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Miss Fire when Applying Brakes at Idle.


bmwbrian

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When I first start the bike and ride off and stop at lights the engine missfires. If the brakes are not applied the miss fire disappears. Has anyone else experienced this? and If so did you get a solution for this?

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Depends upon the brakins system of your particular model. If equipped with the servo brakes, the servo motor draw could be enough to reduce voltage to FI components with a weak battery. Other year I'm guessing wiring harness problem.

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The bike is a 2005 R1200RT with integrated brakes. One more thing. Once the bikes warms up, I'm thinking the battery is fully charged, this stops happening. Does the computer control the alternator output at idle? Maybe this is what's happening?

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IS it a true misfire, or jsut a rough idle. The Boxers are known ot idle rough below a certain RPM. The servo motor could be putting enough load on the motor (through the alternator) to pull down the idle speed or even battery voltage.

 

If the battery is getting weak it might also cause a drop in operating voltage. Although normally I'd expect the ABS light and battery light come on in that case.

 

When is the last time valves were checked and idle synch was done? Thsi might also effect performance at idle.

 

I'm just making sure it's a misfire. Misfires make a distinctive exhaust note in the OEM exhaust... almost like a soft poping sound... as opposed to the "normal" low idle "shudder".

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Applying the stupid power brakes causes the motor to stumble at idle when cold. Not really stumble but like it's working harder. Add to that the stupid wheeezzrrrrinnnggg C3PO servo sound and you've got a real symphony. When the motor warms up it doesn't react to the brakes as much at idle.

 

After two model years of the 12RT, insightful BMW engineers came to the brilliant conclusion in 07 that motorcycles don't need no power brakes. Love my 06 RT. Hate the power brakes (although they are effective).

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John brings up a good point that I failed ot mention. When cold, and engine has much less NET power output available, especially low RPM's. This is because much of the energy released in combustion goes towards heating up the engine block, heads, transmission, fluids, etc.

 

I think it's a matter of the automatic idle not adequately compensating for hte additonal load. BMW probably needs ot change the ECU to raise the idle speed a little high at colder temperatures or keep it at a faster idle a little longer. Another solution would be to specifically raise the idle when the brakes are applied.

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Mine works perfectly so the system does what it was intended to do. It works so MUCH better than any other motorcycle I have owned including the 2005 Boxer Cup, which requires choke to keep it running when very cold.

However, from my viewpoint as a former automotive development engineer, I can see that it would take less than some other bikes to get a poor idle out of it when the brakes are applied when very cold. A marginal battery, marginal charging system, slightly too-low idle speed, dirty fuel injectors, etc. Extra lights or other big electrical draw might contribute also.

BTW, I noticed improved idle out of mine each year when I have added Techron FI cleaner. I have recently gone to adding that twice a year to keep the injectors and combustion chambers clean. That is suggested additive by several auto manufacturers although several really new designs don't seem to show noticeable benefit - like my Honda Ridgeline.

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Yesterday morning I did some more experimentation on what is happening. The brakes are actually putting a load on the alternator which is bringing the idle down. When warm this still happens but not as noticeable. The idle when warm only drops around 100 rpms. When cold around 300 rpms. So I guess it's the nature of the beast.

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When I start my 05RT from cold it (usually) seems to start on one cylinder then pretty quickly goes to two.

 

This consistent with others? I notice this because I have a fairly steep driveway departure situation and do not want to stall on the way out- usually with my passenger.

 

I have been told from the start with BMW's, do not warm up, just start and go... in your opinion is this true under all conditions?

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Yesterday morning I did some more experimentation on what is happening. The brakes are actually putting a load on the alternator which is bringing the idle down. When warm this still happens but not as noticeable. The idle when warm only drops around 100 rpms. When cold around 300 rpms. So I guess it's the nature of the beast.

I haven't ridden my RT for 3 weeks due to the flu that won't go away. This morning I walked out and started it for the first time. The idle went to 1250rpm. I applied the brake for the first time after the bike was running for no more than 5 seconds. I applied it several more times for various lengths of time during the first two minutes. I could hear a slight change in speed during most brake applications but could not get it to drop to 1200. Adding the load of running the high beams (3 big bulbs instead of 2) made no change at all. Don't forget that "cold" around here this morning is about 53 degrees.

I think your bike is not running as well as it should and you are seeing a big drop with the load.

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