Question about 2005 Honda VT 750 DC Shadow Spirit

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Vaccum lines from carb, where do they go?

There is vaccum lines from the carb that comes out from a T. Where does this line hook too? does it just sit out to the open air?

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Check the fuel petcock to see if there's a similar-sized fitting.

Most carbureted motorcycles use vacuum lines for one thing only: To apply vacuum to the fuel petcock so it'll turn the fuel supply on ONLY when there is vacuum being applied.

Though the same bikes also typically have positions on them that override the vacuum safety system.

"Vacuum safety system"?


Unlike a car, most bikes carry their fuel ABOVE the carb, and instead of a fuel pump, use gravity to feed the carbs. If you've got a petcock that's always open, only the float needles in the carbs hold back the fuel flow, and these are prone to being held at least partially open. The tiniest speck of debris can do it.

And the results can range from annoying to catastrophic.

If the fuel is flowing into a carb while the bike is parked, you can fill a cylinder and, when you hit the electric starter with good battery power, you can bend a connecting rod since liquids cannot be compressed. The piston will slam into the incompressible liquid and if the starter's stout enough, there goes a rod.

Or if the fuel leaks into a cylinder whose exhaust valve is open at the time, the bike will start but sometimes you're left wondering what that loud bang was and why are you feeling so warm and where on earth did that muffler go?

You should get this taken care of quickly because if I'm correct about this line, not only are you having to use manual override positions on the petcock (not a biggie -- the vacuum part is a frequent failure point -- just turn the petcock off when you park it), you have a vacuum leak which can result in at least poor running and though the leak is quite small, there's the possibility of running too lean and burning a hole through a piston, which kinda sucks.

Posted on Jun 01, 2010


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