My son found a 740 that had been sitting for about two years. Flat tires, dead battery, rotted exhaust. $100- plus $200- in parts and I have been driving it with no problems (until yesterday) for the last year and a half. Now when cold it starts and runs fine for about 15 minutes. Then it starts to misfire. The misfire gets worse and it wants to stall as I try to add power. Any ideas?