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Jeff Policky
Jeff Policky, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 2425
Experience:  Yamaha Gold, Suzuki Gold, Honda Bronze, Polaris Bronze, BRP
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Yamaha virago carb slide won't go up how do they get air to

Customer Question

Yamaha virago carb slide won't go up how do they get air to raise up like most bikes this is on a xv750
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Gary replied 1 year ago.
Hi there,Your carb is a CV (constant vacuum) carburettor, same principle as an SU carburettor.https://www.google.com.au/search?q=cv+carb+theory&num=100&rlz=1C5CHFA_enAU516AU516&espv=2&tbm=isch&tbo=u&source=univ&sa=X&ved=0CCsQsARqFQoTCIPStp2ShscCFaMkpgodW_EIEA&biw=1277&bih=595With the engine off, does the piston move freely all the way to the top with your finger?If so, then what is the wrong?The piston moves to whatever height the engine requires.Gary
Customer: replied 1 year ago.
ok well what makes it go up there has to be a way to bench test the slide either with vacuum or pressure
Expert:  Gary replied 1 year ago.
If the slide is free to move from bottom to top with your finger, then it passes the first test.The piston should reach near the top of it's travel at maximum horsepower.That is the idea of a variable choke or venturi.If the piston/spring is too light and gets to the top early, the fueling is lean, and throttle responsive is soggy.If it doesn't get to the top, the engine runs out of breath early.You could see the operation on a dyno or a go-pro camera etc.Air rushing past the bottom of the piston slide creates a vacuum above the piston, atmospheric pressure below the piston lifts it to an equilibrium of the two pressures.Gary

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