Main air jets and what they do?

[Vloukole]

I bought new jets cause the old ones were the wrong sizes. Mains were 115 , pilots were 45 with stock airbox and exhaust

[steveb922]

Yep, sounds like some one fiddled. Mine runs 92.5 mains, and 40 pilots. I can't tell you what effect the new pilot air screws would have, because they have a different numbering system to Mikuni. Basically, a smaller jet would allow insufficient air at idle/low-range, whereas too large a jet would cause it to run lean.

[Vloukole]

Yes that is the sizes I bought. Lets say the stock size is 160 cause that is what the carbs had in and I change them to 180s. Will the bike run fine?

[steveb922]

Personally I would stick with the 160s. That's already two sizes up from what I have, but as HH mentioned, that is probably something to do with local regs.

[Vloukole]

Is yours 140s?

[steveb922]

No, 150, but if i recall correctly, the sequence goes 150, 155, 160 etc.

[Vloukole]

That is true. Jets R Us also lists them like that. What is the stock size then cause the manual says nothing about pilot air jets?

[steveb922]

Stock for UK and US is 150. http://members.dslextreme.com/users/...at_height.html

I wouldn't be overly concerned about having 160s - they may well be the stock size for your market.

[Vloukole]

Okay thank you. Will give them a go.

[JTGS850GL]

You keep referring to the "manual". What manual are you using? If you're not using the Factory Service Manual then I would recommend downloading and using it instead.

[geol]

Don't mean to add to the confusion but those are pilot air jets. On BS series carbs with Main air jets, there are smaller holes set back into wells cast on the intake flange below the main air jet wells. You question about how the bike would run with a slightly different size. It won't make much difference if you use a slightly different size. The air from these jets augments air that goes under the slide (aka cutaway) to give the pilot circuit some air while the bike is at idle (that is, the slides are all the way down as far as they will go). Small changes in the size of the pilot air jet won't affect operation much unless the slide is changed to one with a HUGE cutaway (mostly seen in VM carbs such as the first GS750s and then you need a MUCH larger pilot jet to compensate for the increased air which also slows airflow velocity through the cutaway at idle. On a BS carb, no one ever seemed to huge a very large cutwaway.

The effect of the cutaway and pilot air jet can be observed if you put a set of VM33s on a smaller displacement bike, like a GS750. In this case, the venturi itself is too large for the bike's displacement and you will need a far larger pilot jet than would be normal and cutting down on the pilot air jet (they can be swapped on a VM33 carb) is necessary and even then it will bog right off idle if you open the throttle quickly.

The main air jet is often not used as it augments airflow to the main circuit when the bike is at idle.

OK, all that said, going from 150 pilot air jets to 160 won't cause any difference that you will be able to detect.

[Vloukole]

I am using the Factory Manual.

Thanks for all the info.
Btw I will start to call the Pilot Air Jet a Pilot Air jet...