When I put the muffs on the lower unit for water in the driveway, it starts on the 2nd pull every time. When I put the boat in the water, I can pull until I pass out and it won't start. The only difference I can see is that there is more exhaust back-pressure in the water due to having to push the exhaust through the prop while under water. Last year the motor got 2 new coils and new points and plugs. I'm thinking a carb problem. Any suggestions what to look for or check? The carbs have not been touched in the 5 years I have had it.