After getting my bike all sorted and riding it a ton, it is back to its old tricks of hanging idle when the engine is very warm (>20-30 minutes ride time).
The conventional wisdom here on gstwins.com seems to be that "hanging idle" == lean idle mixture. I would prefer to understand why. There is a post in the usenet archive that seems to disagree with this sentiment and says precisely the opposite: http://alt.motorcycle.sportbike.narkive.com/oHTaYnXK/blip-throttle-and-revs-hang-rich-lean#post4
Now, the reason I bring this up is that I hope to foster a real, technical discussion that will get to the root cause of the hanging idle, not just anecdotal "I turned the screws 3.5 turns out and it fixed my hanging idle" typical posts. I want to understand the mechanism of why the idle speed hangs up, and precisely why that points to a lean mixture.
In my case, the anecdotal evidence is that I now have the idle mixture screws at approximately 4 turns out which is basically wide open and I still have hanging idle with size 40 pilot jets. So this is why I am inclined to disbelieve the conventional wisdom that says it's running too lean. It's nearly impossible for it to be running lean at idle. I'm going to try to experimentally figure out what works on my own bike but I'd like to get some detailed discussion of why exactly hanging idle occurs:
One theory: the idle speed (throttle plate stop position) is set too LOW.
This causes high vacuum (air pressure difference across throttle plate) which does two things:
1. keeps the slide up and prevents the main jet from being closed by the needle
2. pulls more fuel from the pilot jet
I have no theory of why the idle speed may eventually return to normal after some delay. It would require the vacuum condition to change after some period of time once the throttle is closed. But it does make sense that "blipping" the throttle will cause it to eventually settle because you are creating vacuum disturbances by rapidly changing the throttle conditions perhaps enough that it eventually allows the slide to return and the main jet needle to close.
IF this is the cause, then the solution will be to open the idle stop (tighten the idle adjuster screw) and adjust the pilot needle screws to set the idle speed. Opening the idle stop would reduce vacuum and prevent the slide from holding up AND prevent it from pulling too much fuel (above idle) from the pilot jet. Once the vacuum condition is fixed, adjust the pilot jet needle (idle mixture) to get the appropriate idle speed.
Thoughts on this theory?
Another theory: idle mixture / pilot needle set too RICH
This allows too much fuel to be pulled when vacuum is high and keeps the revs high until the vacuum is reduced by some mechanism ... ? I have no theory for why eventually the revs go down to a more-like-idle speed.
To me this creates a vicious cycle where you try to adjust the idle speed on a knife edge where there is just barely enough air to go with the overabundant fuel and keep it from stalling at idle but not enough air to let it rev to 3K rpm. There may be a narrow range of throttle stop position where the amount of vacuum is low enough to allow the slides to return closed, high enough to pull fuel from the pilot jet, and there is enough air bleeding past the throttle stop to keep the engine running without revving too far above idle. But when the engine is very warm the air density in the carburetor throat changes and you wind up with way too much fuel for the available air and it revs too high.
If the idle mixture is actually TOO LEAN, then how does this make the idle speed hang up? I can't come up with a reason why this would happen, so I am hoping you guys can fill in the gaps.
Please, someone explain to me where my logic is wrong or help me better understand these carburetors, and explain to me how exactly a lean mixture results in the dreaded hanging idle.
I'm going to go and try to figure this out experimentally myself, just to see what works, but I would really like to understand the why and I think it'd be good information to have in this forum.
I running errands today so I don't have time to go into detail. But I didn't see it mentioned: the intake boots between the carbs and head (and the orings in said boots) cause the majority of wandering idles.
Boots / orings get old, dry out, crack, and you have an air leak. Air leak allows air into the cylinders, the carbs have no choice but to allow SOME fuel in as well, so the idle increases.
Quote from: Big Rich on November 29, 2016, 08:58:34 AM
I running errands today so I don't have time to go into detail. But I didn't see it mentioned: the intake boots between the carbs and head (and the orings in said boots) cause the majority of wandering idles.
Boots / orings get old, dry out, crack, and you have an air leak. Air leak allows air into the cylinders, the carbs have no choice but to allow SOME fuel in as well, so the idle increases.
I can believe that, but this is not my problem. The boots and o-rings are brand new and confirmed to not be leaking. There are no vacuum leaks. I have been through that mess.
That's sort of understandable how vacuum leaks can cause idle issues, but I still don't understand how it causes the idle to be fast then return to normal. It seems it would cause it to be fast period, and then you can't adjust the mixture easily because extra air gets in from unexpected places or vacuum is reduced (in the case of intake boot leaks).
Anyway, one other thing I read, which might be true, is that if the problem gets worse when the engine gets warm then it is rich, and if it gets better when the engine gets warm then it is lean. That makes sense.
Nobody is biting on my call for explanations of how hanging idle is caused by lean mixture or otherwise, so I'll continue to talk to myself here and hope someone will chime in.
After reading a ton more on the internet regarding this issue, the consensus among shadetree motorcycle mechanics the world over is that after blipping the throttle you can observe the following behaviors and their cause:
- the idle drops below the set idle speed and returns to the correct idle speed after a brief moment, the idle mixture is rich
- the idle hangs above the set idle speed and returns to the correct idle speed after a moment, the idle mixture is lean
This may explain some of the confusion, because the common "hanging idle" symptom that I find when reading a ton of this forum and others which is not diagnosed by the above is while riding the idle stays high (3Kish) when coming to a stop and returns to normal after blipping the throttle. I think this condition may be explained by an overly-rich idle setting, which may be caused by the confused home mechanic (like me) making the idle mixture more and more rich in order to combat hanging idle presumably caused by lean mixture. In other words, the stimulus for the test is wrong. If "blip the throttle" is step 1 in the hanging idle rich-or-lean test, then in my case, this test is inconclusive. If step 1 is "ride the bike and come to a stop" then my bike produces a "hanging idle" which I now think is caused by a rich mixture.
Time will tell if I can get out and adjust it, run some tests on it today or before the weekend long rain forecast arrives tomorrow.
Buried in the GSTwins.com wiki page (http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting-LeanOrRich?) is this:
Typical Rich Conditions
...
- The engine's idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.
So this condition is different from the "hanging idle" that is often presumed to be caused by a lean mixture?
Anyone want to join in a technical discussion of a) what precisely is meant by "hanging idle" and b) how is it caused by lean or rich mixture?
While I don't know that I have enough technical knowledge to join in a "technical" discussion about the causes of irregular idles, but I can definitely comment on the terms. I think you're on to something here: I think that it is extremely likely that some people will come on to a moto forum and describe the rich-style "sticky" idle, and then well-meaning bike enthusiasts such as myself or you will assume they are referring to the lean "hanging" idle and give incorrect advice. I'm not sure what can be done about that, other than for us to to make sure we ask more clarifying questions about what symptoms their bike is showing before advising people to start messing with their carbs in one way or another.
Quote from: qcbaker on December 01, 2016, 12:45:37 PM
While I don't know that I have enough technical knowledge to join in a "technical" discussion about the causes of irregular idles, but I can definitely comment on the terms. I think you're on to something here: I think that it is extremely likely that some people will come on to a moto forum and describe the rich-style "sticky" idle, and then well-meaning bike enthusiasts such as myself or you will assume they are referring to the lean "hanging" idle and give incorrect advice. I'm not sure what can be done about that, other than for us to to make sure we ask more clarifying questions about what symptoms their bike is showing before advising people to start messing with their carbs in one way or another.
I agree absolutely.
But I still want to get to the bottom of the real causes. This could really be a big help on the wiki. Let's face it: for most of us here, these are aging motorcycles, many 2+ decades old. They are going to fall into the hands of old guys like me who don't mind working on them (besides adding LED lights and removing mud guards) so a little more concrete information would be very helpful.
On that note, I went out and counted turns on my carbs' idle mixture screws. I have probably turned them out about 3/4 of a turn since I got the bike back from the shop in effort to cure whatever this sticky idle situation is. I had made an assumption that they must be set really rich by now, and boy was I wrong.
Both carbs had almost exactly 1.5 turns out. That means when I got it from the shop, they were maybe 3/4 turns out. That's how the shop set it up. The bike has 40 pilot jets. No way in the world 3/4 turns can be right. I set them to 2.5 turns out and I am going to go test ride later today and see how it does.
This may completely rebuke my theory! Which would be awesome, because correct, verifiable information is the best kind.
More later. Gotta go test ride. After I get done with work today.
So after resetting my idle mixture screws to 2.5 turns out, I went for a short-ish test ride. It was interesting.
First of all, without changing the idle speed setting at all, the bike was idling at like 2500 rpm. So this gives me a theory:
When the idle mixture is too lean, we wind up setting the idle speed way too high just because there's not enough fuel available at actual idle for it to run. The air coming in for idle should be coming in through a little air bypass orifice in the throat of the carburetor, not through the main throttle valve. But if it's too lean, then we wind up setting the idle speed setting (throttle stop position) too high, which will cause more air to flow over the pilot jet and draw enough fuel to keep the bike running. When you initially drop the throttle, vacuum his higher and that causes it to draw too much fuel through the pilot jet and it idles high until the vacuum equalizes some through the air coming across the edge of the throttle plate. This is my present theory as to how we wind up with "hanging idle" when it's set to lean.
Anyway, I adjusted my idle back down to around 1100 rpm and it only has a very very slight idle drop now. It immediately returns to about 1400-1500 rpm when you close the throttle and then idles down to 1000-1100 after about 10 seconds. Perhaps my idle mixture is still too lean. I may try a quarter turn more rich and test ride again.
Anyone want to comment on my theory above? Is the usenet article wrong and the collective internet wisdom right? Is the wiki correct in the statement about rich condition causing it to not return to idle?
OK, another half turn rich made the idle work pretty much perfectly. It still returns to maybe 1200-1300 rpm and takes a few seconds to get all the way down to right about 1000, so it might still be a hair lean. But it's so close I am not going to mess with it. That's a total of three turns out.
So, I think this debunks the usenet archive post I quoted above suggesting "hanging idle" is caused by rich mixture. For me at least, it was caused by lean mixture, just like the common advice on GSTwins.com always suggests.
Now, the question is, why? How?
Thanks!
Quote from: mr72 on December 01, 2016, 07:18:12 AM
Nobody is biting on my call for explanations...
This would gather a lot more attention in the general discussion rather than the projects sections.
This area is more for builds than troubleshooting. But I guess it is the proper area for "tech talk".
It's just not a popular section, I think.
Since we know vacuum leaks cause hanging idles and we know vacuum leaks lean the mix it can be a given that lean=hanging idle. For most people that alone is a satisfactory answer. Like you, though, I am intrigued by the science and why behind it.
I'm far from knowledgeable about it, but I have some ideas. If I'm at least close maybe someone can springboard off of me.
Gasoline requires a pretty specific Air Fuel Ratio (henceforth AFR) to ignite properly, and in a carb there's only three ways to "adjust" this. The first, obviously, is the mixture screws. The second is the choke. The third is to increase or decrease air velocity over the jets.
Changing the jets themselves isn't an adjustment, it's not something you can do "road side".
You choke a motorcycle to deliver more fuel when it's cold because the air entering the cylinders is dense and you need to provide the carbs with more fuel to maintain that proper AFR. Once it's at operating temperature this is no longer needed because the thinner, warm air coming through the carbs requires less fuel to be at the proper ratio. So we turn the choke off and the engine is now running at the AFR controlled by the jets and the velocity of air through the carbs.
But, in this hypothetical case, the jets are NOT sized appropriately and don't allow enough fuel to be drawn out.
An engine can only work with so much. The jets aren't delivering enough fuel so the engine is weak and doesn't want to idle. The only way to increase fuel intake at this point, aside from putting the choke back on, is through air velocity, and that is adjusted at the idle screw.
And this ties into the concept someone mentioned with the throttle butterfly being cracked too much in an effort to set idle speed.
You rev the engine, velocity is high, it's drawing off a lot of fuel, you close the throttle but the valve doesn't CLOSE. The air velocity isn't abruptly stopped, and since it picks up fuel longer than it should the idle hangs.
Something to consider, too, is how cold air and hot air behave in an engine. In an intercooled turbo car the concept is that since cold air is dense air you can force more of it into the cylinder. This, in turn, needs more fuel. More fuel means more power.
Hot air is the opposite in that it's more expansive, so if the air is really hot it is actually going to be forcing the cylinder to use LESS fuel and become weaker.
Compounded with the fact that a higher presence of oxygen causes fire to burn hotter, a lean condition will also generate more heat which will cause the cylinders to consume less with each impulse. Consuming less of a less potent fuel mixture is only an uphill struggle.
Am I right? Am I wrong? No idea. It makes at least SOME sense to me.
Bear in mind I suddenly and for no reason awoke at 4am today when I went to bed at around midnight, and I was out and about all day yesterday on like 3 hours of sleep. My brain could be making all kinds of stuff up right now.
I'll probably be doing some poking about on the interwebz and if I'm way off I may revise or simply delete everything I just said...