I have a Briggs and Stratton 18.5 with a Nikki Carb. Mower runs great for 20 minutes then shuts off. If I turn key completely off it will start back up, but when engaging blade it stalls out. I have cleaned carb, checked the solenoid, but not sure if the rubber tip should be lose. I recently replaced battery, starter and solenoid near the battery. Checked values and gas cap. What else could be the problem?