Motorcycle Repair: Honda CB360 exhaust pipe, honda cb360, worn piston rings

Question
Dear Bill,
When I removed the exhaust pipe from my Honda discovered that there is a thick carbon sediment inside the tube, so the pipe's inner diameter is decreased well. However the engine is still working  well with them.
Is there any preferred method to remove the carbon from the exhaust pipe without get the pipe broken?
If there is no any, in which inner diameter it is advised to replace the pipe to not blocking engine?
Many thanks for your answer in advance.
BR. Karoly

Answer
Karoly, please note that the header pipes on Honda twins are double-walled to prevent chrome discoloration, so what you see may not be as critical as you think. You didn't supply any actual photos or diameter measurements, but I wanted to mention that to you, anyway.

Black, sooty carbon is generally due to carburetor issues and/or can come from inaccurate ignition timing settings which affect carburetor calibration and function. If the carbon deposits are more brown and crusty, that is an indication of oil consumption/burning due to worn piston rings and/or valve stem seals. Clean out a small portion of the carbon and see how deep it really is first, then take other actions.

Any kind of flexible wire brush on a shaft can remove some of the deposits or you can chip away at the closest portions with a hammer and chisel/screwdriver. If you have access to a sandblaster, that would take care of a great deal of it. Just remember to dump out as much sand from the exhaust as you can, afterwards.

Once you have addressed the carbon condition on that pipe, check the other side to see if it is the same or different. If one side is heavy and the other side more normal, then you have some troubleshooting and/or repairs to do to the affected side.

Sometimes, when bikes sit for prolonged periods, moisture can build up inside the cylinders, causing water corrosion damage to the piston rings and cylinder walls. Adjust the valves first, then check the compression readings to see if they are both close to specifications. If one side is more than 15% lower, then there are problems to address inside.

Bill Silver