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Discussion Starter · #1 ·
Hey everyone,

Im a new rider who just purchased his first bike! Just replaced the gas in the tank, charged the battery and put some new spark plugs in. The bike only seems to want to start when the petcock is set to the PRI position, which I’m quite sure isn’t normal? It’s a 2003 Vulcan 800. Any information and help is appreciated!
 

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First to check, would be vacuum line from petcock to carb. There's also a vacuum T in that line that runs a vacuum rebreather that takes block venting and recirculates it through the air box.. start there, I would say. Open to opinions/suggestions by members🍻
 

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Discussion Starter · #3 ·
Yeah, based off other similar threads I thought it might be the vacuum line. I had just placed new clamps on it because the old ones were shot and no longer usable. I thought I might have tightened the clamp too tight and accidentally killed the line somehow, so I loosened it up a bit and so far so good. The real test will be in the morning after it sits in the cold garage for the night to see if that really was the only issue. Messing with any type of air box might be over my head. I'm very slowly learning as I go here. Never been a gear head or wrench monkey before lol
 

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I agree with Vulc98 - Your issue is most likely the petcock, probably the vacuum is not openning the valve correctly.
FYI - Mine was replaced with a petcock that doesn't even use the vac line. It is just a valve with Open/Off/Reserve. The only risk is if I fail to turn it to 'Off' AND the float valve in the carb fails to seal properly.
Also, a warning... I just looked to see how much a replacemnt is. You can find generic's on ebay an Amazon for $12. Some of the reviews say that the Open/Off/Reserve is labelled incorrectly, meaning when you have it 'Open' it might actually be 'Off'. If your's is a cheap replacement then this might be occuring to you.
 

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Hi Cap,
Did the tank come with a new fuel tap or did you reinstall the original?
The fuel tap is easily disassembled and you can check to see if the ON and REServe ports are clogged.
I would also suggest buying a fuel tap rebuild kit and retain the vacuum operated feature.
 

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when there's air going thru the carb, this creates a vacuum and that vacuum goes thru the tube from the carb to petcock, inside the petcock with the petcock dial set on ON, this vacuum pulls on a rubber diaphram with a small spring attached, this spring then releases pressure on a rubber stopper over the gas hole thus allowing fuel flow, either there's no vacuum in the hose or that rubber stopper inside the petcock is not opening up to allow any fuel to flow with dial set to ON, with the petcock dial set on PRI, the fuel will flow non-stop even with the motor not running, or there could be a vacuum leak in the hose, sometimes older bikes have rubber dry rot because over time rubber can dry out and crack causing leaks
 

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Just to be clear, the bike should start in the on position, but will run out of gas and stall because the vacuum isn't opening the petcock.
Then once the carb bowls are empty you'd have to fill them with the PRI - prime position.
 

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Discussion Starter · #13 ·
Thanks for the responses everyone! My battery ended up dying on me again so I replaced the battery and so far it’s been starting normally, even in the ON position. It was probably a bit warm from starting it earlier though. Tomorrow will be a test to see if it might’ve been a low battery issue, but I’m thinking you’re all right about their being a vacuum issue. Right now bike is running which is all I want right now. Will update tomorrow to see if there are any more issues (after a cold night in the barn)
 

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when you put the petcock on PRI and the carb bowl is filled up, and then you start the bike then turn the dial to ON, the bike can still run because there's gas in the car bowl, but if there is indeed a vacuum problem, the bike will run until the carb bowl empties of gas, so keep your bike running with the petcock dial set to ON and see if the bike will run of of gas or not, maybe after 15-20 minutes (don't know how long) if the motor does indeed shut off for lack of gas then you have a vacuum problem, if not then there's no vacuum problem
 
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Discussion Starter · #18 ·
Sadly the weather here isn’t permitting any riding. She failed to fire and remain fired when in the ON position, but once again fired after just a few seconds in the PRI position. I’ve got a friend who is much more knowledgeable about motorcycles who will hopefully swing by on the weekend and help me look into this potential vacuum problem. Will update once again when there’s been some kind of change. Thanks again everyone!
 

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So if you can't start in "on" setting but can on "PRI" setting I'd guess the fuel is leaking out of the carb when it is sitting, therefore emptying the carb bowls.
You put in "PRI" and the bowls fill and it starts. After it starts you switch to "ON" and it continues to run - right?
You dont have a vacuum problem unless the engine dies when set to "ON".
So where is the fuel going?
Does it smell like gas around the bike?
Is the oil full and does smell like gas?
 

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Discussion Starter · #20 ·
So if you can't start in "on" setting but can on "PRI" setting I'd guess the fuel is leaking out of the carb when it is sitting, therefore emptying the carb bowls.
You put in "PRI" and the bowls fill and it starts. After it starts you switch to "ON" and it continues to run - right?
You dont have a vacuum problem unless the engine dies when set to "ON".
So where is the fuel going?
Does it smell like gas around the bike?
Is the oil full and does smell like gas?
When I switch the bike to ON it does continue to run but I’ve not had it running for more than 20 minutes at a time so I’m not sure if it’s gonna stall out if it might take longer than that.
There is no smell of gas around the bike and there are no leaks that I can see.
Ive yet to check the oil in all honesty. Something I’ll get around to today since it’s the weekend.
 
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