OK, so another update.
I’ve continued struggling for revs and power with the Puch head, so after translating the work of some trailblazers in Japan with these bikes, I went back and gave the factory head a shot. Then viola, - back past 10k rpm again with a bit more speed. It then became clear that all I needed was some more compression.
However, I was surprised that the custom 46mm cut NU50 head (like this one: https://www.treatland.tv/honda-nu50-46mm-custom-cut-head-p/46mm-cut-head-urban.htm), would affect compression so much. It wasn’t until I decided to fill all three heads with liquid did I realise the difference in volume. From left to right, oem head -> custom 46mm cut NU50 head -> 46mm Puch head.
The oem head came in at 7.5ml, and the other two were around 10ml. The real question for me was how the custom NU50 head gained so much volume when the only expected change was machining the outer rim to 46mm. Well… it looks like the reason owes to the bowl being machined too! For whatever reason, the bowl section has a lot less meat than the factory head.
As standard (49cc), I calculate that the CR would have been something like 6.5:1.
Once I went to 70cc and the bigger heads, CR would have gone to something like 7:1, but for whatever reason still ran shit up top.
With the current oem head paired with the 70cc kit, CR is probably 9.3:1 ! Maybe that’s why the bowl is machined out of the NU50 custom cut head, - to keep something similar to a factory compression ratio?
Either way, for the moment I will continue to run the oem head, until such time that I can machine down the Puch head to give the double benefit of extra cooling and more compression.
Part 2 of my experiments to follow next…