A 355 is an awesome motor .What do you want to pull for a max r.p.m. I used to run 6,000 in my 355 . If your missing at 4-6500 it means your motor is either running out of spark or fuel . Check ignition system Plugs, wire's, cap , rotor. and check your fuel supply hoses , filters, pump. If you had a bad valve you would know it . It would smoke and go through a lot of oil. Also check your choke and make sure it is opening all the way for high r.p.m's