I've got a 2001 FXSTD carbureted I've been working out bugs with the Mikuni carburetor and I talked to a tech at Mikuni.. he told me to check the gas tank vent check valve in the vents hose line. I disconnected it between the dash and under the tank. I attached a tube to the line and checked both ways.. the passage to the tank was clear both ways but the line running through the frame wouldn't let air be sucked backwards. My question is on the carb bikes dose the tank vent air out of the hose or suck it in!? Talking to the Mikuni Tech he made it sound like it needed to suck air in to let the gas fall out of the tank.. if that's the case I think my check valve might be in backwards does anybody have any experience with this??

 

Oh and I checked the gas cap and it's venting fine.. it let's air in no problem

 

I'm not sure what bugs you are trying to work out, but I suspect there was a miscommunication between the two of you, or a mix up of terminology..

I hope I remember this correctly. If not, I'll be corrected quickly....

This is how the Harley fuel tanks vent....

They have a vent hose nipple high on the front of the tank. A hose connects to it, and runs to either a charcoal canister for CA models, or to the atmosphere on all others. The CA models need to go through a charcoal canister, because it is felt that releasing the vapors from a fuel tank into the atmosphere will cause pollution. All vent hoses have a vapor valve near the end. That vapor valve only vents vapors when they are under slight pressure. The pressure can build when the tank is full and parked with a hot engine, or just in really hot weather. If it was just an open hose, with no pressure needed to vent, sloshing fuel could enter that nipple, and drain out that vent hose.

They also have a right side vented gas cap. That vent is a one-way inbound vent. That is where air enters the tank as fuel leaves the tank. Without that vented gas cap, you could get a vapor lock that could slow fuel flow out of the tank.

So that vent line check valve (vapor valve) doesn't affect the flow of fuel from the tank, the vented gas cap does...



Your statement, "Mikuni Tech he made it sound like it needed to suck air in to let the gas fall out of the tank." will make sense if by "gas tank vent check valve" the tech was referring to the gas tank fuel valve on carbureted models, otherwise referred to as the gas tank fuel petcock.

In 2001, the gas tank fuel valve #61338-94D is a vacuum activated fuel petcock. It has a diaphragm that needs vacuum from the carb to allow fuel to flow, even with the valve turned on. The vacuum activation is like a fail safe, so if someone fails to shut off their fuel valve (petcock) after shutting off the engine, and the float sticks open, the fuel valve (petcock) will not allow the fuel to flow, flood the engine, and/or drain the tank.

The OEM fuel tank valve (petcock) has a vacuum line to a nipple on the carb. Once the engine starts, vacuum from the carb activates the diaphragm in the fuel valve to open and allow fuel to flow from the tank.

This fuel valve can fail due to a bad diaphragm in the valve itself, a crack/split in the rubber hose between the carb and petcock, or if some work was done & the vacuum line wasn't reconnected.

Many people, myself included, replace the OEM vacuum fuel valve (petcock) with a Pingel or other brand petcock that just functions as a valve, with no vacuum back-up. I suspect that may have been done when your bike was switched to a Mikuni from the OEM Keihin CV40 carb.

First step is to check your fuel valve (petcock) to make sure it's the OEM fuel valve (petcock) with the diaphragm. If it was changed out, it can only be your problem if the petcock screen inside the tank is plugged. That can happen from dirt debris, or if your tank liner is peeling. Large pieces will slosh around and affect the flow of fuel through the petcock screen.

if your petcock is good, and there is nothing blocking the petcock screen inside the tank, then your gas cap vent may have failed...

I hope this helps.... good luck with your diagnosis..

 

I think he was trying to rule out vapor lock in the gas tank.. he was right about the hose, the nipple that comes out of the top of the gas tank underneath the dash and runs down to a check valve at the bottom.. I just don't think that the way that functions that it would be the culprit for vapor lock in the tank seeing as how the gas cap vents as well.. I guess I'm wondering now what the pipe/hose on the top of the tank running to the check valve dose!? is it an overflow for fuel?? I don't think the tank would need to breather seeing as how the gas cap is already vented right!?

 

Oh and I don't have the charcoal canister.. the vent pipe runs from the top of the tank all the way down to a valve underneath the bike in front of the rear wheel.. thanks for your help by the way

 

Just for Info.
Many people and many different bikes, remove the"overflow check valve", due to crud build-up over time and causing venting issues. My '02 does not have the check valve.

 

Oh and as far as my fuel petcock goes I do have the vacuum operated one but I just replaced the fuel petcock with a OEM brand new one

 

If I remove the check valve wouldn't that let gas come out of there if it overflowed!? I wouldn't want gas coming down in front of my tire while going around a corner..I still don't understand if that's an over flow line or a fuel tank vent? Dose it even have anything to do with vacuumed in the tank!?

 

As mentioned, lot of people run without that vapor valve... It is possible to get some gas without it, but I doubt it would be a lot of fuel, and it's not a problem to the many who run without it..

The line on the front of the tank is designed to be a vent line, not a fuel overflow...

If your vapor valve allows air to go through it albeit slightly under pressure, your vacuum petcock is functioning properly, and your gas cap s venting properly.... you are good to go as far as the components in the fuel tank that affect the fuel flow to your carb...

You never told us what your issue is....... If your issue is a fuel flow issue, and all the above check OK..... Then you need to check:

The screen to the petcock, inside the tank, for blockage.
Check that your tank liner isn't peeling which will allow liner pieces to obstruct fuel flow to the petcock screen as they slosh around inside the tank.
Check the fuel hose from the petcock to the carb. If that fuel line has delaminated on the inside, it can restrict fuel flow.

Once all the above have been checked and eliminated as possible causes for fuel flow/restriction issues... then the only other source of any fuel delivery issues will be with the carb itself...

Good luck with the fix...

 

I am not sure it 100% applies to the HD's
- But it is normally called an "ROV" which stands for: "Roll Over Valve". Its design is to keep fuel from pouring out in the event of a crash. The valve stops the outpour of fuel but lets air in to equalize pressure and prevent a vacuum from forming. Many call it a vent, which it is, and again, many remove the ROV's from all brands of bikes to prevent future clog issues. They are usually located as the HD in the front high or the side saddle high.