It's different with each N14 engine, some really bad and some aren't as bad. I would recommend you get the walnut blasting done on or before 50k miles. Even though the carbon is not to the point where you need the cleaning it will affect the overall performance of the engine.

When I had the dealer perform the walnut blasting at 47k miles it was bad enough to cause misfiring, pre-ignition and detonation which cause your pistons to crack in the long run. Remember if your valves are coated with carbon then so are the top of your pistons which also causes misfiring, pre-ignition and detonation.

After my valves were walnut blasted it looked brand new in there, but not in the combustion chambers! I was still getting pre-ignition and detonation because my combustion chambers look exactly like to picture attached. So I purchase 5 bottles of BG 44K Fuel System Cleaner and after each bottle I pulled the spark plugs to see how much carbon was removed, remember these are oily deposits burned onto the pistons.

After 5 bottles of BG 44K I haven't had a single pre-ignition or detonation since, I even purposely try to get a pre-ignition and can't reproduce it. My pistons started out black, guess what color they are now? Silver. My best advice is don't stop your cleaning after the valves are cleaned, take the next step and make the combustion chambers clean also. You can get a really good deal when you buy in bulk on eBay, I buy my BG 44K locally.

Thank about it, how much would you pay someone to clean and treat your combustion chambers? With that knowledge 5 bottles of BG 44K costs only $100 dollars to remove a large percentage of carbon from your combustion chambers, compare that to the price of walnut blasting.

 

For those that claim to burn not a drop of oil between oil changes, I find it interesting that your valves no matter how little shows some black carbon burned onto the valves. What do you believe that black carbon to be? Oil vapors is what it is.