Thumping idle - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 36 (permalink) Old 09-12-2013, 08:38 AM Thread Starter
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Thumping idle

Not sure what this one is. First let me say that my bike is riding great. Did a five hour round trip last weekend, coming home at between 65-70 smooth as glass. Rode to work this morning just fine. What happens is...

When the bike gets up to temp, which is fairly hot with the needle just below center but not in fan running territory yet, at idle it feels like the idle is almost thumping. The thumping doesn't feel like something hitting anything but more like one or both of the cylinders is producing a powerful stroke.

Any ideas?

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post #2 of 36 (permalink) Old 09-12-2013, 09:08 AM
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Well since it doesn't seem to get worse at increased rpms I would rule out balance dampers. Might be slight pre detonation in the cylinders. But that's pretty unlikely, what octane gas are you running? Maybe try stepping up one grade on your next fill up. Just kinda spitballing here.

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post #3 of 36 (permalink) Old 09-12-2013, 09:32 AM Thread Starter
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Well since it doesn't seem to get worse at increased rpms I would rule out balance dampers. Might be slight pre detonation in the cylinders. But that's pretty unlikely, what octane gas are you running? Maybe try stepping up one grade on your next fill up. Just kinda spitballing here.
I'll do that. I dropped back to 87 because someone here stated that higher octane burned slower while I thought it the opposite. I'll pop in some premium at lunch and see what happens.

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post #4 of 36 (permalink) Old 09-12-2013, 09:47 AM
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What pilot screw setting and what size pilot jet. No other symptoms, right?
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post #5 of 36 (permalink) Old 09-12-2013, 10:11 AM
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Lol those were my next suggestions. I just figure start easy.

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post #6 of 36 (permalink) Old 09-12-2013, 10:59 AM Thread Starter
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What pilot screw setting and what size pilot jet. No other symptoms, right?
pilots are stock with stock pipes, carbs synched BUT as I've stated previously in another post, when the manometer shows even steven, you are at the point where the idle adjust screw does no good because the synch bolt is in just enough to make the idle screw useless.

Basically, the rear carb had the most vacuum before adjusting it down. BUT, I have since replaced the plugs and wires so will adjust again. FYI, the iridiums gave me worse performance on the new e-bay red spark plug wires (They look great though). Someone said something about removing a resistor on the stock cables, where is that? SO, maybe the new plugs NGK-9's are making the idle feel different and maybe that has also changed the synch.

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post #7 of 36 (permalink) Old 09-12-2013, 11:59 AM
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Goofy, we are back to the same thing, one cylinder appears to be pulling harder. I forget, did you do a compression test?
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post #8 of 36 (permalink) Old 09-12-2013, 12:45 PM Thread Starter
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Goofy, we are back to the same thing, one cylinder appears to be pulling harder. I forget, did you do a compression test?
I did and I got like 129 or something but I think I did open the throttle so prolly should go back and do that again.

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post #9 of 36 (permalink) Old 09-12-2013, 02:26 PM
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Quote:
Originally Posted by Goofyfoot2001 View Post
I did and I got like 129 or something but I think I did open the throttle so prolly should go back and do that again.
what was the other cylinder reading?they are supposed to be close.

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post #10 of 36 (permalink) Old 09-12-2013, 03:12 PM Thread Starter
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what was the other cylinder reading?they are supposed to be close.
I'll try and get both again, I think the other was close. Was this hot or cold? I forget.

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