Carb Question - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
From the gas tank to the carbs
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post #1 of 26 (permalink) Old 11-05-2019, 11:51 AM Thread Starter
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Carb Question

Hello Forum peeps, newbie here. I've had the VN750 about a year now. Had the carbs rebuilt by a local shop here in south Florida and the bike is running well. My question is about the open vacuum port in the picture. Pretty sure there was a tube connected but mechanic says not. Should it be capped? Thanks in advance
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post #2 of 26 (permalink) Old 11-05-2019, 12:04 PM
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If you still have the emissions equipment in place, that vacuum port is connected to the air valve system (which has 2 big hoses, one going to the exhaust of each cylinder jug, and 1 small hose which runs to the vacuum port)

If the emissions equipment has been removed you can cap off the vacuum port.

Where does the disconnected hose in your picture go to?

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post #3 of 26 (permalink) Old 11-05-2019, 12:12 PM
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Uber's probably right. Mine has a hose on it that runs off to the left (rearward) and disappears underneath the tank/seat area. My bike is totally stock (except for MCCTs). I'm thinking that black hose shown dropping down may be the one that is supposed to go up behind the right-side air filter.

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post #4 of 26 (permalink) Old 11-05-2019, 12:46 PM
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Good point about the airbox drain hose. Let's try and demystify this a bit, shall we?

Left Carb Vacuum Port -> Fuel Petcock (Left-side of gas tank)
Right Carb Vacuum Port -> Air Valve (Underneath seat, near battery box) -OR- Capped off
Top of Carbs Vent Ports -> Tee-Fitting -> Back of Right-Hand-Side Air Filter Box
Front Bottom Air Intake Box -> *hangs freely beneath bike*

There's a buncha other hoses, but these are the most common ones I recall seeing questions about

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post #5 of 26 (permalink) Old 11-05-2019, 01:09 PM Thread Starter
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I dont think the bike has emissions, not sure what it looks like. The hanging hose i believe goes to a tee between the two carbs? Maybe? Will try to investigate this weekend. If i put my finger on that open vac port the idle goes from 1000 to around 2000. My point to the mechanic was that the port is drawing in unfiltered air to the carb but he didn't seem concerned. The bike is running well and i'd like to keep it that way. If anyone here knows a good mechanic in south Fla let me know please. I did do the mccts, oil an fuel petcock myself but the carbs were to much for me to handle.
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post #6 of 26 (permalink) Old 11-05-2019, 01:28 PM
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Wow, that mechanic is dead wrong and he didn't connect things up the way they're supposed to go.

This should be an issue you can fix, though, unless you want to take it back to the mechanic and tell him he did it wrong and that you've confirmed it with fellow VN750 owners, but at this point I wouldn't trust their word or work since they've already flat-out lied to you. A vacuum leak is a serious issue, any mechanic worth their salt knows that.

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post #7 of 26 (permalink) Old 11-05-2019, 03:50 PM
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The hose that connects to the right side air filter just slips onto a tit on the rearward part of the innermost side of the air filter housing. Essentially this hose simply dead-ends on that tit. If you pop off the two bolts that hold the air filter housing to the frame (one upper, one lower) all you'd need to do is tilt it out a smidgeon so as to get a look at the rear of it. The hose is either there or its not.

The hose is supposed to end in a dead-air space so they (Kawasaki) put it there. If that hose ends in a place where the air moves then the engine can run like crap. But I don't remember if removing assorted emissions control stuff has anything to do with that particular hose or not.


Give me a few minutes and I'll come back with a decent pic of that area "naked" and shows a bit more detail than yours above.

TOC MCCTs
Saddlebags, hard mounted
Fork mounted tool bag (now hard mounted)
Relocated rear turn signals
LED turn signals, brake lights and running lights
LED license plate frame
Engine Guard and Highway pegs
National Cycles Low Boy Heavy Duty windshield
Home made lowers (nice)
Custom seat (made by a local guy)
Iridium plugs (DPR7EIX-9)
Shenko 230 Tour Master tires at 11,123 miles
Splines lubed at 11,123 miles
Luggage rack
Light bar with LED lights
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post #8 of 26 (permalink) Old 11-05-2019, 04:05 PM
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This pic was taken a few years back when I was changing out all of the cooling system hoses and flushing the system. To your needs, it more clearly shows the hose that runs to the air filter housing. Your hanging-down-hose looks suspiciously like this one.

That thick hose that you see running horizontally from behind the air filter area off to the left is part what Uber was talking about belonging to the air valve system. It comes out of the head, which from this angle is behind the visible air filter housing. If your bike does not have this hose (and the corresponding rear cylinder one that is on the other side of the engine) then someone has removed the emissions stuff.

You can also see the small hose connected to your "vacuum port".
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TOC MCCTs
Saddlebags, hard mounted
Fork mounted tool bag (now hard mounted)
Relocated rear turn signals
LED turn signals, brake lights and running lights
LED license plate frame
Engine Guard and Highway pegs
National Cycles Low Boy Heavy Duty windshield
Home made lowers (nice)
Custom seat (made by a local guy)
Iridium plugs (DPR7EIX-9)
Shenko 230 Tour Master tires at 11,123 miles
Splines lubed at 11,123 miles
Luggage rack
Light bar with LED lights
Chris and Shim is offline  
post #9 of 26 (permalink) Old 11-05-2019, 04:19 PM
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Also, while I'm thinking about it, that hose you've got hanging down (that fits onto the air filter housing tit) could use a slight alteration. I'd snip the very end at a VERY slight angle. The issue is that sometimes this hose can bottom out onto that filter housing and again, the engine can run like crap.

By snipping a "slight" angle to the very end, it won't bottom out and "seal" itself. A 10 or 20 degree snip should do the trick. If you cut it at a larger angle, there may not be enough hose there to hold onto the tit.

TOC MCCTs
Saddlebags, hard mounted
Fork mounted tool bag (now hard mounted)
Relocated rear turn signals
LED turn signals, brake lights and running lights
LED license plate frame
Engine Guard and Highway pegs
National Cycles Low Boy Heavy Duty windshield
Home made lowers (nice)
Custom seat (made by a local guy)
Iridium plugs (DPR7EIX-9)
Shenko 230 Tour Master tires at 11,123 miles
Splines lubed at 11,123 miles
Luggage rack
Light bar with LED lights
Chris and Shim is offline  
post #10 of 26 (permalink) Old 11-05-2019, 05:55 PM Thread Starter
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I dont have a tit on my air box. (Man that sentence sounds bad!) See Pic. I have capped that port for now, guess we'll see how it runs. Thanks for all the help.
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