I bought this bike after my KTM was stolen to cheer myself up. It’s a great middleweight all rounder and I like it so much I have kept it for years – 6 to date. I much prefer it to the 600cc inline 4s I’ve owned because the engine doesn’t have to be revved hard to give strong acceleration, and the bike turns more willingly at speed (cos of the narrower crank) making the handling more enjoyable. Bridgestone T30 tyres match this perfectly.
It’s been very reliable in that time with only one intermittent problem with the fuel pump. Very rarely the fuel pump doesn’t switch on – normally it makes a whistle so it’s easy to hear when it hasn’t turned on – and simply switching the ignition off and on again fixes it. No bother. And that’s it – no other problems.
However enjoyable the bike is, it is far from perfect as standard so it has been modified.
The brakes are good but lack some power. The original pads are HH sintered so it seems it is the budget 2-piston calipers: not an easy upgrade. So I replaced the brake hoses with braided steel and bled the brakes, and this improved the braking enough for me to be content.
The seat pushed my hips into the tank so I took off the cover, added many thin layers of foam at the front and re-stapled the cover on. Another easy fix.
These aren’t wide enough so I used some 25mm long nuts – threaded hex tube – to move the assembly further out. The hex tube is covered in rubber tube to tidy up the look.
The front springs are too stiff and the rear shock is a very cheap unit with poor damping control so the back doesn’t have any feel.
At the back I replaced the rear shock with a Hagon and that made a huge improvement. The first Hagon blew its seals but they replaced it free under warranty. Great service. The second shock was even better!
For the front, I added the fork tops from a 1994 FZR600R to add pre-load adjustment. I also added a 70mm long spring (with shorter inside tube) to lengthen the front spring, reduce the spring rate and increase the travel. I fitted Gold Valve Emulators to improve the front damping. Over time I adjusted them, the fork oil and the preload to get the feel I wanted for Hong Kong’s bumpy surfaces. I ended up with 2 3/4 turns on the emulators, standard oil viscosity but 70% of the standard volume, and on the maximum pre-load.
I also made a rear hugger from part of the number plate support and a section of front fender.
The air filter is already excellent so I only modified the airbox intake tubes for more intake roar – I kept the rubber bellmouths but cut off the tube length. This maintains the smooth flow but allows more noise – and it sounds lovely. I hear a slight intake whistle under 4k rpm and a growling roar over 5K. Addictive noise! On the throttle bodies, I connected the two together with a small bit of rubber pipe – this reduced the slight jerkiness on a very small throttle opening in the mid-range. I tried various mods to the pipe that connects the left throttle body to the manifold inlet pressure sensor but none were an improvement. I also removed the exhaust air intake valve assembly, piping, etc. and soldered a 22 ohm 5 watt resistor into the circuit instead. This was mostly to remove weight, create space at the front of the airbox, and stop this valve creating pressure changes in the airbox. The airbox outlet on the top was blocked off and I made a new plate for the top of the engine so it would be flat. The reed valve that sits in the top of the engine has a rubber seal so I kept that part but removed the valve blades and retaining screws.
Although the OEM filter is excellent and cleanable, I ordered a DNA air filter to try, and when that arrived, I opened up the airbox a little more. Although the airbox intakes are adequately sized for the 38mm throttle bodies, they draw air from right above the engine/radiator where the air is hot, and there is a barrier plate immediately in front of them which might be disturbing intake flow. Later airbox designs for this motor pull air in from the side and not the front. So I added a third intake port in the centre, so this increased the total intake area to roughly the same as two 45mm diameter throttle bodies. This is more than enough for the engine but isn’t so large it drops the pressure in the airbox. I also blocked off the crankcase breather and fitted a separate air filter for the crankcase. This is the first prototype of the airbox, which had the centre intake not fitted quite right – notice the 2 halves of the airbox don’t fit together tightly enough.
I also removed the barrier plate from the engine to the radiator, and made some brackets to move the radiator forward and down about 1 inch, the maximum the current radiator hoses allow. I added a fabric-reinforced thick rubber shield over the top of the engine from the throttle bodies to the top of the radiator to create a smooth path for cool air from the front of the bike. I added a grille above the radiator to trap larger bits of dirt etc so they don’t get sucked straight into the airbox and clog up the air filter.
I was planning to get an after market exhaust but first I tried modifying the original. I got a long 10mm drill bit and drilled a small hole from the exhaust output pipe through to the adjacent chamber – this bypasses 2 middle resonant chambers designed to create back pressure in the mid range and limit noise to pass testing. The bypass reduces the tuned back pressure so the engine breathes better throughout the range and especially in the mid range, so the mid range flat spot has gone. It also pulls from lower down in the range without seeming to have knocked anything off the top. It’s all good!
Strangely it doesn’t increase noise perceptibly and because the cat is unchanged it won’t change emissions. However, it does make the engine leaner – a noticeably sensitive throttle and surging. So I added a voltage booster to the output of the manifold inlet pressure sensor – the same design as the one I used for the SYM scooter. 3% extra voltage (1.03 gain) was still a little lean (very slight lean surge) so I tried 4% next and that works great – no surging anywhere and a lively response. Actually, she pulls really hard from about 4,000rpm and there’s no flat spots – a much much better power delivery. The DNA air filter needs a boost of between 2% and 3% so the total is around 6.5%.
The engine is now very flexible, accepting throttle from 2,000rpm up in any gear. Two up and going up hill, it needs just under 3,000rpm to avoid a snatchy acceleration. The noise is a gobble-gobble from 2 to 3, then is a mute whistle until 4K when induction noise rises significantly and the engine pulls harder and louder until the redline. The torque band is so wide that I don’t need to be constantly changing gears to get hard acceleration – like riding a much bigger engine but without the top end craziness.
Except for the DNA filter, this tuning is very cheap to do – a 10mm drill bit, less than 10 bucks on electronics, some tubing, some aluminium, some rubber sheet. Even including the air filter, it’s less than US$100.
Of course, these changes improved the acceleration noticeably so I re-geared the bike with a 16 tooth front sprocket (JTF565, also a 43T rear does the same, JTR478). This makes highway cruising more comfortable and the speedo is now dead on accurate too. First is very tall but still acceptable (just) for car park speeds.
And the rest…
I replaced all the bulbs with LEDs and replaced the turn signal flasher with one designed for LEDs so I didn’t need to use resistors. I also made my own rear number plate support with LED plate lamps.
And fitted Kawasaki’s crash protectors.