piston ring

I have a '96 XP that I purchased in January for a project. It had an SBT motor, but the owner hadn't finished installing it yet. So I picked it up for a project ski. I finished setting up and installing the motor and I've got about 15 hours on it. It's been running great. I had also overhauled...

The motor is backfiring at 5000 RPMs. Not 4500 or 5500, but right around 5000 RPMs. It was reproducible numerous times (by tech). The boat was taken back to the shop to determine there was a fouled plug. The plugs were changed out and the boat water tested again. Still backfiring. The techs then...