I am by no means a small engine mechanic, and I need some help. I was cutting the other day, and went through about 4 tanks of gas. I filled the saw from the same gas can of gas/oil mix each time, so the mix should have been the same. The first three tanks were fine. I noticed, however, that on the last tank of gas, blueish smoke was coming out of the muffler. It got worse as I went, and by the end, smoke was really pouring out. I ran the saw again today, and the smoke was there from the start. The saw runs fine, no stalling or anything like that. I know that a mix too heavy in oil may cause this, but the mix is the same as the first three tanks I used that day. Any ideas what would cause this?