I have a friend with a Slimline pitts on his 91 FX. The engine detonates at the bottom of the downlines. It sounds like the ignition is miss firing. He's used a colder blug and 10% nitro but it still makes that sound.

Another friend has one inverted with the original OS muffler in a UCD3D and it's the most beutiful sounding two stroke you every heard.

They are both the original model with the metal body needle valve.