Crash- Just for the heck of it, pull the mufflers and look thru the exhaust ports and see what the cylinders and piston skirts look like.

There was a bad batch of ignitions a long time ago, C&H will replace it if that is the case.

When you say slow to "run up" can you tell if it is rich or lean? I would try cleaning the carb anyway.

I had a similar problem with my 110 the first time I cowled it. The engine would not want to run up either, it did not sound rich or lean just constipated. I had to cut a hole below the carb and that cured everything. I don't know if that will help yours or not being that this problem just came about.

I would start with the carb/fuel system then the ignition.
Ralph

By the way, when you mentioned one cylinder was puffing , front or back? The smoke was white and not a blueish color?