Modern Vespa

Hi, I'm new to posting on this forum. If this question's been asked already, I can't find it, so please excuse me.

My ET4 when coldish jerks very slightly at stationary idle, as though clutch is partly engaged and it's trying to move forward. Worse, when you accelerate, the engine judders as though I'm in too high a gear, with little power. As it warms up, the problem decreases, to the point where all seem fine. When totally cold, there is no judder and all seems fine too. I think it all kicks off when the choke disenages. I've changed the oil recently, recommended motorbike oil, could this be affecting the variator etc? The bike did work fine for a while, ther problem appeared gradually, I think

Although it might be something else, or a combination of things, you need to adjust the valve clearances to eliminate that as the cause. That's the main reason for poor cold performance.

The other things? The carb may need cleaning/adjusting, the fuel lines and/or rubber intake from the carb to the engine may need replacing, the transmission may need attention (new belt/rollers/clutch clean). However, unless you're sure the valves are set correctly you'll just be chasing your tail with all those.