I doubt if there is carbon buildup on a 2018 engine.
It is still covered under warranty but the dealer will probably have to experience the problem during a test drive cause it is obvious there are not getting any trouble codes retrieved from the PCM.
If it only does it while cold, below forty degrees, you may have to wait until the weather is below forty degrees and take it back to the dealer and ride with them as they test drive it and prove it to them.
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Monday, January 8th, 2018 AT 9:10 AM