RPMs not going down when letting off throttle - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 6 (permalink) Old 02-07-2011, 10:44 AM Thread Starter
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RPMs not going down when letting off throttle

Hello all -

To clarify the title - after my bike has warmed up, and I am riding along, if I let off the throttle, instead of the RPMs falling all the way to idle as one might expect they would, they stay where they are for a moment, and start falling very slowly. If I pull the clutch in they fall more, but if I don't, they stay up.

I am thinking it is some kind of carb issue, as most things like this seem to be. Seems I remember reading somewhere that it might be a result of the bike running too rich - does that sound even remotely possible?

As always, any and all suggestions/insights are greatly appreciated

AZ Kev
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post #2 of 6 (permalink) Old 02-07-2011, 12:05 PM
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If the bike is running fine, except for this issue, then perhaps it is something minor.
  1. Broken or weak return spring
  2. Sticky accel and decel cable (when was the last time you lubed them?)
  3. Sticky carb/throttle pivot points

The above can be check by simple observation.

Chris Glennon - Portland, OR
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post #3 of 6 (permalink) Old 02-07-2011, 12:18 PM
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If the tach is working properly, the RPMs are going to indicate how fast the crankshaft is turning. With no throttle, and the clutch lever not pulled, the tach should wind down as motor or other braking takes effect. Haven't paid much attention to this as I'm not watching the tach due to whatever reason I let off the throttle (approaching cage at intersection, upcoming turn, etc.).

What you are describing sounds normal to me.

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post #4 of 6 (permalink) Old 02-07-2011, 12:23 PM
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If the cables are fine and the throttle is closing, you have a vacuum leak.

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post #5 of 6 (permalink) Old 02-07-2011, 03:05 PM Thread Starter
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Great suggestions

Thanks all - those are some great suggestions, and I plan to go through all of them in a process of elimination to figure it out. I am glad to hear that - from what it sounds like - is probably not something major.

I did notice a bit of a hissing sound... I am wondering exaclty where I might check for vacuum leaks - any tips?

Once again thanks

AZ Kev
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post #6 of 6 (permalink) Old 02-07-2011, 05:57 PM
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Your discription was not that clear...am guessing you mean the engine rpms do not fall when you let go of the throttle...when the bike is in neutral or the clutch lever is pulled in.


If you are moving and in gear...and let go of the throttle...... engine speed will slowly go down...dependant on vehicle speed. So you need to clarify more here.

If the former is true..

Actually a slow to fall throttle (while out of gear) is usually caused by an air leak...making a lean mixture not a rich one.

Check the tightness of the clamps on the intake boots and the ones between the cylnder and the carb. next check the small hoses off the carb.

One way to check of leaks is to spray WD 40 on the suspected area while the motor is running. If the exhaust smoke turns white..you have a leak.

Now... if you mean the bike is moving and in gear...you need to use the brakes to see the engine speed go down


Sometimes it helps to re-set the idle after the motor has warmed up...(to roughly 1100 rpm)

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Last edited by Knifemaker; 02-07-2011 at 06:00 PM.
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