I'm running a p5xs in a mugen mbx5r. I get great mileage about eight minutes
per tank. I am using an novarossi 9886 pipe,short manifold,Odonnell 20% and
a mc8 medium plug; the engine runs great with good power.
The mc59 is an hot plug mainly for small block engines using 20% nitro or less.
If are using 25% nitro with stock head shims, I would try a medium plug like
the stock novarossi #6 or an Odonnell #99. Hope this helps