Won't rev past 6K and low top speed. - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 35 (permalink) Old 03-17-2017, 08:42 AM Thread Starter
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Won't rev past 6K and low top speed.

Hi.

I recently purchased a 89 VN750A. It seems to be in great overall condition and don't have many miles on the clock.
The other day, I had it on the freeway for the first time and things got weird.

In fifth gear, it won't rev past 6K and I could barely push it to 85mph. In lower gears it will rev higher, but each gear seem to top out at about the equivalent to 6K in fifth.

At the lower speeds I usually ride (50-60mph) everything seems fine.

I have very limited mechanical knowledge and I have no clue what the problem could be.
Any help would be greatly appreciated.
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post #2 of 35 (permalink) Old 03-17-2017, 10:09 AM
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There are others here with more mechanical knowledge than me that could conceivably think of other possibilities but I had a similar problem but at the lower RPM of around 4K. Mine was coming out of a three year period of non-use where it just sat in my garage. I got it running again and, after a while, it seemed to run normally. But at 4000 to 4200 rpm it stuttered and basically refused to go faster. In my case it was the main jets in the carbs that were the issue. They were clogged with gunk. Once I had removed them and soaked them in SeaFoam for a while the bike came back to life. It now revs up to Mars. As it happens, apparently, the main jets kick in around that RPM and at lower RPMs only the pilot jets are in use. Now in your case, and I am only speculating here (perhaps correctly, perhaps comically wrong) that a similar soaking of your main jets would prove to be beneficial. Perhaps a simple draining of your floats will work, too, for example. Someone else will chime in here soon with confirmations or alternate suggestions. Then again, it could just be nature's way of telling you that you're going too fast!

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post #3 of 35 (permalink) Old 03-17-2017, 11:02 AM
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Im curious of the fix myself as I have the same issue on my bike,,,only difference i know the jets are fine as I cleaned the carb...my issue is the bike seems lost of power...in truth i think the mechanic ( though he helped ) i dont think he got the lines hooked up rite..But if there is something I am missing,,,im interested to see the answers,,,you guys are quite helpful.
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post #4 of 35 (permalink) Old 03-17-2017, 01:35 PM
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Original post sounds more speed related than rpm, so I would guess the carb vent lines are getting wind, which prevents fuel flow into the carb bowls.

With factory air filters, the vent hose plugs into a sheltered hole in either of the air filter housings. Each carb vent goes into a tee, then the single hose runs to the filter housing. The single hose needs to be cut at a 45 angle so it doesn't seal against the bottom of the hole.

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post #5 of 35 (permalink) Old 03-17-2017, 02:21 PM Thread Starter
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Quote:
Originally Posted by Spockster View Post
Original post sounds more speed related than rpm, so I would guess the carb vent lines are getting wind, which prevents fuel flow into the carb bowls.

With factory air filters, the vent hose plugs into a sheltered hole in either of the air filter housings. Each carb vent goes into a tee, then the single hose runs to the filter housing. The single hose needs to be cut at a 45 angle so it doesn't seal against the bottom of the hole.
Would you happen to have a photo? Google turn up nothing useful and without first hand experience, it makes close to no sense to me.
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post #6 of 35 (permalink) Old 03-17-2017, 02:54 PM
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Have you downloaded the repair manual, I think it's in there.

Here's a couple of pics from google image:

In the second pic, see the slotted tubes on the right end of the housing? Vent hose goes in any of those, on either side of the bike. I think those slotted tubes are normally toward the rear of the bike, but check that.




2003-19k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Me and Cinderella, we put it all together, we can drive it home, with one headlight.

Last edited by Spockster; 03-17-2017 at 02:59 PM.
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post #7 of 35 (permalink) Old 03-17-2017, 08:24 PM Thread Starter
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I think I see what you are saying.
So if the hose in one of those tubes is flat against the bottom, I should just cut the end at an angle?
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post #8 of 35 (permalink) Old 03-18-2017, 09:06 AM
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Quote:
Originally Posted by Biking Viking View Post
I think I see what you are saying.
So if the hose in one of those tubes is flat against the bottom, I should just cut the end at an angle?
Right. The 45 cut keeps the hose from bottoming out in the hole.

A strong enough wind on the vent hose makes positive pressure in the fuel bowl, and fuel won't flow in.

When we earshave, the vent line gets tucked under the tank, either on the nipple at the fuel gauge sender cover, or tied to the frame.

2003-19k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Me and Cinderella, we put it all together, we can drive it home, with one headlight.
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post #9 of 35 (permalink) Old 03-24-2017, 12:29 PM Thread Starter
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Sorry for the delay. Weather has been horrible but I finally got a chance to have a look at it.

While the slotted tubes are there, there are no hoses. Had a look around and found nothing of the sort.
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post #10 of 35 (permalink) Old 03-25-2017, 09:41 AM
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Quote:
Originally Posted by Biking Viking View Post
Sorry for the delay. Weather has been horrible but I finally got a chance to have a look at it.

While the slotted tubes are there, there are no hoses. Had a look around and found nothing of the sort.
Need to find the vent on each carb and see where the hoses go. If no vent hoses at all, water can easily get into the carbs, besides the wind problem.

2003-19k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Me and Cinderella, we put it all together, we can drive it home, with one headlight.
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