I have a stock 260 hp merc. engine i have added thru hull exhaust and can't stop the water from entering the riser exhaust outlet. This is on my 1986 thundercraft,manifolds,risers and spacers were checked and do not leak. Any thoughts thanks

Do you have flappers on the end of the pipes or inside them?

What is the down angle from the risers to the transom?

Is it new exhaust? if not you could have a crack in one or both that will only leak one hot.

Over 16 inches of length the pipe drops about 3 inches. They are new 4inch 7 degree risers.I have stainless steel exhaust tip with flaps inside.

Does it leak on both sides? What manifold and riser are you using? Does water pass thru the joint or around it with a hose?

Yes it leaks on both sides,the engine is glycol closed cooling,the sea water is piped to the riser not through the flange.They are aftermarket center outlet manifold and 4 inch sierra risers.