Hello Martyn
Martyn to get the NSR above 65+hp one thng you will need is $$$$$$$
First question is it for road or race?
1. Derestrict the engine (electrical) this is a MUST
1.a. (for race) buy a HRC wiring harness with derestriction box it also advances the timing.
1.b. If for road, buy a posh derestriction box.
1.c. if for road the cheapest way is to do a wire splice.
When you do one mod the rest must be done.
2 Get a aftermarket exhaust Tyga is the way to go.
because you have allowed more gases to ecsape you need to let more fuel in so.
3. (for race) buy a pair of pj38 carbs and jetted it properly.
you have nw allowed more fuel to get in so you also need more air to flow.
3.b. for road buy a hrc or jha jet kit.
4. mod your airbox to allow more air to rush into your carbs.
now if you still have money
5. port the exhaust ports to 1990 rs250 specs
6. buy a set of HRC or Tyga stuffers and cabon reeds.
7. lighten your flywheel for increased acceleration
8. buy a set of rs250 coils NOT CHEAP
9. do a f3 head conversion
now if you want to go further
10. machinen the heads to get higher compression and run avgas.
11. take the bike down to the drag races and pink slip your bike against any 600-750cc rember change the gearing and if all goes well you will have a few new bikes to sell.
But the best way is to just buy a 90-92 rs250 engine and pop it into the frame it's alot cheaper