Inactivating the Airbox Intake Flap

Many TDM900 users are experiencing engine problems at low rpm : engine jerkiness, irregularity of operation and overall a lack of engine torque.

Although the behavior of a twin-cylinder engine is very different from a four-cylinder one, it should be noted that the admission system of the TDM900 is equipped with an active device which is largely responsible for these problems. The reason why this device is present is because of the EURO normalization about intake noise under 4000 rpm.

This page shows you several methods to disable this device and improve the operation of the engine at low rpm.

All these methods are fully reversible, all lead to the same result : de-activating the air-intake flap.
• The presence of second market exhausts usually worsens the situation because they result in a lean air-fuel mixture. In this case the work-around proposed here will not always give a satisfactory result.

• The fuel mapping was changed in 2007, mainly because of the new EURO3 standard and also to try to fix the low rpm jerkiness issue.
Inactivation of the air intake flap will therefore give a less significant result with a 2007+ TDM. However the mod is worth a try as it's quick to do and reversible.



Below 4000rpm the ECU controls a solenoid which in turn controls a pneumatic actuator. This actuator closes a small flap placed in the main inlet air-duct of the airbox. Admission is then done through a smaller secondary air-duct.

The pneumatic control is carried out by the depression in the throttle body #2, by means of a hose and a pulsation damper device.

The solenoid is placed on the underside of the air-box.
The actuator is placed on the upperside of the air-box.


There is a similar device on some Honda bikes ( VFR-VTEC / FireBlade / RC45).
It is also present on some cars : Honda VTEC / PGM-FI, BMW, Mercedes, Lotus, Ferrari, Volvo, Toyota, etc.

The arguments used by manufacturers are varied, but the general policy is to pass a mandatory (EURO standards on admission noises below 4000 rpm) for a customer benefit : "increased speed in the ducts admission ", " improved cylinder filling at low rpm ", etc. Yamaha says that this function "improves conductability and reduces admission noises".


Below 4000 rpm

• Solenoid is activated by the ECU.
• Air-intake depression is directed to the actuator flap.
• The flap closes the main air-duct.
• Air flow goes through the smaller secondary air-duct.

Above 4000 rpm

• Solenoid is inactive.
• Air-intake depression is blocked.
• The flap opens the main air-duct .
• Air flow goes through both air-ducts.

What the rider actually experience

• Engine's smoothness is not very effective at low rpm.
• The engine seems less torquey under 2500-3000 rpm and does not seem to behave correctly.
• engine jerkiness appears around 4000 rpm.
• Users often think all those problems are due to a faulty ECU or injection issues.

Some explainations

• Engine is starved of air below 4000 rpm by the closed flap.
• The flap is under the control of the ECU, which triggers it around 3500-4000 rpm.
• Flap works in an full open / full closed mode, there is no progressive control.

An atmospheric engine will only draw as much air as it needs through the air filter. Any device that block the airflow will introduce a perturbation.

On any electronic fuel-injected engine the fuel-air mixture is permanently adjusted by sensing the atmospheric pressure and the depression inside the throttle bodies.

This fuel-air mixture is always optimal, no matter the altitude level or which air-filter is in use, without requiring any manual adjustment. In case the air-filter element is dusty, it will simply lower the engine efficiency the same way a closed air-intake flap do.

On the other hand, when the air supply is hampered by a dirty air filter or a partially obstructed intake pipe, the engine will still be fed by a correct mixture but its power will diminish significantly.

Why disabling the device ?

Disabling the air-intake flap is likely to bring some benefits :
• Better smoothness at low rpm.
• Low rpm torque increase, great on mountain roads.
• Better, faster rpm increase, even from as low as 2000 rpm, without knocking.
• No more jerkiness around 4000 rpm.

Please note that inactivating the air-intake flap will not give you more bhp / top speed. Only usability & efficiency will increase.

Here are three different ways to achieve inactivate the air-intake flap.

To achieve this, there are several methods, depending on whether we act at electric or pneumatic level.

Procedure A : disabling the solenoid

This is the quicker, easier and in my humble opinion the neater way to disable the device : a mod on the ECU connector to disconnect the wire that controls the solenoid.

• Remove the left-hand side fairing to access the ECU.


• Unplug the ECU connector.
• Locate the three white strips on one side of the connector.


• Unlock pins by gently pushing the white strips inside the connector using your nail or any flat screwdriver.


• Locate the Brown / Black wire #8 on the top row, right-hand side, see drawing below.


• Remove the wire from the connector and provide insulation to the pin - do not put it back.


• Lock pins by pushing down the two white strips that protrude on the other side of the connector.
• Warning : make sure that all other pins are still fully inserted inside the connector body before locking them.


• Replace the connector on the ECU.
• Replace the fairing.
• Switch on the main switch : no error code appears on the dash as the solenoid command is not monitored by the ECU.
• Done !

Procedure B : disabling the pneumatic actuator

This mod is more difficult but it's easier if done while servicing the air filter element :
• Remove both side fairings.
• Remove the black plastic covers under the tank.
• Unscrew the tank front screws & rotate the fuel tank backwards to gain access to the air-box.
• Unplug the hose (C) from the flap actuator (G) on the top of the air-box.
• Important : blank the hose off. I suggest to fit a bearing ball, a screw or a plug of the adequate size inside the hose.


Procedure C : disabling the depression hose on the throttle body #2

This procedure is somewhat complicated by the lack of accessibility, but no fairing / fuel tank removal is needed.
• Remove the hose (2) from the right-hand side throttle body depression plug.
• Blank off the hose with a steel ball.
• Replace the hose on the throttle body depression plug.
• Alternatively, fit a rubber cap like the one found on the the right-hand side throttle body (1).



Additional stuff

Now that the flap is disabled, all the elements that compose it become useless and may be removed during the next air filter maintenance operation.

All the parts are located under the air filter box :


1 - hose from the injector body #2 : disconnect and blank off.
2 - surge tank : remove.
3 - solenoid : remove.
4 - connector : remove.
5 - hose to the flap actuator : remove & save.

You may reuse the hose (5) and plug it into the injector body #1 (left-hand side), then make accessible the body injector #2 hose. This gives easier access when synchronizing the throttle bodies.

Do not forget to blank off both hoses with a screw or a steel ball, place them between the chassis and the oil tank for easy access.

(image) (image)

Thanx to Alan Eccles / Carpe-TDM forum, for information and advices.
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