Old kamma doesn’t burn. One of the great ironies in the history of Buddhism is that some of the teachings the Buddha explicitly attacked are nowadays attributed to the Buddha himself.

"As we sit here meditating, we’re engaged in a type of kamma. It’s called the kamma that puts an end to kamma. But this doesn’t mean that it burns away old kamma. Old kamma doesn’t burn. One of the great ironies in the history of Buddhism is that some of the teachings the Buddha explicitly attacked are nowadays attributed to the Buddha himself. One of these is the idea that simply by learning how not to react to anything in the present moment, you’re not just refraining from creating any new sankharas, you’re also burning away old sankharas, old kamma.

But there’s a passage where the Buddha actually attacks the idea that you can burn off old kamma. In fact he gets quite satirical about it. He goes to see some Jains who believed that they could burn off old kamma by submitting themselves to different kinds of austerities, that the pain in the austerity was the burning of old kamma. He said, “Can you measure how much kamma you burned today? How much kamma you burned yesterday? Do you actually see the kamma burning?” Of course the answer is No. “And what about the pain caused by your austerities? What’s that?” They replied, “This is just the old kamma burning up.” He said, “Have you noticed that when you don’t do your austerities, there’s no pain?”

Here he’s making the point that when you experience the results of kamma, it’s not just the results of past kamma. It’s also the results of your present decisions. If the Jains didn’t practice austerities here and now, they wouldn’t be experiencing those pains here and now.

In fact, your present actions are the most important part of the kammic mix. This is the lesson of the kamma of meditation. Our present decisions, what we’re going to do with the mind, what we’re going to focus on, how we develop it: That’s the kamma we witness as we meditate. So you want to focus on doing things that help you understand what’s going on in the mind right now. What is this process of intention? How does the mind create an experience of pleasure, how does it create an experience of pain, out of the raw material of old kamma?"

~ Thanissaro Bhikkhu "Equanimity Isn’t Nibbana" (Meditations5)