Don't tap the muffler. What engine type, displacement? If you're losing fuel pressure it's because there is a leak in the lines somewhere. Check all bolts and tighten down good. Also make sure the lines are secure at all connection points. Tie wrap them all, even in the fuel tank itself. Make sure you have a gas conversion stopper also, as the stopper that comes with the tank is usually for glow fuel only. Walbro carbs do have an internal pump, so the tank position should not make a difference. I am by no means an expert in running these engines myself as I to have just gotten started in gas engines. I am just reitterating what I have read in the forums and mags. Perhaps someone else will give you a better idea of what the cause might be. Hope this helps. Ben