Had my first motor problem with my approx 1 year old Yamaha 15hp 4-stroke a few weeks ago and figured I would share the problem and how it was fixed. I am by no means a motor guy, I know very little about this stuff really, but I figure by posting it maybe this info will help someone else down the road.

The symptom went like this...

When you started the motor, no matter if it was warm or not, it would sound rough. With the throttle in the start position, the engine would run for anywhere from 30 seconds to 4 minutes sounding rough. After a while though you would hear it go from a rough sound to a fast smooth idle.

If you waited for this fast/smooth idle, then throttled down to the lowest idle speed before shifting, it would not stall. If you throttled down while it sounded rough though it would stall 9 times out of 10. The problem developed over time but was pretty consistent by the time I took it in.

The solution was to clean the carbuerator. There was apparently some buildup on the carbeurator was causing it to act this way. In my case, it was covered under warranty and repaired no problem, but if it happened to you, you could probably fix it yourself with some carbeurator cleaner. Assuming of course you know where the carbuerator is and how to get to it :) So that's not complete info on how to fix it but it might help someone somewhere down the line.