Have 18hp Fastwin that starts fine but won't stay running at idle, spark is fine bulb stays firm, gas is in line to carb, (I put on clear line so I could see fuel) but it still won't idle. It runs a little while then it just dies like no fuel. I rebuilt fuel pump and carb two years ago. Could it be bad fuel? I run 50 to 1 and I put stabil in last years fuel, and topped off this year. Never had this problem before.