[T@BD]

With respect, shadow and xlint I disagree with much of what is being said.

Shadow, the sensitivity of valve adjustment is extremely overplayed, and is not a real issue on the GY6. Adjusting, and re-adjusting your valves is thrown out as a cure-all rather than encouraging and coaching new owners on decent troubleshooting steps to pinpoint the root issue. Although important, the valve lash settings aren't super sensitive or finicky, and aren't something that requires extensive experience to get right. Early on I mistakenly ran a couple of GY6 builds with over 1/8" gap valve lash. Bent a valve stem quickly, but it fired up with no fuss and ran better than a stock 150 engine. Don't get me wrong, valve lash is critically important, but it's not one of the first step in figuring out an idle issue. Pripyat is on the right track checking for intake leaks first.

xlint, damage from valve float isn't something I've ever witnessed or heard to be an actual occurrence on the GY6 (though it is mentioned a lot in theory). Do you know of any specific cases of valve float causing damage to GY6 engines? If so, who did the post-mortem breakdown and reached that conclusion? Buggy models with the 27/24 large valve routinely and safely hit 12,000 RPM in stock form directly from the factory.

On both of these counts, I feel that bits of info like these are repeated to the point that they become common knowledge, but untrue.

Also Pripyat on the sprockets: The larger sprocket on the reverse, 39T axle sprocket, and 22" rear tires all work very well together and are a good match for most rider weights and terrain. Though it sometimes comes down to preference, it's very unlikely that you will need to change anything with gearing after adding reverse.

With that said, after big bore and stroker (depending on a lot of skill factors) you will have enough power to run the 31T, gain top speed, and still have significantly increased low end power over stock. I don't recommend removing the 39T until after you go BBK and stroker.

9200rpm is normal and your method is (creative, but) sound. But 9k is nowhere near the top limit even on a bone stock 150. You're safe to keep pushing it further (though I can't speak to the quality of the eBay head holding up). At 11,000rpm you'll hear a very cool "spooling" sound from the cooling fan get louder before power starts to (safely) fall off. Damage at high RPM begins to be an issue when you build specific Big Bore and Stroker combinations.