So I spent the day dealing with the new slingshot racing screw on gas cap. I found out that if you tighten it down all the way, there is no way for air to get into the tank and you end up with no more fuel flowing from the tank. What I found was that if you screw the cap all the way down, it eventually bottoms out on the base, preventing any air from these check valves from flowing into tank. So you need to have it not all the way screwed down, but enough to have the o-ring on the cap seal it. If you do this, there is not enough tension on the cap to just unscew itself from engine vibrations

To fix this you need to do a few things.

1. With the cap and base off the bike, unscrew the cap.
2. You will notice that there is a small set screw and ball bearing underneath the cap. You will want to unscrew the set screw and remove the spring and ball bearing. Careful not to loose these as its very easy to drop them and they dispear fast(don't ask how I know,spent 20minutes looking for them).
3. With a small round needle file, open up the hole in which the ball bearing seals(not to much though, you still want the ball bearing to not just fall through). There is also a drilled passage that is just offset from the main hole. This allows the air to go past the ball bearing and up through this half-hole. Since the setscrew and these hole are very close it really restricts the air. Take the needle file and open the half-hole up to allow more airflow.
4. Clean out any metal shavings and clean the filed portions to prevent these from getting into the tank eventually.
5. I completely removed the spring, but you could cut down the spring to make it less stiff.
6. Re-install ball bearing and spring(optional) and apply some loctite(blue color or medium strength) to the setscrew and screw it in until its just a bit below the surface(it will be adjusted later)
7. Clean off the cap of any metal debris that might have got on it during the working.
8. Screw on cap until it bottoms out and you can not tighten it any more. YOu need to turn it about 1/8 turn back from this, this is the ideal spot for the cap to be. The o-ring on the cap is sealed against the base, but there is still clearance between the bottom of the cap and the base for air to flow through to prevent the tank from getting vacuum locked.
9. I used a permanent marker on both the cap and base to mark where they should line up for the ideal mark.
10. Unscrew cap, unscrew the set screw a small amount so that the top of the set screw is just above base. Screw the cap back on until it wont turn anymore, even with a bit of force. The objective is to line up the two marks you just put on, or to get the cap to the ideal spot. Readjust the setscrew until when the cap if frimly tightened and stops at the ideal position.  What this is doing is using the set screw to stop the cap from turning down any further, but since its adjustable it be adjusted so the cap is just right to allow air to flow into the tank. When you try to tighten the cap it pushes down on the setscrew allowing you to but a bit of torque on it to prevent the cap from backing out during riding.
11. Once the cap is where it should be, let the whole thing sit overnight so the loctite will set and harden.

The whole system should work as it should now without restricting airflow into the gas tank and the gas leaves the tank.

Pictures to come!

Interesting, mine came with the set screw raised up already... 'course I turned around and lowered it back down so it wouldn't scratch the bottom of the cap.

If you flip the cap over you'll find a second set screw.  That screw covers the spring and ball for the 'vaccum relief' valve assembly.  The set screw, spring and ball that you mention under the cap are for tank 'overpressure relief'.  If you're going to defeat one, I'd go with the vaccum relief one, as it's more likely to open to allow air in anyways.  I'm likely going to cut the spring down on mine so it 'just' seats.  That'll raise the spring rate, but the decrease in preload should make it more sensitive anyways.

You are correct that the ball bearing that faces up towards the base of cap is for over-pressure relief. However, I was talking about the set screw right next to that ball bearing. That set screw is the one that puts pressure on the spring, which puts pressure on the ball bearing that seats on the underside of the base. This is the vacuum relief circuit.

I think you've got it backwards.  With the cap assembly installed, twist on cap removed, there is a ball bearing and a set screw looking at you.  The ball bearing that you see is the vacuum relief unit.  When there is a vacuum, air pressure outside presses against the ball bearing, which then compresses the spring as the bearing moves down slightly, opening the air passage, allowing air to enter the tank, equalizing the pressure.  The set screw under the cap works the over-pressurization relief.  For that ball to move the tank needs to be pressurized, which will press against the ball, yada yada yada.  If you remove the spring on either, they become ineffective valves and just allow air by at all times.

Thats why I was suggesting just cutting the preload on the vacuum relief spring instead, making it open sooner at a lower pressure differential without defeating it entirely, so in the case of a crash it will try to slow the flow of gas at least a little.

Quote from: Kurlon on April 13, 2008, 02:08:30 PM
I think you've got it backwards.  With the cap assembly installed, twist on cap removed, there is a ball bearing and a set screw looking at you.  The ball bearing that you see is the vacuum relief unit.  When there is a vacuum, air pressure outside presses against the ball bearing, which then compresses the spring as the bearing moves down slightly, opening the air passage, allowing air to enter the tank, equalizing the pressure.  The set screw under the cap works the over-pressurization relief.  For that ball to move the tank needs to be pressurized, which will press against the ball, yada yada yada.  If you remove the spring on either, they become ineffective valves and just allow air by at all times.

Thats why I was suggesting just cutting the preload on the vacuum relief spring instead, making it open sooner at a lower pressure differential without defeating it entirely, so in the case of a crash it will try to slow the flow of gas at least a little.

With all due respect you have it wrong. The vacuum tries to pull air into the tank correct? The ball bearing visable with the spin on cap removed is not the vacuum relief because as the air tried to flow into the tank it would push the ball bearing against the seat and seal it. THe pressure difference(with the tank being at a lower pressure) would push the ball bearing up towards to the cap, thereby sealing it. Its the ball bearing that faces towards the ground when the cap is installed that is the vacuum relief. When the vacuum is generated in the tank, the difference in pressure lifts the ball bearing off the seat and allows the air to flow in.

Trust me(I am a mechanical engineer), if you look at it closer you will see what I mean.

I'm still not seeing it.  The ball bearing facing us under the cap would have low pressure under it, high pressure above it... why would that press it UP and seal it, what am I missing when I look at this?

Edit: All I can think of is possibly a lifting body like effect once flow is established around the ball?

You have the pressure backwards. When the fuel leaves the tank, it creates a lower pressure than ambient, so the pressure on the ball bearing on the side that faces up is lets say 10PSI and then the ambient pressure is 14.7PSI. So the ball is being forced up sealing.

The way you have it written above  would be the case when the tank is over-pressurized from the fuel getting hot, there by creating a positive pressure on the top side of the ball, forcing it down and releasing the pressure. Thats is why I am saying the ball bearing the is facing us under the cap is the "over-pressure" relief valve.

You have all the right concepts, what you are confusing is the fact that when the tank drains it produces a "vacuum" or a lower pressure than ambient.

See, that's whats got me baffled, I agree with the theoretical numbers, 10psi on the inside, 14 and change psi outside when the tank is in vacuum.  What I don't get is why the higher outside pressure isn't pushing the ball down, unsealing it?