Ignore the Mikuni.
Stuttering, surging and backfire sounds on the weak side to me. So go up one size on the main jet and see how you go.
Quote from AMAL tuning instructions:"If at full throttle the engine runs "heavily" and does not pull at full power, the main jet is generaly too large. If the engine fades and perhaps deteriorates, with possible detonation, then the jet is too small. With the correct size jet the engine should run freely at maximum rpm giving good power. When testing for the correct main jet, give careful attention to the sparking plug readings; the correct colour will be light brown or chocolate brown surrounding the central electrode." Tuning the AMAL, Dellorto and Mikuni all have the same sequence, so using the AMAL method is fine for your Dellorto and the Mikuni.
I thought that you had a good colour on your plug from the image that you posted. But, have you increased the main jet size since removing the tube to the air filter? If not, then I would suspect that going up one or two main jet sizes would be benificial.
As an aside, since fitting my homemade exhaust/silencer that has quietened the rasp, but I have had to go up from a 200 main jet to a 220 and dropped the needle clip by one groove and turned in the air screw by ⅜ of a turn towards the seat and may have to richen further. FICKLE BEASTS!
If you have jets for the Mikuni (Chinese? to test, then get OME jets once found) then have a read through DanC' posts in the 'Desmo 450 carb' thread [Desmo_450_carb]. He has a Mikuni 30mm working well on both a 450 and a 350 and he lists their settings.
Keep at it, Bill
