Stock jetting is part of your issue, it's way , way too lean , by running premix you have leaned it out even more., if the choke shafts have been removed it has to be rejetted see below and don't try to rejet it using the O-side specs and the stock airbox , it won't work and no one has been able to make it work..for stock airbox follow the first set of specs but to use them the cat converter has to be removed which if it hasn't been already it's part of the problem. They retain too much heat and melt things in a bad bad way .
Myself is correct those 800's and 1200's are like a Swiss watch , they will not tolerate any imprecision . He is also spot on about debris, anytime a reed valve motor pukes , you have to pull all the reeds out and clean out the debris, if you don't as soon as you fire it up it goes right back through the engine, I know it's a pain to do on these but this is what happens when you don't.
I don't use those SBT cylinders they are crap , they don't dissipate heat like the stock cylinders do , cast iron retains heat , Aluminum dissipates heat , these engines were designed with plated cylinders for a reason, they ruined them when they installed those thick cast iron liners in them., the heat dissipation is so bad that they have to use a lot of extra piston to wall clearance to keep them alive probably double what the stock clearance is and they rattle like a freight train , absolute junk IMO.
The following was copied from Bamafreeride where I store important info like this :
Bit off topic but if anyone is ever wanting STANDARD European spec carb settings they are as follows. I have given myself a headache trying to source these but I have had them confirmed from 2 x Yamaha dealers in the UK
Main Jet 135
Pilot Jet 92.5
Low speed Screw carb 1 = 1 1/4 carb 2 = 1 1/2 carb 3 = 1 5/8
High speed screw Carb 1 = 7/8 carb 2 = 3/4 carb 3 = 7/8
The european specs skis have a decat plate from factory, no exhaust temp sensor and no plastic cover over the stinger.
The are set up differently from USA Skis.
Oside Bill jetting specs
1200R Aftermarket F/A's (Flame Arrestors) chokes intact
110 Pilot jets, 1 turn out on low speed adjusters
120 main jets, 1-1/2 turn out on high speed adjusters
1.5 Needle and Seat, 95 gram spring (Dull Silver) 42-45 avg. pop-off pressure.
1200R After Market F/A's chokes removed.
110 Pilot jets, 1 turn out on low speed adjusters.
125 Main jets 1-1/4 turn out on high speed adjusters
1.5 Needle and Seat, 95 gram spring (Dull Silver) 42-45 avg. pop-off