfall of Lectron/Ei carbs). Enter the corrector which adds air in proportion to the amount of air going into the carb.So if your having low end rpm problems the air corrctor isn't the place to look. It's the slide cutaway and idle mixture screw that give air to the low speed circuit.
There is no real 'correct' jet for this just remember the smaller the jet the richer the mixture(taking away air), as I have a VM36 that had no corrector jet and has been drilled out a bit. If it is running well then that's the correct jet.
Has the engine got low speed running problems?

[Freakshow]

if in doubt you can leave it out.
all the ones i have seem are usually stamped #2.0 anyway if thats any help

[oldyzman]

Yep, Ive been through this discussion a few times> i have heard people say if its a 250cc or over then leave it out. It seems that the size stamped on it reflects the hole size in mm diameter. case ive measured a 0.5 and  2.0...
Brett