Champion (Briggs & Stratton) Classic 35 - smoke problem?

I have just picked up a second hand model and when I got it going I noticed two things that could be an issue:

1 - it idles along just fine, but doesn't very strong. The engine runs smoothly and consistently but doesn't have much pull at all (I know it's only 3.5hp but I still feel it's a little lacklustre)

2 - after around 10 minutes it started to spit out quite a lot of white/grey smoke. It carried on mowing just fine and the smoke calmed down but it does come back, and constantly puts out a little smoke. My previous (Mountfield) mower didn't do this so it seems like it's the sign of a problem.

Any help is appreciated.

Thanks