Many times it's just drilled to size.  2.5 should mean 2.5mm diameter... 

(spec sheet often has the "phi" symbol after the number which stands for "diameter")

Hi,
I guess I need to be more specific..
My carb does NOT have an air bleed jet installed in it..
Bike is fat on bottom . Went from 50 to 45 and dropped needle one to top clip. It is better but not right.
I will try a 40 pj and a leaner jet needle but wondering if air jet not ibeing nstalled will cause this. I would think it would make it leaner but not 100 percent sure as I never ran into this.

I am trying to find out if 250E yz had an primary airjet for needle jet? Mine does not but carb body hole is threaded.
this machine has had many issues so wondering if someone left it out? ?

Ok..
Think i know why there is no air jet. Bleed type needle jets use them but this is not a bleed type so probably will not have an air jet just an open bore. I was thrown off by the threads in the bore...
Joe

You were clear.  And so was I.  Passage is often drilled to size.

Odd that it's threaded, but not hard to check diameter of hole with drill bit close to correct size.  

Air jets can be found on carbs with primary needle jet or air bleed...  not common on Enduros or YZ's, but the design of needle jet does not determine if it had a jet installed or not. 

Finally, jet or no jet, it would not effect the problem you've described. 

As counterintuitive as it sounds, a larger air jet with a primary needle jet (shrouded type) RICHENS the mixture at full throttle, high RPM.  It doesn't do much, if anything, at lower RPM and lower throttle settings...  with the possible exception of closing throttle at high RPM...  and again, it will draw more fuel, not make it lean.

One other thing is you are dealing with a race bike!  It was designed to be used at full throttle as much as possible...  not putting around the pits or riding slowly on a trail. 

Cooling and lubrication are both provided by the fuel...  and it's always seemed like Yamaha jetted the race bikes very rich at idle and just off idle which gives the engine a nice little cooling/lubricating boost when off the throttle.  If you are racing it hard as intended, they run great.  But yes, too rich for riding around slowly. 

One caution...  a friend had a YZ back when I was racing in this era... can't remember if he had a 76 or 78.  We used to race and trail ride.  He rejetted the low throttle setting circuits so it wasn't so rich just riding around for fun.   The next race, he stuck a piston. 

The pilot circuit...  and even more so, the needle, both have an effect at full throttle!  We never re-checked the main jet after he leaned out the pilot jet and the changes he made were enough to make it too lean at full throttle.

The largest airjet for the Mikuni VM series is 2.0. The airjet controls mixture at 3/4 to WOT. Needles and needle jets wear from dirt and vibration, becoming richer over time. I suggest a P-6 or P-8 needle jet and a 6F15 needle in the #2 position.

Without reding all just yet, old MX bikes wear out the needle jet--the emulsion tube where the needle slides up n down continualy. Can see it by eyeballing, looks egg shape. Replace that & the needle & should sort the midrange.
Funny what shows up when simply googling the words "air jet yz250e 1978".
Not sure its relevant as i've never seen a removable air jet in a 38mm round slide carb.
www.z1enterprises.com/motorcycles/Yamaha...carbureted-jets.html
 

Ordered a 6f15 to replace the 6f16...
   no wear of needle and nj shows no elongation.
i have been riding racing bikes since the 70s and bike is to fat on bottom..i race the bike in AHRMA Expert class so I believe i am riding the bike properly...i bought the bike used and i believe someone put it together to flip it and if i listed all the fk ups you would laugh....
yamaha has specs on carburetor parts page. That is where 2.5 air jet was listed.
i may for the hell of it order a 2.0 airjet ...
Thx...JA

 

JA-  Taking a step back...  What exactly do you mean by "rich on the bottom"? 

I'm asking because carb tuning is 100% about how far the throttle is open, not what RPM the engine is running.  So if the engine is rich at lower revs but the throttle is wide open or even halfway open, you won't fix it with the pilot jet as the carb is on the main circuit at full throttle and the midrange circuit at half throttle. 

Also, don't discount needle jet wear.  You can't always see the wear in the needle jet. 

One other remote possibility is the condition of the starter (choke) plunger.  If it leaks, it could provide extra fuel under some circumstances...  and no, it won't richen the mixture the same everywhere. 

Ignition timing and ignition system can also do so weird stuff.  Timing is easy to check/set/verify with a timing light and Yamaha had the wrong spec in 78 manual.  It was also perhaps their earliest attempt at an electronic "advance and retard curve" and it's not unheard of for electronics to act up.   I had a friend with a motoplat or SEM ignition that apparently had a weak spark at lower RPM...  it sounded rich as hell (literally four stroking) at low rpm and could not "jet it" away.  And if you tried, then it was too lean at high rpm.  I helped him install a VAPE ignition as a last ditch effort and the problem went away.  (We did notice the timing light seemed to be skipping or erratic at low rpm)

Hope you figure it out soon...  video of what it's doing might be very helpful too...  we're all flying blind here and can't see and hear what you can.