My 15hp Yamaha 2-stroke outboard made a very loud metallic bang while under heavy load. What was it?

Question

I'm thinking a back-fire into the crankcase. It was quite a violent sound. I was trying to get onto a plane but the load was too far aft and we had been sitting on low revs for an extended time.

Answer 1

It may have been a backfire. I have heard the sound you speak of. I know it not the piston hitting anything as there are no valves on a 2-stroke, they use Reeds. It may have been a Reed stick open and allow combustion to backfire through the carb. They make a carbon cleaner called "Seafoam" may try running some of that through the engine. Many Auto-Parts stores have it.

http://www.seafoamsales.com/

Preignition and Detonation are some reasons it make a sound. Preignition is premature burning of fuel. It can be caused from carbon deposits, overheated plugs or cooling not working properly. Detonation is like the sound we hear a car make going up-hill under a load. It is usually caused by low octane. But, timing, fuel/air mix, cooling can also cause detonation. Both of these can cause serious damage. So, if it keep doing this you need to find out why and get it fixed.

Answer 2

well, if you're lucky, one of your spark plugs has glazed.  you can clean it or replace it.  if you're NOT lucky, you should see bits and pieces in the lower cowl.  after a backfire, always check the flywheel key for sheer.