how much vac to open fuel tap?

ok i what to drain gas tank on work table.

going to put fuel tap to (on) and then put tap to (res)and then with (mighty vac)pump open valve.

what i need to know is how many lbs 2-3 or just till gas runs out,my harley takes 6 lbs

what really am after is my reserve amount 1/2 gallon like manual says or 1 gallon like its seems to me when fill up only takes 2.5 gal or little more.
book says 4 gal tank total
i would like know for sure i travel alot and this a pain azz not really knowing.
thanks for help.......... PS aint no fun running out on reserve.

Interesting. I have no idea about the vacuum needed, but somehow -6psi seems like a lot.

Once you have the fuel level below the ON tube, you will not need the vacuum. You can turn the petcock to PRI and gravity will pull the fuel out. As you have surmised, the factory capacities are only approximate.

ok sorry its only 25 in Hg of vacuum on the harley i had to dig out my service manual
but thanks on the prime thing.
i just have little more gas to go then hit reserve. thats why i am using mighty vac thing to make it act as to run out in the (on) position

P21 wrote: ok sorry its only 25 in Hg of vacuum on the harley i had to dig out my service manual
but thanks on the prime thing.
i just have little more gas to go then hit reserve. thats why i am using mighty vac thing to make it act as to run out in the (on) position

That can't be right. 25 inches of mercury is close to a perfect vacuum. 25 inches of mercury is also about -12 psi.

When running, a typical bike will have between 8 and 15 inches of mercury. The petcock seems to be able to open pretty far below that.

ok fellow riders the results are in! (up-date)

ok the reserve is 1 gallon geez :evil:

and it only took about 0.5 in Hg to open fuel tap

well now i know the rest of story i feel much better thanks for the help guys.

so in the end i would say she holds about 3.7 gallons sounds about right.