Tuesday, December 20, 2005

Tuning 2) Idle, Base CO and Throttle Body Balance

-contributed by Brad with Moto-One Australia
MV Dealership, Workshop and Custom Eprom tuner
Moto One
1432 Dandenong Rd. Oakleigh
Victoria Australia LMCT 7644
Ph: (03) 9568 0100

Please contact Moto-One directly if you are interested in their custom Eprom tuning for MV! Their information can be found here:

-additional notes JamesC in Ital.

Step 1) of this process is setting up the TPS, found here:


Step 2) after the TPS (and optional eprom replacement) is this current document below.

*Optional step 3) of this process would be eprom replacement for tuning found here:

The idle trimmer is a little metal square just next to the eprom with a plastic rotating pot inside. This adjusts the idle mixture, working the same as the electronic idle trimmers in the 1.5M and 5.9M ECU. It adds or subtracts a given pulse width from the map fuel number across the whole map, therefore having a much greater percentage effect at small pulse widths.

This trimmer is adjusted by rotating it between the ends of its travel. The total travel is 270 degrees (3/4 of a full turn) as shown in the next photo so if you manage to make it go all the way round you’ve wrecked it and it’s time for a new ECU. Be very gentle! As with a mixture screw, clockwise is lean, anti clockwise is rich. The mixture screw is also very sensitive so it doesn't take much to effect a large change of CO %. The mid point, where the trimmer slot points directly at the eprom socket, is nominally zero. Best to use a non metallic screwdriver when adjusting this especially when you have the engine running and the ECU circuit board is live. You don’t want to be shorting anything out.
In the photo you can see the trimmer is set at about 60 degrees lean (which looks identical to 120 degrees rich).

As a starting point on the Brutale I recommend setting the trimmer around 35 to 45 degrees lean (clockwise from the mid point). This is the setting used when developing the Moto-One Brutale eprom and has proved to be consistant thus far. In fact, if this doesn’t give the right idle mixture CO% setting for your bike it’s worth rechecking your TPS setting or having a look at the fuel filter, that sort of thing. Other bikes may start at a different baseline position. Best to measure with your exhaust gas analyzer to be sure which direction you need to go. Below shows an '02 F4 750 which was zeroed exactly perpendicular to the eprom.

-JamesC , click to enlarge

To check the idle mixture CO% we need to fit probes to the two take off bungs in the LH and RH collectors of a brutale.

The Brutale is unique amongst the MV for having two take off points – the F4 models have only one as illustrated below.

-JamesC, factory manual

The issue with the Brutale is that while the exhausts are paired 1-2 and 3-4, the main fuel map and offset fuel map are paired 1-4 and 2-3. This means that checking in each header still gives a combined reading, not a dedicated ‘main’ and ‘offset’. It is still well worth doing, however, as they will vary when the air bleeds are set for balanced vacuum. The F4 models only having one sample port obviously provide only a 'main' reading.

At this point it’s just the usual balance and idle mixture setting. Initially wind all the air bleeds in fully – the air bleeds are the little brass screws at the fronts of the throttle bodies.

Now set the balance between the two butterfly pairs using the central balance screw.

The vacuum readings from cylinders 2 and 3 are the ones I use for this – I ignore cylinders 1 and 4 at this point. Usually I balance this at idle and find it stays fairly consistant when the throttle is opened to raise the RPM. Now this may seem easier said than done, but we use little right angled screwdrivers that do the trick amazingly well – see the photo below. Well worth buying if you do a few MV as we do. Access to the central balance screw is easiest form the LH side. If you don’t have one of them, getting someone to hold the tank up provides access.

Once you have this done wind the air bleeds out around half to three quarters of a turn (one on F4 750, one and a half or so on a F4 1000), balancing the settings using the vacuum readings of all 4 cylinders. This range should give the correct idle speed. There is no need for the air bleeds to all be at the same turns out – it would surprise me if they were. The vacuum is what matters here.

The factory specifies the following air bleed/bypass screw ranges as ideals:

F4 Oro: 1/2 turn to 1 turn air bypass opening. Idle 1250 +/- 50 RPM. CO 4 +/- .5%
F4 750 2000/01: 1/2 turn to 1 turn air bypass opening. Idle 1250 +/- 50 RPM. CO 4 +/- .5%
F4 750 Senna/2002/2003/2004/SPR/SR: 1 1/4 to 1 1/2 air bypass opening. Idle 1150 +/- 50 RPM. CO 3.5 +/- .5%
Brutale 750 2003/2004/2005: 1 1/4 to 1 1/2 air bypass opening. Idle 1150 +/- 50 RPM. CO 3.5 +/- .5%
Brutale 910: 1 to 3.5 turns air bypass opening. Idle 1200 +/- 50RPM. CO 3.5 +/- .5%

Because of the unique EBS system on the MV 1000 the procedure is somewhat different for that model (1000S, Ago and Tamburini). The factory specifies that you first adjust Cylinder #2 starting at 1.5 turns and try to stay as close as possible to that position while balancing the other three cylinders. The range for cylinder #2 is 1 - 2.5 turns. The range for cylinders 1,3,4 is 1 - 3.5 turns. End idle result should be 1200 RPM +/- 50.

Warm the bike up before setting the idle mixture. Most manufacturers specify engine temp of at least 65 degrees must be reached before setting idle mixture, but MV specify the idle speed and mixture be set with the radiator thermo fans running. The fans usually come on around 103 degrees C/215F, and luckily they warm up pretty quickly.

Now hook up your gas analyser to the take off from either collector on a Brutale or your single collector on F4. On the Brutale, once you have a stable reading on one hook into the other collector. We’re looking for around 3.5% CO as an idle mixture – certainly between 3 and 4%. You can use the idle trimmer to set the mixture in this range, but as stated above it should remain in the range of 35 to 45 degrees lean. For the Brutale, if the readings between each collector take off vary by more than 0.5% or so use the air bleeds of each cylinder pair to balance the mixture. This results in the vacuum balance being off, but don’t worry about that.

Winding the air bleeds in richens the mixture, winding them out leans the mixture. Make the necessary adjustments to either or both cylinder pairs as required to keep the idle speed and mixture correct– you will end up with the vacuum of cylinders 1 -2 balanced, and the vacuum of cylinders 3 – 4 balanced, and most likely a variation between the two pairs. This variation is not important (within reason). The air bleed position/idle speed/idle mixture interaction can be a bit of a chasing game at times, but you get used to it.

For a chart correlating the CO% to a/f ratios refer here:


The ECU position on the Brutale makes it a real pain to get to, but it’s worth keeping the ECU under the front of the tank and having the ambient air temp and pressure sensor connected. Hanging the ECU out the side with this sensor disconnected will give a varied reading based on the assumed values for ambient air temp and pressure the ECU will use when it’s not receiving input from the sensor. Using the idle trimmer setting mentioned above should ensure there’s little need for a second helper holding the front of the tank up so you can get inside the ECU.

The ECU position on an F4 allows for easy access to the trim POT. Key in off position, flip the ECU upside down and rebolt it to one of the ECU mounts on the subframe. Remember to keep the ground strap in contact with the ECU when securing it for the CO trim work described above.


At this point you have reset the TPS baseline, set the idle stop using either TPS voltage or the ECU degree reading, set the balance, idle speed and mixture. This is all pretty simple stuff – the TPS baseline setting is something we do at first service, the balance, idle speed and mixture something we do every service.

-blogged/edited JamesC


At 4:20 AM , Blogger kiwifruit said...

thanks for all this great info!


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