HP4 50 kit gear ratio question

[allu]

I'm building a v50 special for my daughter, and without going crazy, am looking to add a little to the performance - both acceleration and top speed. Maybe dad also gets to ide it occasionally

My plan is to install a HP4 top end, a Primavera gearbox (with the shorter fourth gear cluster), 19.19 and BGM touring pipe. All quite moderate in the big picture.

On splitting the cases I found a 22/63 (2.9). My worry is this may be a bit long and the hp4 will struggle.

My question is, does anyone have a decent suggestion for what primary gearing would pull best - or is the 22/63 a good place to start?

[henri]

22/63 is v90/v100 gearing ,i'd say a hp4 will be struggling with that .on 102 kits the lowest i'd go would be 24/72 ,3/1 sets , usually straight cuts .i'd guess that with a 19/19 carb ,torquey touring pipe an short 4th a hp4 will pull that .H

[jonnysnatchsniffer]

you could always skim 1 or 2mm of top of barrel and move cylinder up 1 or2mm, and get squish to 0.7mm, open up the inlet pad a little or cut 10mm off the crank
and widen the exhaust port a little but only above the transfers, this will make a couple of horses more, the hp4 does have a half decent port arrangement compared to the 50 special barrel, you can also radius the edge of piston as its an easy way to get more power, cant do this if it has a lesbian top ring tho
another thing to do is get a 50cc polini or dr kit, while not being mental it will at least make it safer to ride and still keep the primaries you have with the short 4th