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Discussion Starter #1
Hi All
Anyone have any ideas as what the problem might be with the ST1050, it will start and run with throttle but it cuts out every time when it should just return to tickover
TIA
 

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Yes, the Stepper Motor/TPS is what controls idle speed on a 1050. Generally speaking it's not a maintenance item and will usually remain trouble free. However, you have an issue so the link that Fredsprint provided is a logical place to start.

Before you get in there doing detailed checks you can do a quick analysis to help diagnose the issue. You say it "will start and run with throttle but it cuts out every time when it should just return to tickover".

Questions:
  • Will it start without touching the throttle grip - either when warm or cold?
  • When cold will it hold a high idle - again without touching the throttle grip?
  • Does it only cut out after you rev it or does it refuse to idle at all?
  • Can you hold it at idle speed (1100 rpm as I recall) by holding the throttle grip?
The answers will help determine if the stepper is not holding the throttles in the correct position or whether it's another issue where the bike just won't run at idle speed at all - even if the stepper is working correctly.
 

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Discussion Starter #5
I have only just picked the bike up this evening and the previous owner has told me a little more about it's history

The bike will not start without any throttle
It will not hold any idle hot or cold
Will not hold idle at all
Can't seem to get it to hold at 1100rpm

A couple of other things I have found out about the bike this evening:
Reg/rec was fried, replaced then the second one also fried, Stator changed then new reg/rec fitted all seemed ok,
When held at revs the bike overheated and spat coolant out, not sure where from,
Hope this extra info helps in some way
 

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The bike will not start without any throttle
It will not hold any idle hot or cold
Will not hold idle at all
Can't seem to get it to hold at 1100rpm
The stepper motor should hold throttles open slightly for cold start, warm-up and to maintain idle once the engine is warm. The first three answers suggest it's not doing that but if you cannot hold idle speed with the throttle, that makes me think the stepper motor won't either - even if it's working correctly.


  • I would start with the easy step first - fresh fuel.
  • Next, verify that the stepper motor is working. When the ignition is turned on, you should be able to hear a slight whirring sound as it cycles (once the fuel pump has pressurized)
  • If you don't hear the stepper cycle then you'll need to lift the tank to take a look. See if the idle speed control lever that Westozbob mentioned moves when the ignition is turned on. The throttle butterflies will open slightly but you won't see those with the airbox still in place.
Once you get beyond those things it has less to do with maintaining idle and you start checking for things that make the bike cut out at lower rpm.

ISC (Idle Speed Control) Stepper Motor & Throttle Cam. You should see that arm move when ignition is turned on
 

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Discussion Starter #7
The stepper motor should hold throttles open slightly for cold start, warm-up and to maintain idle once the engine is warm. The first three answers suggest it's not doing that but if you cannot hold idle speed with the throttle, that makes me think the stepper motor won't either - even if it's working correctly.


  • I would start with the easy step first - fresh fuel.
  • Next, verify that the stepper motor is working. When the ignition is turned on, you should be able to hear a slight whirring sound as it cycles (once the fuel pump has pressurized)
  • If you don't hear the stepper cycle then you'll need to lift the tank to take a look. See if the idle speed control lever that Westozbob mentioned moves when the ignition is turned on. The throttle butterflies will open slightly but you won't see those with the airbox still in place.
Once you get beyond those things it has less to do with maintaining idle and you start checking for things that make the bike cut out at lower rpm.

ISC (Idle Speed Control) Stepper Motor & Throttle Cam. You should see that arm move when ignition is turned on

Thanks for all that info, I will try and take a look at the stepper motor and other bits tomorrow and let you know how it goes.
 

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Definitely go with fresh fuel to test and check vacuum hoses also. Mine would only start on starter fluid when I got it. Hoses were toast along with fuel. Yellow nasty and also change the fuel filter.
 

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Still the TPS is a possible culprit. Worth checking w/ TuneECU or a voltmeter the TPS position and or wiper voltage when butterfly closed or practically closed (aside from other suggestions).


Fred
 

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Discussion Starter #10
Just a bit of an update, tank removed and old fuel is drained, I removed the airbox to expose the stepper motor, It moves back, then forward when the ignition is turned on, however I did notice that a couple of times it sort of hesitated then moved again , very slightly but it wasn't a continuous movement, not sure if that makes any sense, also the throttle cables are a bit stiff and the butterflies don't return to closed after opening the throttle.
So, new cables are going on tomorrow evening, I have also today received another throttle body with tps and stepper motor so I think I may change it over just to see if there is any difference in the motors,
then refit airbox, tank , new fuel and give it a try, fingers crossed
 

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the butterflies don't return to closed after opening the throttle.
On the 1050 the idling is managed differently from the 955 where additional air was brought aside from the butterfly.
On the 1050 the stepper slightly open the butterfly. and regulates the idling that way.


I would have start by applying the champ procedure (also depicted in the workshop manual) and see if voltages and free play were OK.

However if you change the TB you'll have to apply it anyway.



Fred
 

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Before you refit that airbox put a little blob of Hylomar or similar on that one IACV line which isn't secured with a clamp. That sucker can pop loose and cause problems.
 

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Discussion Starter #13
So I changed the throttle bodies, replaced the throttle cables, fresh fuel, still the same, the bike will only start with throttle, can't get it steady at any revs, the management light is also on now, I didn't notice that last time , yellow light on the dash
I think it is time for the manual to come out,
Can anyone guide me in the right direction for what I need to purchase for the Tune ecu
 

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As I wrote you CAN'T change the tb w/o plugging a computer to your ECU.

First go to the TuneECU site and read the instructions, then chose in regard to your model whether you go the pc route or the android route. For a sprint I would go pc. Keep in mind you can't purge abs w/ the pc version.

Pc means kkl vag cable or other compatible usb interfaces.
Android mean blutooth elm327 compatible interfaces.
You may use a usb adapter plus an OTG adapter cable on android but it depends on you android version and it is more funky.

For the rest read Champ sticky.

Fred
 

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Discussion Starter #15 (Edited)
Didn't read that bit, everything ordered and now just waiting for the bits to arrive and then download the tune ECU.

Give me a carburettor
 

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Discussion Starter #17
Plugged in a code reader today while I wait for the tune ecu to arrive
Got Code P010B and stated Mass air flow sensor short circuit,
My question is would this cause the idling issue, I know I need to get one but jus wondered if this could be my problem
 

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Plugged in a code reader today while I wait for the tune ecu to arrive
Got Code P010B and stated Mass air flow sensor short circuit,
My question is would this cause the idling issue, I know I need to get one but jus wondered if this could be my problem
Yes, it could. The Sprint doesn't have a Mass Airflow Sensor. Code P0108 would refer to the MAP Sensor. That's located on top of the airbox.

From my experience, the engine will still start and idle with the MAP sensor disconnected. However, a high voltage reading may be throwing things too far out of range. A faulty MAP sensor will definitely affect idle and you have idle issues so, fair to say, it's at least part or maybe the whole problem.

If you don't have another MAP sensor you could try disconnecting it and see what happens. It's quite common for owners to forget to reconnect it after removing the airbox. Usual result it poor idle and rough running. When you refit a new one don't forget to reconnect the vacuum hose as shown in the photo.

 

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Discussion Starter #19
Yes, it could. The Sprint doesn't have a Mass Airflow Sensor. Code P0108 would refer to the MAP Sensor. That's located on top of the airbox.

From my experience, the engine will still start and idle with the MAP sensor disconnected. However, a high voltage reading may be throwing things too far out of range. A faulty MAP sensor will definitely affect idle and you have idle issues so, fair to say, it's at least part or maybe the whole problem.

If you don't have another MAP sensor you could try disconnecting it and see what happens. It's quite common for owners to forget to reconnect it after removing the airbox. Usual result it poor idle and rough running. When you refit a new one don't forget to reconnect the vacuum hose as shown in the photo.


I will double check everything tomorrow just to make sure I have reconnected everything, thanks for the quick reply:smile2:
 
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