You shouldnt have an issue making 5 minutes with a novarossi engine. The 2660 pipe is very efficient and actually many use the pipe on OS engines to make time. We need to look elsewhere to get time....

Firstly how short of fuel time are you? Is your tank full size (many are undersize out of the kit - Serpent is a particular tank thats about 5cc too small out the box). Clutch setup properly - not slipping? Is the engine lean enough - if the engine has just come off break-in then the engine will get better as it fully breaks in and you can go leaner.

Honestly you should make 5 minutes easy on that pipe and engine. I can get around 5:30 from most Novarossi .12 engines on the 2660 pipe with both the 5 ring and 6 ring manifolds.