Tuning the Suzuki Bandit 1250

This is a topic that comes up in bits and pieces all over the Net, so it's worth recording my experiences in one place.  This is what worked for me.  If you aren't confident, competent or know what you are up to, don't do this.  Do it at your own risk.

ADJUSTING THE THROTTLE POSITION SENSOR (TPS)

Firstly, it's worth getting some baseline figures on the Bandit's injection system.  Purchasing a dealer mode switch part number  enables you to check the fault codes and the throttle position sensor (TPS) setting.  It should be -C00 with the cursor in the middle position as shown below.  If the cursor is elsewhere (i.e. up top or down low), the TPS is slightly out of adjustment.

Any other codes, check the manual.

To adjust it, you will need the bike in dealer mode.  The connector is under the right hand side cover with a white dust cover on it.

Connect the switch (you can short the plug with a paper clip to achieve the same effect), turn on the bike and turn the switch "on".  If all is well you will get -C00.  If the cursor is anywhere else, find the TPS, which is on the left hand throttle body, and slightly loosen the two security Torx bits, which are type T20H with the hole in the middle.  Do not remove them, and do not remove the TPS from the throttle body.

You will see the cursor move with even undoing it, so take time and a little patience to get the cursor in the right place.  DO NOT overtighten it, because you may crack the housing.  In theory you should adjust it with the throttle stop screw between the throttle bodies, but it's buried deep and I don't know anyone who has managed to get to it.  Is it critical?  Suzuki say good is C00 and don't mention the cursor.

BALANCING THE THROTTLE BODIES


Do this after you have replaced the plugs and cleaned/replaced the air filter.  I don't plan to list those steps here, as if you don't know how to do that. you shouldn't be messing with this.  Balancing the throttle bodies will give smoother idle, smoother low speed running and reduced surging.

To do this, you will need a 4 carb balancer.  I use the Morgan Carbtune Pro:

There are various schools of thought on balancing, but I used the Suzuki method listed in the service manual as "Use of Mode Select Switch".

You put the bike on the main stand, and remove the tank and prop it up so you can access the throttle bodies with the fuel hose and fuel pump still connected.  I use a Metrick metal (www.metrickmetal.com) tank prop:

  Connect the dealer mode switch, and run the bike until it is warm.  Wait until the fan kicks in (don't adjust anything if the fan is running though).  The dealer mode switch automatically sets the bike in synchronisation mode.  (Some people don't use the switch and apply vacuum to the IAP valve, but I don't think it's necessary).

Stop the engine and attach the Carbtune lines to the vacuum ports on the manifolds.  The number 1 line is shown marked below - pull it off an replace it with the Carbtune line and repeat for the other three.

Make sure the mode switch is"on" and run it.  Check the 4 readings are the same and use number 2 cylinder as the base reference.  Getting the 4 columns even is the aim.  The speedo will show "C13" which means the IAP isn't getting vacuum because it's disconnected from the manifolds.  Ignore that

The levels are adjusted with the air screws on the throttle bodies.  The air screw by the number one injector is shown below.  Turning it clockwise makes the column rise.  Don't unscrew them too much - small turns are all that are needed.

Give the throttle a "blip" when you think it's right, as you may have to fiddle a bit.

If you get it right, smooth running results.  If your bike idles poorly afterwards, a common fault is due to leaving the vacuum lines disconnected from the IAP sensor.  Any vacuum leak means she won't run.