Air Jet on Primary Type Carb

[jradnich]

Not much is mentioned about the air jet in the Mikuni manual except that it helps atomize the fuel. So bigger jet=more air thus leaner mixture or bigger jet=more air helps pull more fuel thus richer mixture? And at what throttle openings does it have the most effect.

Second question. In the Yamaha manual listing carb specs for this engine I see 2 abbreviations I don't recognize BP and PO.

Yes I'm trying to sort out a carb. It's on a 73 MX360.

Thanks, John.

[teazer]

The answer is that the main air jet changes the mixture as revs rise, but with a Primary type needle jet, it doesn't have much, if any, effect because it just provides air to help break up the fuel into smaller droplets as it leaves the needle jet.  On a Bleed type, the air mixes in the needle jet, so a larger jet means a leaner mixture.

That's why they are typically a 2.0 or 2mm diameter to provide as much air as is needed, and in many carbs, the air jet is left out altogether.  On a Primary needle jet, the height of the spray bar or Primary choke as it's called plays the same role as an air jet in a bleed type.  Not so much because iof the air but because a tall choke sits closer to the center of the carb where air flow is faster so it has more effect.

Where are you seeing "BP" and "PO".  The needle jet is a 159 series I believe and it's size is P-0 which is 2.650mm diameter each letter increase equals 0.05mm and each number increase is 0.005mm so P-0 is 2.650, Q-0 is 2.700, Q-1 is 2.705, Q-2 is 2.710 and so on.  Could that "BP" actually be 159 by any chance, if you are looking at the needle jet?  That would be the "series" 159 is used in VM34 carbs and uses a large hex main jet. #188 is almost identical but is drilled and tapped to take Large Round main jets which flow differently so Mikuni gave them a different thread to stop us from mixing them up and using the wrong type of jets.

[jradnich]

Here's the chart where I see the abbreviation BP and PO.

And thank you for the info on air jets. I'm trying to get the engine running correctly. Stock jetting is rich (4 stroking) except at WOT. I have reduced the PJ from a 50 to a 45 and moved the needle clip one step leaner. It feels better (smoother) at steady throttle but has less power when I open the throttle. Stock jetting easily wheelies in 3rd gear. One step leaner nd it barely wheelies in 3rd. It's a MX360 with stock porting, factory chamber and silencer. Premix  32 to 1. Only changes are a Uni foam filter and Powerdynamo ignition (so I have a lighting circuit).

[teazer]

Interesting chart.
I suspect that PO = Pilot outlet ie the small drilling above the pilot jet. 
BP might be something to do with the pilot circuit so Bleed Pilot ie air jet size perhaps

The rest all translate OK

Mark: on the choke boss indicates the bike it was set up for
M.J: Main Jet
J.N.: Jet Needle
N.J.: Needle Jet size
C.A.: Slide cutaway
B.P:   
P.O: Pilot outlet
P.J.: Pilot Jet
A/S: Air screw turns from lightly seated.
V.S.: float Valve Seat diameter in mm
S.J.: Starter Jet, pressed into the float bowl