Exhaust Smoke coming out the inlet?

[brainbug007]

I started my engine up over the holidays to let it run for a few mins as it had been sitting for about 6 months. I noticed once it got going that around every 5-10 seconds I'm getting what looks like a puff of smoke coming out of the #3 inlet port. Any ideas what this is likely to be and how to fix it? I thought it could be a timing issue but wouldn't that mean it should be happening out of port #2 at the same time?