If it's Kjet, I can only think of two things.
The cold start injector is dumping too much into the intake or failing to shut off. The simple test is to unplug the connector and see if it helps. Note that this isn't controlled by the coolant temperature sensor, it's a separate temperature switch located further aft on the head. Pull it out, attach a meter to the pins to check for continuity. Then stick it in a cup of boiling water. The connection should open circuit as the sensor heats up.
The other thing it could be is a sticking plunger in the metering head. If this sticks in a high location then it will dose the fuel as if the throttle is open.
Get a long 3mm Allen key and put it into the mixture screw next to the big rubber air sock, and use at a mild angle it to lift the adjustment screw and as such the whole air plate up and down. If it's sticking you will either feel resistance and or a "tap" as the air plate hits the plunger. If working properly it should follow very smoothly and quietly.
It's worth also checking the air plate itself to make sure it isn't fouling on anything and sticking in a high position.
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