Question about fuel flow in VN750 - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 5 (permalink) Old 05-11-2011, 11:11 AM Thread Starter
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Question about fuel flow in VN750

Ok, I am one of those people who likes to know how things work. And since I am not able (due to space and other restrictions) to tear my bike down and look myself, I am hoping one of the awesome, knowledgable people here can help me out.

What I am wondering is, how exactly does the fuel flow from the gas tank to the carbs - and more specifically, how does the whole "vaccum" system work? I have looked through the service manual, and can see that there are three hoses that come off the petcock - one to each carb, and one is a vaccum hose. My assumption is that vacuum is created somehow, which draws fuel through the petcock from the tank and into the carbs. But how is the vacuum actually created?

If these sound like silly questions, forgive me. I am just hoping that by having a better understanding of how this system works, I may be better able to diagnose, and maybe even resolve future issues.

As always, much gratitude. Happy riding all...



AZ Kev

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1989 VN750
7800 orig mi when purchased Jan 2011
Turned 11,000 mi Nov 2011
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post #2 of 5 (permalink) Old 05-11-2011, 11:18 AM
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That vaccuum is the cut off or rather the cut on for the fuel. The petcock is spring loaded to off with no vaccuum. When the engine starts to turn with the starter, the third hose that goes to the carb intake, over comes the spring and you get fuel flow. Every other time that the piston goes down you get a vaccuum inpulse. It goes so fast that the valve stays opened. Hope this helps.

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post #3 of 5 (permalink) Old 05-11-2011, 12:05 PM
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To be more precise here, fuel flows to the carb via gravity.... It is not sucked out of the tank .... The vacuum line to the petcock is just acting as a switch, opening the valve to let fuel flow when the engine is running and closes the valve when the engine is not.

Older bikes did not have a vacuum shut off, you turned the fuel on yourself ... And could turn it off . The "float" in the carb controls a small needle valve that let's fuel in... When the float rises as fuel enters, it reaches the top and shuts off the flow of gas. Much like the float in your toilet.

Because on occasion you can get debris in the fuel that makes the needle valve stick open, it was common practise to turn the fuel off after you stopped to avoid leaking fuel.

The vacuum line on the vulcan does this for you when the engine is stopped.. That is the only difference between it and and old style petcock.

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post #4 of 5 (permalink) Old 05-11-2011, 03:29 PM Thread Starter
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Thank you both for the replies. You guys NAILED that explanation in a way that even a cretan like me can understand! Haha.

Since I am on a roll here - how does the reserve work on Vulcan? I had a Suzuki GZ250, and it had a little "pipe" in the tank that the gas flowed through when the petcock was in the "On" position. When the fuel level dropped below the top of the tube in the tank, you had to turn the petcock to "Reserve" which would switch from drawing the fuel from the tube to another 'port' on the bottom of the tank. At least that's how it was explained to me.

Is that basically the same setup that is on the VN750? And if that's the case, is that pretty much the setup for most bikes -at least the older ones?

Again - thank you all SO much for explaining this stuff. I am an IT guy, it is probably akin to me trying to explain to someone who is computer illiterate how to clear their browsing history! Lol.


AZ Kev

Arizona Kev

1989 VN750
7800 orig mi when purchased Jan 2011
Turned 11,000 mi Nov 2011
Saddlebags
Taillight relocation kit
Chad's highway peg mounts
Emgo Pegs
Aftermarket mirrors
Dunlop D404's

Otherwise, all stock


"But it's a dry heat - like the inside of an oven"
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post #5 of 5 (permalink) Old 05-11-2011, 04:19 PM
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yes, reserve has a "pipe" that is like 1/2 inch tall, and the "on" pipe is like an inch and a half tall, both sticking up from the bottom of the tank where the petcock is.

On odd thing about the bike is there is a "hump" where the tank fits over the frame, and the tank tends to slant a bit to the rear....meaning even when the bike runs out of gas on reserve, there is still a few cups of fuel at the back of the tank and on the other side.

I have heard of folks stopping from running out of gas while on reserve, leaning the bike over to the left, and getting enough fuel in the carb to go another mile or two........

KM

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