Pekka's ZXR750J/K


Pekka Kuronen from Finland has put a BigBored, Race Kitted engine from a ZXR750R (K) into his ZXR750J
Here's his recipe for 130 rear wheel bhp!
Here's some info of my bike:

• '91 ZXR750J
• Stock J1 fairing and body
  
• Stock front and back suspension (back 'pump' could be better)
  
• Stock wheels
• Stock J1 Gearbox
• the engine is "just a little bit modified" ;)

The Engine mods are:

• Engine from ZXR750R (K)
• Racekit exhaust full system
  
• Racekit cams (plus kit valve springs, stainless retainers)
• I'm not sure whats the exact timing of cams (Done by an expert) but it's around 102,5 in.ATDC / 104 ex.BTDC degrees (cam lift center)
• Ported and modified head
• Keihin's FCRs 39mm (mains 145 (a bit too big with airbox), 60 pilots (too big, but tested and worked for the bogging), airjets 200, air and pilot screws 1,5 turns open)
• Wiseco's 765cc pistons (72mm diam.)
• I'm runnig with stock airbox and filter, but I am thinking of putting kit velocity stacks (plus home made airfilters on them) and removing the whole airbox.
• Stock J1 gearbox with 17-45 sprockets
• Racekit ignition (rev limit 13600 as I remember) plus stock timing rotor

I got the engine's dyno results from the expert who sold the tuned kit engine to me and the best run were DIN 138hp@12250rpm, 80.2Nm@12250rpm and SAE 134.8hp@12250 and 78.4Nm@122500 at the back tyre.
It pulls better than stock through the rev area, but I have to be careful not to twist the throttle too much under 6000 rpms, otherwise it bogs down. Must be the adjustment of the carbs (the pilot jet and accelerator pumps, I'm gonna work on that during the winter and get the best results when the snow melts in April).
On the race track the gearing feels a bit too long and I was thinking of putting a smaller front sprocket on it. This is because the power is so high in the rev range.

The engine is currently being rebuilt. After this is done I will post the latest Dyno figures.

More on this later

Pekka