Briggs 12hp model:281707 type: 0412 01 code: 90051011 Started up for 1st time and would stall after awhile. Oil seemed to be diluted with fuel!! Drained and refilled with new oil...appears to be running better. My question is after sitting all winter it happened...what caused it I am sure it will happen again, so Id like to take care of it now. What to look at?