Understanding coasting - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 10 (permalink) Old 02-26-2012, 11:36 PM Thread Starter
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Understanding coasting

I think I'm getting close enough to having my carbs running well enough that I can start going on rides. The only thing I'm still seeing is erratic coasting idle. When coasting sometimes it will drop down to idle, sometimes it will hold at 2,000 RPM. When I come to a stop 90% of the time it immediate drops down to a normal idle but occasionally will stick at around 2,000 for a bit. I've done enough work with these carbs that I really don't think I've got an intake leak and I don't see any change spraying brake cleaner around.
However, I understand that we have a coasting enrichment circuit. Can you explain how that works? In cars I'm used to see a solenoid that takes in manifold pressure and an electric signal from the speedo to determine when to kick the carb into coasting mode. Certainly our bikes are not that complex. So how does the coasting circuit know the difference between coasting and stopped?

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post #2 of 10 (permalink) Old 02-27-2012, 12:45 AM
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By "coasting" I am assuming you mean "closed throttle". Which in my riding style means sitting still and just idling.

If you are moving, you should be in a gear and at some open throttle position. The only time you would be under closed throttle is when you are stopped.

So, you are saying your idle speed is not consistent. Have you tried adjusting the "air screw" on the carbs?

Are you sure ALL the hoses clamps etc from the air box to the the carb mouth are tight and not letting in some air - other than what passes through the air filter?

What is your idle speed set at?

Sorry , but you keep saying "coasting" and I'm not sure what that means to you.

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post #3 of 10 (permalink) Old 02-27-2012, 01:37 AM Thread Starter
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"Coasting" as in no throttle and clutch squeezed, such as approaching a stop. In that situation I'll usually see about 2,000 RPM. One I've come to a stop it will drop down to proper idle (most of the time)
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post #4 of 10 (permalink) Old 02-27-2012, 06:13 AM
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I have endured the same symptoms as you are describing here. I may be a million miles wrong with your issue,,BUT,when the same thing was happening here i used the idle adjuster,,took the idling revs to 1000. Previous to that i was only idling at slightly over 1000,,but it made a difference here (dont really know why it did ) !. Just my thoughts !!.

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post #5 of 10 (permalink) Old 02-27-2012, 06:19 AM
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X2 ^... I had the same issue and I adjusted my idle screw down... the problem is fixed 95%... I still have that 5% phantom high idle for a second.. But for the most part bringing it down to spec worked.

Accomplishments;

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post #6 of 10 (permalink) Old 02-27-2012, 07:46 AM
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Check that your decelerator cable isn't binding or has too much slack, and that the throttle stop is lubed and returning freely

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post #7 of 10 (permalink) Old 02-27-2012, 09:59 AM
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So, you are saying you do not downshift ( engine breaking ) that when you are approaching a stop.... You pull in the clutch lever and brake until the bike stops. (do you shift gears down while braking or wait till you stop?)

When you finally stop, it usually falls to proper idle speed. (1100-1150 rpms)


When I approach a stop, I down shift through each gear, and only at the last few feet before I completely stop do I pull in the clutch lever...we are talking about less than 2 seconds here.

If its waiting till you completely stop before it falls to idle, your clutch might be out of adjustment. As mentioned, re-adjusting the idle speed does seem to work sometimes....although if you can't get the engine to to fall to idle unless you adjust the idle speed below the reccomended amount above... There is something Wrong.

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post #8 of 10 (permalink) Old 02-27-2012, 10:55 AM
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CV carbs are a strange animal, the slides or one of them may be sticking in the bore .

The throttle plates only open up the air side of the carb ,vacuum signal raises the actual slide up in the bore lifting the needle of the seat,you may not have a problem in the idle circuit but a lack of return to the seat on one or other of the carb halves,if none of the above suggestions work I would give them a check.But I agree with all the above. Just giving you one more to check that can cause this symptom.Happy hunting !




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post #9 of 10 (permalink) Old 02-27-2012, 11:31 AM
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Are your carbs setup correctly if they are out of balance this can sometimes cause the problems you are having
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post #10 of 10 (permalink) Old 02-29-2012, 10:07 PM
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If you are completely off the throttle, and have the clutch pulled in, basically you are at idle. With the clutch in, it makes no difference whether you are moving or not. Does it do this when sitting still, idling in gear with the clutch in?

"Coasting" is what I call "trailing throttle" which means closed throttle while in gear and clutch out, to utilize engine braking. Under that condition many bikes will pop and backfire slightly, even when everything is set right. That is something I do all the time when slowing down and speeding up. I will have the throttle open while climbing a hill, then close it or partially close it when starting back down. I also do it when coming to a stop sign or light. I usually don't start downshifting until I have reached a fairly low speed, unless I need to lower my cruising speed, and need to go down one gear to prevent lugging the engine.

But none of that sounds like your problem. I also don't think it is an air link. It sounds like something is wrong either in the throttle linkage, the throttle itself, the cables, or it could also be an issue with the slides or diaphragms. I personally hate CV carbs, They were originally designed as an emissions control device, they do allow the carb to operate over a wider altitude range without rejetting, but they can also be a RPITA when things aren't right.

I would check your cables first, to see if they might be binding. The Vulcan has a dual cable setup, something else I never liked. It causes twice as much cable friction. I removed my decel cable (not only on the Vulcan, but on 2 other bikes as well) and am just using one cable, to open the throttle, and the spring on the carbs closes it. The throttle snaps closed much faster and easier with just one cable.

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