I recently had my 95 GSR (of which I am the original owner) to the shop for 90K service. The timing belt and water pump were done.

A few weeks later I noticed that the inside of the exhaust pipe tip was starting to show some carbon build-up, as if the engine was burning rich.

Previously the color was always a very light gray, almost nothing there at all.

Can a timing change result in this carbon build-up? I'm wondering if the guy who did the work might have adjusted the timing differently than it used to be.

I appreciate any insight y'all might have.

 

Yes, incorrect timing could cause a rich burn. I don't see the need for him to adjust ignition timing at the distributor, but he could have done it. If the cam timing was off, you would know. The car would idle and run very rough. Pull your plugs and check them for signs of a rich burn, and get a timing light to check your timing.