Topic: Have to switch to reserve after about a half tank?WTF?

[JS550]

My 75 750k, with F tank, for some reason has to be switched at about a half or so to reserve? I put a brand new,OEM screen tube on the pet cock. What could be causing this? Ive pulled the petcock out & it looks fine. And will it hurt to run on reserve for more then a few miles?
Jim

[TwoTired]

Do you also have in line filters, too?
Describe the fuel line arrangement from valve to carbs.

Is the gas cap vent blocked?

[JS550]

No inline filters, one line from petcock to a T that splits two lines, down to the carb rack. Never thought about the vent? But if that was blocked, wouldnt it stop the reserve also?

[TwoTired]

Reserve feed is from the bottom of the tank instead of the stand pipe.  This gives the reserve outlet the extra head pressure related to the height of the stand pipe.  If you do have a blocked vent, you will probably have problem before the tank completely empties, as the head pressure reduces, corresponding to fuel level above the outlet.  It's about the weight of the fuel in a column over the the outlet.  The vacuum can only lift so much.

In theory...

You sure the stand pipe isn't assembled backwards?  You did align the flat in the valve with correct orientation, right?

[OldSchool_IsCool]

You can diagnose a plugged cap vent by opening the cap instead of switching to reserve next time.  This will release the vacuum and allow fuel to flow again if the vent is clogged.  If after opening the cap you still have to switch to reserve, then you can rule out a blocked vent.

[Bob Wessner]

You say you have to switch to reserve at "half a tank or so." How are you determining this? How many miles, on average, have you gone when you have to switch?

[JS550]

Reserve feed is from the bottom of the tank instead of the stand pipe.  This gives the reserve outlet the extra head pressure related to the height of the stand pipe.  If you do have a blocked vent, you will probably have problem before the tank completely empties, as the head pressure reduces, corresponding to fuel level above the outlet.  It's about the weight of the fuel in a column over the the outlet.  The vacuum can only lift so much.

In theory...

You sure the stand pipe isn't assembled backwards?  You did align the flat in the valve with correct orientation, right?

Hmmm....good question. I know what Im doing tomorrow now! I didnt know about the orientation. Thanks.

[MCRider]

To your last question, will it hurt to run on reserve more than a few miles? in my opniion and experience, it won't hurt anything to run it on reserve until it runs out of gas.

And if you'll notice, I did say "experience".  

It is all the same gas after all, and I couldn't say why it would be designed to do anything but flow out until its gone..

One side issue, if you've got some debris down low that wouldn't ordinarily be disturbed running it on main, it may get disturbed on reserve. But that's a different issue and there shouldn't be any debris there anyway.

That's what I think.

[JS550]

You say you have to switch to reserve at "half a tank or so." How are you determining this? How many miles, on average, have you gone when you have to switch?

Well, when my bike starts to sputter, I switch it to reserve & it stops sputtering. (not being a smartass. This is really how it goes) Ive checked the tank & its always at about the same level.

[OldSchool_IsCool]

Well, maybe there is nothing wrong at all.  Lets say you start with a completely empty tank.  Tip it upside down, nothing in it.  You top off with say 4 gallons of gas.  You will burn only about 2.5 to 3 gallons of it before the fluid level drops below the top of the stand pipe.  You must switch to reserve to get to the last 1 to 1.5 gallons in the tank.  Does that sound like what is happening?

How many miles can you ride between fill-up and the need to switch to reserve?