I used them when I put the BR pipes on a few months ago. With the motor at idle I held a rag over the pipe to feel back pressure. I then held a hose by the gasket area and I did not hear a leak. But I touched the pipe with the hose and it melted to the pipe, what a mess. With the pipes only about two feet long it is hard to trace the noise. Someone said to spray something, I do not remember what, into the carb to get the motor to smoke and then look for leaks. That I have not tried but you could probably use outboard motor fogging spray since that stuff smokes like hell.
So far so good.