A GX340 pumped up to 16-17hp will not likely reach 100 mph. You can setup your gearing for 100 mph based off of what the speed calculator says however it doesn't mean the engine will be able to pull that gear ratio. We are aware of many karts and mini bikes making much more power and cannot reach 100 mph. Not knocking your quest just trying to to share information that may help.
There are a few things to note when comparing a GX200 to a GX340. The 340 weighs nearly double what the 200 weighs. A 200 pumped up to 18hp will walk all over a 340 making 18hp. The rotating assembly in the 340 is much bigger and heavier than the 200's rotating assembly. Because of this the 340 is a slow revving engine compared to a 200. On the track the 200 will easily out perform the 340 at this HP level. The 200 will also spin more rpm.
To take a 340 to 17hp there are a few things we would consider. We'd start with an upgraded carb and exhaust. Next would be upgraded valve springs, billet flywheel, increased compression and of course governor removal. And we would base this engine off of a GX390 instead of a GX340. The two engines are physically the same size however the 390 makes a couple more HP. An even better choice would be the 420cc clone engine. These can be purchased at Home Depot online for a really good price.
Another important piece of information is the 7000 rpm you mentioned. The 340, 390 and 420 engines can reach 7000 rpm but just about everything in the engine needs to be upgraded. You need heavy valve springs for that rpm. The stock cam cannot handle the heavy valve springs so you need an aftermarket cam. You definitely need a billet rod and flywheel. And all of the valve train needs to be upgraded to handle this kind of rpm. All of these parts for the bigger engines cost much more than they do for the GX200. So your looking at a lot of upgrades and a lot of money to get 7000 rpm out of a 340, 390 or 420.
I'm not trying to discourage you. I would love to see a forum members bike do a legitimate 100 mph. Just sharing some of our knowledge on these engines.
