Slow progress, finally have the godawful skunge cleaned off the 3KJ... took a few goes, in the end the heavy duty de-greaser won all the prizes. (naphtha, ethanolamine, propane and butane).
Taking the cylinder off revealed a piston in very good condition, some really sharp, and quite jagged port edges, a reasonably large boost port, quite high transfer ports.
The porting calc I use says (if I have measure everything accurately) 168, 119, with blow down of 24.5. Exhaust port is already at 68% width of bore, stock.
My plan is to use a 0.8mm base gasket, raise the exhaust port roof by 2mm's, keeping the existing shape, clean up the transfer port roof edges and this should come out at 180, 122, blow down 29, looking at peak power at 8000rpm, which afaik is where the stock pipe will run out of juice. Probably polish the exhaust port, but not remove any material, it's the D shaped port. Also smooth the skirt edges of cylinder and piston.
Interesting that the boost port entry in the base of the cylinder is so huge, and the path in so small over the cylinder skirt, maybe this accelerates the air/fuel flow??
The KN Planning pistons I bought seem well made, but the skirt has a dual window instead of the single. Can anyone recommend the best way to open the skirt up?
They were pretty cheap ($16usd) so no great loss if they don't work out. The 3KJ piston is looking fine anyway, so could just steal the rings from the KN Planning one. Haven't checked clearance yet.
Will also delete the head gasket, and use copper spray or not, as the squish is wider than 1.5mm currently. 3KJ has the wavy head, and I haven't looked but I think the 5BM will have the proper squish band.
I sourced some second hand gears and kick start assembly from an Aprio locally.
Main order of parts has arrived. Still need to get the rest of the bearings for both scoots, and then oil seals, rubber dust covers, o rings some nuts and bolts (screws) etc.
Hope I made the right choice with the yellow torque springs.. bit of a guesstimate there. Lots of steep hills round here though.
Have regulators for both already, and a tail light assembly for the 3KJ to replace the lights hanging off the back of the rear tray.
*https://www.aliexpress.com/item/4001119609662.html?*
Bought a really good selection of main jets from here. Only need maybe 3, but cost was the same in the end, and someone recommended them as being good ones.
*https://www.aliexpress.com/item/32242718025.html?*
I'm thinking of keeping the CV50 as Zino suggested, because it has a buggered exhaust, something heavy is rattling around inside, and it has the front disc brakes, do a slight restomod on the CY50. I'm going to have to reshape the nose on that one, as there is a big crack, and a little bit missing. Don't really want to repaint it.. so thinking of removing the damage, and reshape instead...
Bought them both for 700NZD or 430USD, so I figure I can sell one, and have a refurbished, fully unrestricted, road legal runner for free.. minus all the time spent lol...
CV50 was originally sold, restricted, with a 2.5kw (3.3hp) power figure I think. The engine should be able to do 7hp afaik with all the inbuilt restrictions removed so at around 75kg it is probably going to be fast enough for these parts. Power to weight *should* be pretty close to the PGO with 70cc.
with the provided measurements of 792-16.6-30.
