I bought a Monster in February. It was a bit hesitant when riding it home, when pushing it a wee bit. I was trying to hold back for first wee while to let the engine settle in. Checked the plug gap and was only 0.04mm, should be between 0.06 and 0.07mm. Next planned to check valve clearances etc. I assumed they were the same as the Yamaha YBR125. Spark plug change and valve clearances were my immediate plan, and ensuring all bolts were tightened properly. Both rear shock upper mounts were loose, missing washers. Also air filter side panel mounting screws were both very loose, one was almost out! Build quality not great so far. Also sump had way too much oil in it, well over the maximum mark. I was pleased as I got it for a very good price. Bike had lain for ages unused by previous owner, had only done 2 miles when I bought it. This could have accounted for the hesitancy at 6000RPM. It was like it hits a wall, seemed like fuel starvation. Changed plug, and adjusted valve clearances, made no difference. Added some high performance fuel and Redex, and noticed some improvement. Blanked off the SAI system at the cylinder head as it can account for problems such as this. Also stripped and cleaned the carburettor. Possibly just needed to get a few more miles on it. Just bought it for a bit of fun during the winter months, was out first few days when temperature only 2 degrees!! Planned to keep my Harley for the nice dry sunny days, not many of them, only managed 200 miles last summer. The hesitancy at 6000RPM continued to be a problem, so swapped the main jet from the stock 92 to a 96. Now runs as smooth as you like. Goodness knows why it was fitted with such a small main jet at the factory. I also have a 98 jet and may fit it together with a cone filter. So anyone with a Zontes 125 that is not running as well as you would like, just swap the jet. Takes 5 minutes and costs about a quid. No need to remove the carburettor, just remove the 3 bottom screws holding the float bowl and remove it.