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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Liverpool Brick on October 05, 2012, 06:29:37 AM
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Hi I have a'91 K75S 75k miles and its been good to me, until recently. It won't start from cold when the choke is in the first position, I have to shift it on to the second position. This is new, it always started on the first position. In the summer I had replaced the filters and plugs and fitted a new battery but thats all - no tinkering with stuff. Any ideas what the problem is?
Thanks in advance
Andy
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When was the last time the bike was tuned-up, Andy?
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The lever marked "CHOKE" is not a conventional choke but only a means of increasing the idle speed till the engine warms enough to idle at the normal settings.
Any enrichment of fuel for starting is done by the ECU according to ambient temp and engine temp and when you push the starter button the ECU gets a signal to enrich futher for easy starting.
Wether the choke is in position 0, 1 or 2 it gives no fuel enrichment to aid starting. I think it was a sick joke by BMW but thats the way it is.
With the L Jetronic system Bosch recommends that you do not open the throttle while starting. My K100 was very susceptable to opening the throttle when starting in that it just would not fire at all until the throttle was closed and left alone.
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How dirty is your air filter? The choke lever opens the butterfly valve in the throttle body to let more air into the motor during cranking. The air flow meter senses the amount of air and the L-jetronic triggers the injectors. A dirty air filter could be "choking" your airflow...
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The 'Choke" as previously notes is only a throttle advance. It is not an actual choke Did you gap the plugs before installing? When was the last time the O2 mix was checked. If the the computer thinks the mixture is too rich, it will lean itself out possibly the reason you have to use position 2 rather than 1 or none as the increased throttle opening would provide more fuel (richer mixture)
It may also be due to dirty injectors, a 91 with 75K on the clock may need the injectors cleaned.
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Hi Guys and thanks for your input,
Firstly the bike has not been "tuned" since I've owned it (2 years). I gapped the plugs as per spec. but will recheck and will get the mixture checked. It sounds not too serious unless its an ecu problem which I think is unlikely. I will also check idle speed when warm etc. and adjust as necessary. If you can think of anything else . . . .
Thanks again,
Andy
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Andy,
Get a Carbtune. http://www.carbtune.com/ (http://www.carbtune.com/) They're right down the road from you in Belfast.
Super easy to balance the throttle bodies (http://www.motobrick.com/index.php/topic,364.0.html) on your S too since there's nothing to remove.
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Hi Tim,
You learn something new everyday - I shall be in Belfast on Tuesday and will go see them. Thanks
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An air leak would make the bike run/start lean. Have a look at the crankcase breather hose and do the brake-cleaner-test.
Keeping the throttle closed during starting ensures the ECU enriches the mixture, this is one of the functions of the TPS.
Only adjust the throttle/synchronise the bike when you are certain you don't have air leaks, the valve clearance is OK and the air filter is new/clean.