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Discussion Starter #1
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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Retired USAF (IYAAYAS)
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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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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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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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Discussion Starter #5
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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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.



 

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Discussion Starter #7
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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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.
 

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Discussion Starter #9
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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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.
 

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Discussion Starter #11 (Edited)
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.
Sorry if I sound like a complete moron here, But could you point me in the right direction, as to where those hoses are supposed to be?
I have spent the last hour on google and can't find a single understandable overview of this thing.
 

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Sorry, just now saw your post. Below is a pic I made a few years ago, not that great but might give you an idea of what to look for.

Probably best to take the tank off, that's how this pic was taken. There's three types of hoses on the carbs, 2 fuel, 2 vacuum, and 2 vent. The vacuum lines are on the sides of the carbs, facing outward and can be seen easily. The fuel and vent hoses are located to the inside, between the two carbs. So you need the other two hoses that aren't fuel lines. You may only see the hose nipples with no hose on them if yours were removed.

The vent hoses are the only ones with a tee in them, if it's still in stock form. The tee doesn't have to be used, it just makes one less hose running somewhere.

See if this pic helps any, if not, post back and we'll figure it out.

 

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Discussion Starter #13
I think I found the vent hose. Haven't had a chance to get the tank off (weather has been bad and I have no indoor work space ATM), but I dug around as best I could and found a loose hose that seems to start at about the spot indicated on the photo above.

It does not go anywhere. Was just sitting on top of the carbs and seems to be a bit too short to reach the airbox.
 

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Discussion Starter #15 (Edited)
So getting that hose to the airbox should be enough?
Seems a bit too easy. Especially since it's not loosing power but simply stops going any further at a specific point. It gets to about 87mph (about 6K RPM in 5th) and simply won't go any further. And those numbers are consistent.

In any case, since the hose isn't long enough to reach the airbox, I will fit a small filter to it tomorrow and see how it does with a bit more restriction.
 

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Discussion Starter #16
Tried putting on a small filter that is about as restrictive as I guesstimate the airbox would be. No change whatsoever.
 

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FREEBIRDS MC CENTRAL NY
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if not long enough to reach air box you can put it on the spare nipple on the fuel sender

Sent from my LGL34C using Tapatalk
 

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Discussion Starter #18
if not long enough to reach air box you can put it on the spare nipple on the fuel sender

Sent from my LGL34C using Tapatalk
Which are where? I have not found anything remotely resembling a useful overview of the carbs with parts named, so I have no clue what is where.
 

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FREEBIRDS MC CENTRAL NY
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the fuel sender is on the bottom side of the gas tank

Sent from my LGL34C using Tapatalk
 
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