After doing research, the cheap studs (stretch easily) that come on these engines torque to 3-5 foot pounds (6mm) and to 12-17 (8mm) foot pounds. Converting to inch pounds equals 36 to 60 for 6mm, and 144-204 inch pounds for 8mm. I trash all the head, intake, and exhaust OEM studs and replace with US grade 5 (grade 8 is overkill). Now a grade 5 can take more torque then the OEM studs, but the case can't. I use 50 inch pounds for 6mm and 150 inch pounds for 8mm and red locktite, and haven't had a single problem. On my first build and before I changed the studs out, I blew a head gasket. To be honest I don't know if it was the cheaply made studs or they were not torqued to specs. SBP sells a thin gasket and I've had issues with them. The piston would hit them (slant head), as the diameter of the bore hole was smaller (about 1/16 to 1/8"). I don't know if they were a bad batch or what. Make sure that the head mating surface is true (flat). Out of 3 engines I haven't seen one that the head DIDN'T need surfaceing...As far as I'm concerned!!. Factory engines usually have 2 head gaskets...Remove one and save.
Picture 2 Showing partial shave. RED lines shows milled area and BLUE shows unmilled area.
Picture 3 Showing completed milled head...same thing was done to the cylinder.
Now for something you are going to think I'm out of my head...Increase your plug gap up to .040...I have two engines that run much better with a .040 gap.
We have experimented with a boost bottle...To tell the truth it made a noticeable difference on the two of the bikes I built (had to turn out the idle screw out at least two turns). One we didn't experiment with as a before and after test. I don't know if that was the reason we went to .040 on the plug. Much better top end now. We have a hill in the neighborhood and I use it as a testing guide.
The bottles are easy to make (PICTURE 1)from PVC. Each end cap holds 5cc and 3-1/8" of schedule 40 holds 57cc's...for a total of 67cc's. Obviously this is for the so called 80cc engine. If you use schedule 20 the length needs to be shorter as the inside diameter is larger.