what do they mean main air jet 1.0 - 2.5

[trailie]

Hi I'm restoring a little 125 but I've no knowledge of these bikes. This one is from 1984 and has matching 10W frame and engine numbers. The cdi unit is marked 10W-10 but the carb is stamped 12A00. Is it possible it came like this or has the wrong carburettor been put on at some point?

Looking at the haynes book at the differences between the two carbs, the main air jet is listed as 1.0 for the 10W and 2.5 for the 12A.

What are they refering to? It's obviously not the main jet size. Is it an internal dimension which can't be changed?

[mellow]

I think the main air jet is located in the rear end of carb where airbox rubber goes on.Its a small round thing at the bottom of carb.
If i remember correctly though my 125 one is blanked off,as are most.It works with the air screw i think
Or i could be completely wrong

Have a read of the carb articles in the Technical section m8

[trailie]

Thanks, I've modified this post to try to make more sense of it all. I think you're right when you say it's at the back of the carb. Looking a bit more carefully at mine, and blowing through it, I see there's an air passage from the side of the carb body direct to the needle jet. See pictures:






At first I thought the air jet was behind the big plug, metering this flow, but I now feel this airway is just a vent.

Is the main air jet behind the small plug ?


Initially my thought was that since the 10W and 12A share the same carb body I could effectively make one into the other simply by changing both the needle jet and main jet. But this won't work because I can't change the main air jet to suit.

Does that sound about right?

Anyway it's too late now this evening, i'll look at it again tomorrow and try to suss it out.

[lovelydog]

Good pictures mate,better than Haynes manual. Someone on here will definately be able to point you in the right direction regarding this issue,sorry I'm no help,but great pics anyhow.

[trailie]

Thanks, but now I definately am going to bed, m' brains fried !!

[trailie]

Well I couldn't let it rest until I had it figured out. I found a Mikuni VM manual and it describes the function of the main air jet. Here's a couple of extracts.






So it's purpose is to meter the flow of air to the needle jet and aid atomization of the fuel, therefore it must be suitably matched to the needle jet your using.
It's location would be behind the larger of the two brass plugs. My only concern about this is, the air hole, being angled upwards, is a perfect place for dirt and grime to collect. If it gets blocked up then this important part of the main circuit won't function correctly. They could have cast the body a better shape to protect from ingress of dirt.



There is in some Mikuni carbs (probably others) a pilot air jet, doing a kind of similar job in the idle circuit. It's location would be behind the smaller plug. But I don't think it's a part of this particular body.

I hope I've got this right.

BTW here's a link to the manual
www.mikuni.com/pdf/vmmanual.pdf

Ralph

[Deleted]

yes the air jet is behind that brass ball bearing and is fixed permanently, there should be a white plastic 90degree elbow with a tube that plugs into where you red arrow is pointing (air rushes in here),(trying to get a picture but might have to photo one). it cannot be changed internally but then again does not have to unless you are modifing the carb with a ledar kit (see special notes below).....



oh and i can send you a pdf parts fishe of the entire bike with part numbers if you want, pm me your Email for it ok

[trailie]

Thanks, that's more good information. Funny enough I saw a drawing of the carb with that plastic elbow on. It's obviously been missing for a long time on this bike, i'll try to find something in work that would do in it's place. I now know enough about it to put it back on the bike.
Thanks for the pdf, will be in touch soon.