Are there two different kinds of GY6 QMB139 Engines?

I have owned three different scooters, two are 07 models and the other was an 08 model. They all three have the same engine, the GY6 QMB139 Honda Clone. It seems like the 07 models have half the power the 08 does, they all three have the same belts, variators, roller weights and CDIs. I cant figure out how to make the 07 I have now run and have the power like the 08 I had did. Now all three of these were made by different manufactures (07 Linwood, 07 wildfire, 08 NST) would that make a difference I mean the engines all appear the same, have interchanging parts, but the only thing is the 07s dont have as much power or even top speed as the 08 does