I have a John Deere SST18 with a "18hp" B&S Vanguard. It has run fantastic from 2001 until recently. It runs fine, but blows black smoke for about 20 minutes. Then it falls on its face power wise. I take it in the garage, clean the plugs and it's fine again, but only for 20 minutes. A mechanic told me he thought the coils were probably getting worn out and I should replace them. I did and it ran great...for 20 minutes. Is it time for a new carburetor or is there something else I should check first? I am perfectly capable of throwing parts on a machine, but as far as rebuilding a carb, it's not gonna happen. I can put a new one on for less than I can pay someone to rebuild it