Boxer Knocking Combustion

I had a similar issue with my Boxer not too long ago. The symptoms you describe – reduced power, jerking, and smoke – sound very familiar. Have you checked for any unusual noises coming from the engine, particularly around the cylinder heads? Could be detonation, or 'engine knock', which happens when the air/fuel mixture in your cylinders ignites in an uncontrolled manner. Is this happening constantly or just under load?

The knocking sound does seem to get worse when the engine is under load, especially going uphill. If it's detonation, does that mean there might be carbon deposits in the combustion chamber?

Yes, the fact that it worsens under load does suggest it could be detonation. Deposits in the combustion chamber are a likely culprit, as they can create hot spots that pre-ignite the fuel-air mixture. I would recommend taking it to a workshop and having them check the combustion chamber for excessive deposits. Ignoring it could lead to further engine damage. I paid about 425€ for the job.

I'll take it to a workshop and get them to check the combustion chamber. Hopefully, it's just deposits and nothing more serious. I will post the solution once it's sorted.