I can go ahead and tell you from the start a 70 jet is way to small if it's a 70 cc. Friend of mine had a 92 jet on his express with a 12mm carb and sift seized it numerous times before I had the wide and to dial it in. With a 15mm carb I would say at least a 90 to 100 jet. His wasn't water cooled and didn't have a temp sensor either but I guarantee his was about 450 degrees. Even if it's water cooled it can still be damaged by running to lean even if it's at 200 degrees. Cars thermostats open at around 165-185 degrees. "Most Hondas I've worked on" and peak temp around 220 before the ecu "hondata s300" runs in limp mode with a check engine light. Up your main jet and do some plug chops and you should be fine. My friends express was a 1 speed so it revved really high at 40 mph.