RM125B carb mod - your thoughts?

[John Orchard]

I just pulled the carb apart on an old RM125B that I have aquired, I find there is a piece of red plastic tube, what looks like WD40 spray-can nozzle extenion tube, about 10mm long shoved into the main air bleed, with what looks to be aprox a 1mm ID.  The main jet in the carb is a 150 (stock is a 230).

This change will give a leaner mixture at full throttle/low rpm and I suspect a close to correct (read-original) mixture at full throttle/high rpm?

So, I am thinking that MAYBE the person that did this has an understanding of fuel slopes and the effects of the main air bleed verses the main jet and has actually made a calculated modification  BUT I doubt that there are many people in this world that actually know the effects of the MAJ and maybe they just shoved this bit of plastic tube in when the stock MAJ has vibrated out and has had to go down to a smaller main just to get it to run?

What say you chaps?

[Tahitian_Red]

Is that the Mikuni Power Jet conversion?

http://www.ebay.com/itm/New-Mikuni-Power-Jet-Kit-VM-and-TM-30-44mm-Carbs-/370400475356?hash=item563d96b8dc&item=370400475356&pt=Motorcycles_Parts_Accessories&vxp=mtr

[Lozza]

If the tube was placed in the air corrector (hole at the bottom of the bell mouth) it counters the natural richening of the main jet as rpm increases. The air corrector is a size jet , so of stock jet is 50(0.50mm) and now has a 100(1.00mm) it effectively has leaned the top end off a fair bit.