Top end cutout on the RD

[Striker1423]

Every once in a while I attempt to hit top speed down my road. Usually no traffic and several miles of run to test on. I noticed two things. I can hit 100mph on the speedo (which isn't 100 for sure). But, I couldn't get it to go over that.

At 95-100mph:
1. If I try to fully open the throttle, the bike just quits accelerating and actually slows down until I back off a bit to just a hair over 3/4 throttle. It sounds like it's bogging at the top, just like it runs out of ass. If I roll back to 3/4 it picks back up accelerating to around 100mph and that's it.

2. As I go down from full throttle (and that bog), it'll pick back up and decel for around an 1/8th of a twist of throttle just fine, but then seemingly cut out out of nowhere for the tiniest bit (say a 1/16th of the throttle) as I continue to roll off, before picking back up just fine past that spot in the throttle. 

So, it goes throttle up to 3/4 or just past, and the bike sings just fine at top speed, roll up to full... bog. Roll off slow, decel ok, cutout, then decel ok the rest of the way down.

I'm curious what causes that weird cutout. It honestly feels like the ignition or fuel just gives up at that one spot only. The top end bog is jetting for sure, I think it's a bit rich still.

Specs:
Dave F modded carbs
Needle clips 3rd position (was thinking of shimming to 3.5?)
Main jets #200
Pilots: 27.5 1.5 turns out

All other rev ranges appear OK. I should note, I do get some pretty whacky surging at slow speeds (carb sync must be off).
 

[m in sc]

when it does it, kill the fuel, pull over and pull a bowl. see how much is in the carb. if its full, ign. if its empty or super low (my guess) fuel feed issue. the hybrid did that, turned out the seat in the needle and seat was too small, went up to a 3.0 and solved. was literally sucking the bowls dry

[Vintagewannabe]

when it does it, kill the fuel, pull over and pull a bowl. see how much is in the carb. if its full, ign. if its empty or super low (my guess) fuel feed issue. the hybrid did that, turned out the seat in the needle and seat was too small, went up to a 3.0 and solved. was literally sucking the bowls dry

What do you mean hybrid?

[m in sc]

lc, white rd.

[Striker1423]

Would it hurt to do the needle and seat swap anyway?

[m in sc]

there's no downside

[Striker1423]

Do you know which one to order? I'm seeing the retained spring ones, but nothing for the regular vm28.

[SoCal250]

The Mikuni part for the VM28 does not come in a 3.0 size, as the range only goes up to 2.8 for the VM26/26 series.

Float valve assembly (metal tip):
VM26/26-2.5 - OE part  (Alternate with Viton tip: 786-15005-2.5)
VM26/26-2.8

The VM34/39 series seats used in the larger VM carbs, which do come in a 3.0 size will not fit because the threads are 1mm larger dia.
Hopefully Mark can share which 3.0 part he used

[m in sc]

it wasn't a stock carb. however, like said, it sounds like its starving for fuel. might be the petcock or the cap. I used my example to explain, it was in that case, the needle seat diameter.

[Kawtriplefreak]

It might be the tank vent.