Are you sure it hasn't blown the coupler, or the coupler come loose ? Might check that out. Other than the manifold gasket, or manifold being bad, I don't see anything down there to allow exhaust out. Has it ever been used in salt water ? Is the backfiring happening when you let off the throttle ?
I would do a compression test and make sure there isn't something going on internally. Any other odd noises coming from the engine ?

 

Sorry I haven't gotten back sooner, been very busy…. I'm not sure what the coupler is, and it is in the saltwater a lot, I go fishing off of it… Here are a couple of pictures I have taken… I seem to be constantly burning up pistons, and I cannot figure out why that is, I have eliminate the oil injection because of burning up pistons, I have been mixing my oil 50:1, or a little better just to have a little more oil in that gas, but that doesn't seem to be helping much… the spark plugs look good, not dry at all… Also, this time the head has some pitting on it from the burnt piston… Can this be fixed, or can I still run it with the pitting? I can seem to find a good head for it… I think the skirt that has broken could be the problem with the backfiring, but not sure…. I am just about to give up on this ski, I seem to be working on it more that I am enjoying it… I will be throwing in new pistons and gaskets to see what takes place… On a good note, I bought a new ski two months ago, and I have been enjoying it a lot…. 2015 Yamaha V1 Sport….

 

Prob is, but i'm betting on more damage than meets the eye hear. That pitting appears to be from broken ring or failed bearing. What do the skirts look liken domea look like and what color are the plugs ? You need to check the condition of the case bearingsn rod bearings and wrist pin bearings to locate this issue. When is the last rime the crank seals were replaced ? How are the water jackets in the barrels and head ? All clear ?

 

WOW, man you are really talking over my head... I bought the ski two years ago, and have been patching it together as it brakes down... This was my first ski, and although I don't know much about them, I usually can fix anything that is apparently wrong with something... But when I am unfamiliar with something, I don't know all the other things that might be the cause of the problems, and that could be the things that are repeatedly causing damage to the ski... The plugs looked good, the rings on the piston looked good, the bearings on the connecting rod to the crank feel good, but I cant say I know how the bearing on the crank itself are.... I guess I really need to tear this thing completely down and do a complete inspection before just replacing the piston, and slapping it back together and hoping for the best.... Any words of advice to me before I tear it completely down, like the do's and don'ts?
Thanks

 

If the plugs are colored tan, there is no scoring on the piston skirts, then your looking at either piston slap from bore to piston mismatch (to much piston to cyl wall clearance), or bad bottom end that could be from a bent rod.
The damage to the top of the piston could be from pieces of the skirt (if it's the same one). If not, the bottom end is probably failing. If the engine has never been rebuilt, it's probably time if there's alot of hours on it. This will include new crank seals, and getting the crank rebuilt. If you don't want to mess with it, there is a place on the east coast that a few members have been having good luck with. don't remember the name. You will have to hunt for it.

 

well, I found where the problem is... Now I have to see if anyone can weld this thing up....

 

Prob a piece of the piston wedged between the case and crank. I would be good if you can find the piece of that and have it welded back in from the out side, other wise you are going to be doing alot of shaping.