I have an odd question, I am curious about the air jet specs on the early rt1 models, and the dt models too.  There is no spec listed for the early models, except for the early dt1's having a 0.5 listed.  Otherwise they had a dash listed up until 72, where the dt got drill 2.5, and the rt 2.0.  Did they block off the air bleed entirely in those years?  I don't have my original carburetor anymore for my rt1b, so I'm curious.  Tuning is favoring rich currently wot at high rpm, and I'm curious if it has to do with the air jet being open?  when it should be closed or restricted?

If I confused anyone, or can elaborate further, please let me know!

I've never heard of or seen a blanked air jet on a Mikuni. 

Specs I've found show DT1 models had 0.5mm air jet and RT1 models had a 2.5mm air jet. 

But you don't have a stock carb?  Many larger Yamaha models had a needle jet with a longer or shorter shroud sticking into the airflow.  Shorter shroud runs leaner at high air velocity (high rpm and full throttle) than the needle jet with a taller shroud.  The shroud makes a significant difference.  Air jet has less effect.

159, 164, and 182, and 188 series needle jets are similar but different dimensions and may be able to be interchanged.  (Some may require a different jet setter as there were 2 different threads used.)

I don't know what you have but the RT1 had a 188 needle jet if I remember correctly.

Finally, rich at high load is not always bad on an air cooled two stroke.  Fuel also provides cooling.  I found out the hard way that full throttle jetting that seems a little rich during a short full throttle burst ends up being perfect when running WOT for a longer distance.  Easy to go too lean if you're not (or can't) do a long full throttle pull with engine fully warmed up... 

 

I just dug through my needle jets, and a 188 series definitely came out of an rt3 32mm carb.  I see the height difference, compared to a 480 series.  I am currently running a 480 series, which is shorter.  The needle jet orifice would be higher too, with the 188 style.  So wouldn't that have an effect, similar to moving the needle down?  Then the shroud helps to richen at higher rpm/wot?  I definitely see two things that will change, thank you for pointing this out!

The rt3 carb is recessed deeper for the needle jet.  The carburetor I used was not like that.  In other words, the needle jet is still flush with the bore of the carburetor, with the 480 style.  If that makes any sense.  So the main jet would be less immersed in fuel ever so slightly in my case, and the shield height and needle jet orifice should be in the same position.

I have never seen a definitive guide on needle jet designs.  Most commonly you stick with the series the carb came with. 

I do know that Mikuni made a lot of very similar needle jets with different features and slightly different dimensions (not just the ID which follows a common measurement...  a P-2 has the same ID of a P-2 no matter the "series"). 

Many times the "oddball" needle jet was something an OEM specified... which makes sense as the OEM is building thousands of machines and they would want to get the tune as good as possible...  and with emissions laws they were forced to. 

Bottom line is we can guess or eyeball all day that one series is the same or very close to another series...  but it's unlikely it is. A mm here and there could make a significant difference.

Bottom line is it really doesn't matter 99% of the time.  The rule of jetting is you give the engine what it needs.  If you carefully test and truly believe your engine could be a little leaner at WOT, then maybe trim the shroud slightly.  Or reduce the air jet.  (Very easy to tap the hole and Mikuni makes threaded air jets in various sizes.)  I've done both. 

Post #5.
[So the main jet would be less immersed in fuel ever so slightly in my case]

If you mean the jet setter below the needle jet it needs to be long enough for the Oring to be compressed past the taper at the start of the 'bowl hole' or you will have a super rich main jet--ie--bypassing the side bowl main jet & straight up the spout.
Those little Oring have caused a lot of headaches over the years when chasing rich running.
Any airway that looks blocked off is usually the air bleed to the pilot jet which will be sucking it's air sideways from the needle jet air drilling near the mouth end of the carb just past the blank off, but controlled by the air screw of course, i think?, without looking at mine. I run a 360 32mm on my DT1.
i keep editing--i bracket words & they don't load, remove brackets & they stay, i dunno just me.

Rt325, I am not using the banjo bolt style float bowl.  Main jet threads into needle jet.

Back to the air jet specs, the dt1e, dt1emx, rt1, rt1m, rt1b, rt1mx all have a dash for the air jet spec.  The prior dt1's show 0.5, and the later reed valve motors show 2.5 on the 250's, and 2.0 on the 360's.  Looked at 3 service manuals, rt1m-rt1mx service manual, 71-73 dt/rt service manual, and the 68-71 dt1/mx manual.  The specs coincide in the enduro manual, the years they cross anyway.  I wish I had an rt1 or rt1mx carb, or a 71 dt1 carb so I could look and see what's actually going on.

I have a book that shows the specs and I posted them earlier.