the cylinder has the two slots down the sides
This particular cylinder responds well to the exshaust port timming mod changing the timming to 150 degrees. It however will do you absolutly no good unless you go with the bigger carb and a much more free flowing exhaust than a stock muffler.
i have replaced the gaskets with thinner ones
This particular cylinder i think you can take the gaskets out completly and should not have a problem with piston to head clearance. you will need to use a small amount of something for sealant though. I like the loctite red anerobic sealant.
i will try to readjust the timming on it my pointer might be off but even with a stock carb it should get better then 6400 rpm michael
Actually that sounds about right. With the setup you are currently running i dont think your gona do much better. You may be able to tune a bit more out of it but not much until ya change the carb.
If you lower the cylinder as far as you can go, install the larger carb, make sure your muffler is free flowing and that should get you to 7000 rpm. It might not though till you do the timming mod.
Also what does your cylinder look like? does it have any scores in the cylinder at all???