Part #1: Purge The Surge
When I bought my current Bandit it suffered from the well documented low speed surging issue. It wasn't terrible but it was there and neither of my previous Bandits did it. Initially, I tried the basics: A full service with new plugs, air filter, throttle body synch and TPS calibration. All of which made no difference. I tried running on E5 petrol instead of E10 which also made no difference. I tried injector cleaner, again no difference. There were no stored error codes and otherwise the bike ran perfectly.
And so a process of elimination began: First I fitted an O2 Sensor eliminator which made no difference. Next up was the PAIR system. I didn't remove it, I just blocked it off but again it made no difference.
And then, after a lot of research, I removed the secondary throttle flaps. This didn't cure the surging either but it did make a noticeable difference: On a standard bike the secondaries shut everything down at 7,000rpm but without them the engine keeps pulling right up to 9,000rpm. The throttle response is more immediate as well. And removing the secondaries and remapping can easily net an extra 15bhp with a slip on exhaust and performance air filter (more on which below).
But none of this sorted the surging and I gradually noticed that the problem wasn't there when the bike was cold, on the cold map it ran just fine, so it was clearly a fuelling issue.
There are various tuning modules on the market which allow for adjustment of the fuel map. The Power Commander is the best known but the Healtech FI Tuner Pro does a similar job for half the price if you don't need the optional extras of the Power Commander.
Set up is simple, the unit plugs into the bikes diagnostic port and then taps into two of the main ECU wires. The Healtech unit itself is matchbox sized and simply velcros to any convenient spot under the seat. It has a USB port to connect to a laptop which, having downloaded the free app, gives pretty much full control over the fuel map. It is recommended to use an O2 sensor eliminator and a Timing Retard Eliminator which forces the fuel system to use the unrestricted 5th gear map in every gear.
The Healtech also allows on-the-fly monitoring and so, with a tablet PC taped to the tank, I was able to ride the bike in the “Surge zone” and see exactly where on the fuel map the problem occurred. It turned out that the problem area was the 2,000–2,500rpm range at 0-2% throttle openings ie the on/off throttle transition. And so I was able to adjust the mixture at exactly that point. I just added 10% extra fuel and the surging is gone. So yeah - the Healtech FI Tuner Pro. Recommended.
Part #2: Moar Powah!
With the FI Tuner fitted and the secondaries removed I was already halfway to a decent tune so I decided to carry on to a full "Stage 2". First up was cutting a large hole in the airbox lid. Next was a K&N SU-6000 filter, this is actually the listed part for the Bandit 1200 but it fits the 1250 and has 20% more filter area than the official 1250 part.
Most Stage 2s use a slip-on silencer with the standard downpipes and I did the same. I looked at various systems and they're all very similar: Smaller, lighter and less restrictive than the OE system. I went with a Delkevic X-OVAL 343mm: UK road legal, cheapish and decent quality. It even looks a bit like the Yoshi can that was once offered as an official accessory.
With the hardware complete it was software time. I started with Healtech's free map for after market exhaust/air filter and it felt pretty close save for a hint of lean surge around 2,500rpm / 35mph which I fixed with a quick adjsutment to the fuelling in that area. It's a work in progress, but for now it doesn't feel too far off the mark.
Part #3: Dialling It In
I ran around with my modded Healtech map for quite a while with no real issues. It felt okay at town speeds and wasn't short of power up top but in places it felt a bit fluffy and I suspected an over-rich mixture. So I bought a cheapo wideband O2 sensor kit to allow me to see what the Air/Fuel Ratio was doing in real time.
The kit comprises a sensor, a gauge and a control box. The control box needs a 12v power source and the other parts just plug into it. As this was a temporary set-up I just connected it straight to the battery positive terminal via an inline fuse and a switch. The earth wire from the unit was connected to the nearest convenient earth point on the frame. This saved having to tap into the bikes wiring loom, although it does mean that the unit must be switched on and off manually.
The wideband sensor has a larger thread than the standard O2 sensor and I could have just drilled out and tapped the original sensor hole, but I wanted the option to return the bike to standard so I bought a used link pipe and welded the supplied boss into that.
With the sensor fitted and the power hooked up it's just a matter of finding somewhere to mount the various bits and hide the excess wiring. I ended up using self-adhesive velcro to attach the gauge and I also velcro'd the control box to the fairing. As these components aren't waterproof, I also wrapped the control box inside a plastic bag. I did consider putting it under the tank but I wasn't sure if the engine heat would end up cooking it. For testing, the PAIR system must be disabled - I just plugged the hose. Job jobbed.
From looking at various dyno charts it seems that 1250s running a Stage 2 make the most power with an Air Fuel Ratio of around 13:1 so that's what I aimed for. As suspected, above 3,000rpm the AFR showed 12:1 (too rich) and above 4,000rpm it was around 11:1 (way too rich).
With this set-up I had no way of logging the AFR, so adjustment was trial and error. For the first attempt I ignored throttle loading and simply adjusted the mixture to be the same for all throttle positions at each rpm. So at 3,500rpm I just set +3% across the board regardless of throttle position. In the real world revs scale with throttle loading anyway and we just need the AFR to be correct where we actually ride, in the green zone of the map, and the rest is largely irrelevant. This is my first attempt:
This was already an improvement with an AFR of 13:1 at 3,500rpm and then 12:1 thereafter. Still too rich but getting there. After that, it was just a matter of tweak it, ride it and tweak it again until the AFR was on target. Below is the finished map, it delivers an AFR of 13:1 near enough everywhere save under hard acceleration where it dips into the 12s, which is pretty much perfect.
Final Observations:
#1 This bike prefers a rich mixture in the yellow area, where the AFR dips down to around 11.5. As stock this bike suffered low speed surging, so that may be a peculiarity of this particular machine, but any attempt to further correct the mixture in this area results in lean surge. And since it runs super smooth on these settings that's where I'm going to leave it.
#2 I’m surprised at how rich the stock settings are generally (maybe they rely heavily on the 02 sensor to trim the fuelling?) and how much fuel I needed to subtract in order to correct the AFR. This is counter-intuitive: I expected to be adding fuel but every Stage 2 map I’ve seen has a substantial subtraction somewhere in the midrange, so it’s definitely a thing.
I haven't dyno'd the bike but similar set-ups produce around 115bhp and that feels about right: As standard, power dropped off at 7,000rpm and now it pulls hard right up to the redline. Job's a good 'un!
