Get to the bottom of front cylindar thump - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 03-24-2014, 03:49 PM Thread Starter
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Get to the bottom of front cylindar thump

I've seen several people that seem to have the same or similar condition I have. At idle, my front cylinder seems to thump or just pump harder than my rear. (that sounded kinda funny).

So my thoughts are:

1. Plugs, but no, I've used two different kinds, iridium and regular and even iridium on the rear and regular on the front.

2. Synch - maybe but when the carbs are sync'd it seems even worse maybe.

3. Slow jet? Hmmm not sure but not sure I want to take out the carbs either.

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post #2 of 12 (permalink) Old 03-24-2014, 06:38 PM
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Mine does it. I wish I had known it was normal before I tore my carbs out. 4 times...... When mine idles down its like the front cylinder hits harder than the rear, as soon as it comes back up to about 1200 rpm its picks right back up. Once I get it tuned out this time I'm just not gonna worry about it.

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post #3 of 12 (permalink) Old 03-24-2014, 07:01 PM
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is it really 'hitting harder'? or is it just seeming so, since our engines do NOT fire evenly. One cylinder is firing a long time the other (I dont recall the numbers involved).

maybe its just your sensing a power stroke after a longer delay (more degrees of crank revolution) than the other cylinder (which fires after a shorter delay)

Also, I do NOT see this on my bike (not noticable at least). I idle at pretty doggone close to 1100 rpm, and spots mentioned that by 1200 it seems normal.

maybe your trying to idle too slow (spec is 1100)
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post #4 of 12 (permalink) Old 03-25-2014, 08:24 AM Thread Starter
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I'm usually idling too fast because when I synch the carbs, it takes the idle screw off the level (see video). If that makes sense. Basically synching the carbs is just idle screwing the rear cylinder carb. So let me think. Maybe I need to synch this again this weekend and I'll try and video that.

https://www.youtube.com/watch?v=Zc59BThqRa4

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post #5 of 12 (permalink) Old 03-25-2014, 10:44 AM
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My bike runs good. My pilot circuit is near perfect synchronization. At idle and all the way to 5k, as long as I stay in the pilot circuit. It takes some small time to go back all the way to idle. Maybe 2 sec. It will go down to about 1400 immediately and then settle to 1100.

Goofy, I watched your video for I think the second time. You have a pretty good handle on what is going on.

Like most m/c, the vn idle rpm is not set by gas, it is set by the amount of air. You can adjust the pilot mixture screws and not change the rpm noticeably.

Either you have a really poor cylinder or an ability to get not metered air. Vacuum leak, etc.
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post #6 of 12 (permalink) Old 03-25-2014, 10:57 AM
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Goofy, A possible source would be a vacuum leak around your carb boots. I checked my boots and couldn't see anything so I took mine to a bike mechanic friend. He found where one was slightly slipped on the intake after spraying a bit of starting fluid around each boot. After giving the slightly crooked boot a twist both cylinders ran much more evenly.

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post #7 of 12 (permalink) Old 03-25-2014, 11:16 AM Thread Starter
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Quote:
Originally Posted by FoolAmI View Post
Goofy, A possible source would be a vacuum leak around your carb boots. I checked my boots and couldn't see anything so I took mine to a bike mechanic friend. He found where one was slightly slipped on the intake after spraying a bit of starting fluid around each boot. After giving the slightly crooked boot a twist both cylinders ran much more evenly.
So start the bike then spray some starting fluid at the boots and listen for a change correct? Am I right that you can also use WD 40?

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post #8 of 12 (permalink) Old 03-25-2014, 11:42 AM
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Quote:
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post #9 of 12 (permalink) Old 03-25-2014, 06:50 PM
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Quote:
Originally Posted by Goofyfoot2001 View Post
Am I right that you can also use WD 40?
No, the simple answer is that WD40 is not flammable enough (among other things.) Ether (starting fluid) is best, but about any flammable carb cleaner will work.
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post #10 of 12 (permalink) Old 03-25-2014, 07:01 PM
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WD might make it stumble, if there's a leak, but it won't burn worth crap. I can't start fires with it.

Water will make it stumble too.

If you have pod filters, flammable spray can get pulled into the open filters and give you a false positive for a leak.

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