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Discussion Starter · #1 ·
Surging isn't the right word, but can't think of a better one.

When I first start the engine, the idle varies about 300 rpm. Up, down, up, down. As the engine warms up, it surges less, i.e. about 100 rpm.

Others have this? Or is mine just rarin' to go? I changed the oil at 600 miles myself, so didn't have the dealer inspect the idle as specified.
 

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I noticed that on mine as well. It will idle high for the first couple of minutes then it will drop to a lower RPM. The second I gun it, the high RPMs start again, but then it throttles high then drop low then back to high again. Once up to temp, it's steady. I wouldn't fret about it.


Dealer did my 600 mile service, an it was performing the same before and after the service. No need to worry. Just saddle up and ride....
 

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Not sure if anyone has noticed this, but the rpm increases about 300 rpm when you let the clutch out a little bit. I assume it is to help keep you from stalling it out when starting out.
 

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Discussion Starter · #4 ·
I noticed that on mine as well. It will idle high for the first couple of minutes then it will drop to a lower RPM. The second I gun it, the high RPMs start again, but then it throttles high then drop low then back to high again. Once up to temp, it's steady. I wouldn't fret about it.


Dealer did my 600 mile service, an it was performing the same before and after the service. No need to worry. Just saddle up and ride....
Yeah, but no. I mean

  1. start the engine.
  2. sit there in neutral.
  3. higher RPM - yes.
  4. But the engine will surge up and down.
Later on, when the engine is warm, I can stop at a light and sit there with the clutch in. The engine will still surge up and down, but not as much.
 

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I think this is a design feature of this bike. My dealer explained it to me when I took delivery on the bike, but I was too excited about getting on it and riding that I didn't pay much attention to what he was saying. After mine warms up it settles down to a steady idling rpm, so I haven't worried about it. Next time I'm at the dealer I'll ask about it (if I don't forget!).
 

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There's more than one chapter to the issue:


  1. "higher idle when cold" is the bike ECU's way of simulating a choke (since this bike doesn't physically have one). It controls the air allowed into the mixture (with throttle closed) by means of the IACV, which has a rather coarse adjustment range. It's enough to start the engine when cold and keep it idling until it warms up, but not enough to please human ears/eyes
  2. "the engine rpm goes slightly up when in gear and releasing the clutch" is, indeed, a feature of this bike's ECU -- maybe that's why I never ever managed to stall it yet :D. If it were open source (like it should be), I could actually point you to the source code line that does that, but since it's not you'll have to take my word for it (which is wrong on so many levels but there you have it)
  3. "hot or cold, the idle rpm reading wobbles around the target value and doesn't just stick there" is also a "feature" of the ECU. In order to do all the needed measurements and math in real time, you would need a CPU 10 times as fast in the black box. Keihin (ECU vendor) being cheap dinosaurs as they are, they reused one of their ancient designs and to fit all the math in, they average everything over 1-2 second intervals and then adjust the injector pulse and IACV position to match. This is a cheap trick to make sure the ECU can always cope with how fast things change in the engine.
Unrelated to the question but related to the context of the answer: another "cheap dinosaur" feature of the ECU is the fact that it only has two channels for controlling ignition coils, thus the wasted spark setup we have on our bikes.
 
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Discussion Starter · #7 ·
There's more than one chapter to the issue:
[snipped]
3 "hot or cold, the idle rpm reading wobbles around the target value and doesn't just stick there" is also a "feature" of the ECU. In order to do all the needed measurements and math in real time, you would need a CPU 10 times as fast in the black box. Keihin (ECU vendor) being cheap dinosaurs as they are, they reused one of their ancient designs and to fit all the math in, they average everything over 1-2 second intervals and then adjust the injector pulse and IACV position to match. This is a cheap trick to make sure the ECU can always cope with how fast things change in the engine.Unrelated to the question but related to the context of the answer: another "cheap dinosaur" feature of the ECU is the fact that it only has two channels for controlling ignition coils, thus the wasted spark setup we have on our bikes.
Dex - you have provided a useful answer to my question, thank you.

My second part to the question - is my wandering engine idle speed common among CTX owners? [note more wandering when cold than hot.] I am surprised that I am the first one to bring this up, as far as I've seen with a search of the forum
 

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Well, my bike does it too :) Probably, people are too eager to saddle up and get going to waste time looking at the tach when the bike idles on the side-stand :D
 

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I guess I really never noticed much. Every bike I've had does vary the idle when first started cold. Doesn't seem to matter if FI or old style carbs, they would do the same bit of wavering +/- a few rpm during warmup. I didn't really pay attention after warmup while sitting at a stop... until this thread was started. I do vaguely recall my ST1100 (carb) would do that +/- 100-200 rpm, my CTX seems a bit more stable but will still sometimes vary maybe 100 rpm up/down. I figure it may have something to do with quality of the fuel in combination with ambient air temp and pressure (windy or not). My cages both do the same and have always that I remember. Never gave it much thought and really don't worry about it.
 

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Mine does it too and I love it, having come from a bike with twin carburetors and a choke. I'm all for letting the bike manage its idle speed and do whatever it needs to do to warm up. I wait for it to settle down after a minute or two and off I go with no second thoughts. :)

Jeff
 
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