Wow that blowdown is horrendous, and is definitely the cause for the lack of higher RPM power. You could try to lower the jug to reduce the transfer duration and increase the timing ATDC to try and help improve the blowdown some. That would buy you a little room to raise the exhaust port. The downside is the port floor being a turbulence zone due to the piston blocking part of it would get worse. Plus doing so would only buy you a couple of degrees at best, and as you mentioned the jug is already decked so that will cause other problems.
You could raise the exhaust port a little (A couple degrees) without likely causing many issues. 160 degrees of exhaust duration isn't too terrible. Another thing you can attempt to do is buy yourself as much evacuation as possible in that limited blowdown by widening the exhaust port at the top. The risk here is if you shape it wrong you will snag a ring.
Another option still would be to ramp the piston on the exhaust port side to get it to open sooner relative to the transfers. You will sacrifice a little compression and part of the squish band as a trade off but it may be worth doing in this case.
Obviously any combination of the three could also be made to work.
Yeah, all that is basically what I assumed.
I know how to do the physical part of porting to where I wouldnt snag a ring etc.. Just not super hip on durations and whats ideal on these.
I do have another jug that isn't decked. It has practically identical durations to this jug. I think I can tweak it in better than the current jug as I have not decked it yet. At least where I'm thinking now tells me I should be able to.
I actually already have a piston that i ramped so I could play with the timing without scrapping a jug, then transfer those found timings to the jug so I can maintain squish and put the non ramped piston back in. Just havent swapped it in yet to play.
I actually love the low end. Its smooth. I can cruise slow, at very low throttle/idle and it has decent torque to keep it rolling nice and smooth, and its got some a$$ WOT right off of a low cruise...it just peters out the last half and revs with no additional power, and hence no speed gained......and just wasting fuel up there.
Its actually not horrible to ride, but having the top end do almost nothing bugs the snot out of me. I could sacrifice a bit of the nice low end to spread the power out a bit more, but honestly, I think maybe just dropping a good ole 47mm top end on may be the easiest way. If I dont get very far on it with the new jug, that's what I will do, and maybe put this engine on a mountain bike for trail riding. It is much better suited for that purpose as it is.
I like to tinker, but we only have so much to work with on these jugs and it looks like I happened to get some that are on the lower end of tunability due to port/sleeve locations. There are a few example of these jugs out there that are pretty close to stock doing very well on the top end.
