Auxiliary Fuel

[RICHO2006]

Has anyone installed a gravity feed auxiliary fuel tank on an '85 era k100? Mine will not drain into the main tank while I'm riding. It will drain while the bike is at idle and when the engine is off but doesn't seem to drain while the bike is moving. I really don't want to install a transfer pump but I may need to. I would appreciate any help.


Thanks

Richard

Commerce, Mi

[cjack]

Has anyone installed a gravity feed auxiliary fuel tank on an '85 era k100? Mine will not drain into the main tank while I'm riding. It will drain while the bike is at idle and when the engine is off but doesn't seem to drain while the bike is moving. I really don't want to install a transfer pump but I may need to. I would appreciate any help.


Thanks

Richard

Commerce, Mi

How is it connected? The connection has to be below the level of the aux gas level and the aux gas level has to be above the tank gas level. Did you make a tee at the return line to the tank? It's possible that the pressure is too high during running for the head pressure developed by your aux gas level. Also make sure that the tank is not pressurized...that the tank vent line is open and clear. The vent was originally connected to the crankcase thru a grey plastic valve to the front top of the engine under the gas tank. That should be removed and the vent hose directed to the atomsphere. The two hose connections under the right side of the rear of the tank can be allowed to have no connections and a small plastic tray installed (if it is not already there) with one hose on it to the atmosphere. Those two connections are the tank vent and the drain for the area around the filler cap of the gas tank to drain water, etc. from it.

[RICHO2006]

Hello,

I cut a tee in the return line below the main tank. The bottom of the aux tank is above the level of fuel in the main tank so there is a height difference in fuel level. The only thing I can think of is that there is some pressure in the return line due to the restrictions encountered by the size of the fuel line and fittings and somehow that pressure changes once the bike is underway because the fuel will transfer when the bike is on the main stand and engine running. Does the fuel pressure regulator adjust the fuel pressure as the engine changes rpm? Is there a lower pressure at idle than say 3000 rpm?

[cjack]

Hello,

I cut a tee in the return line below the main tank. The bottom of the aux tank is above the level of fuel in the main tank so there is a height difference in fuel level. The only thing I can think of is that there is some pressure in the return line due to the restrictions encountered by the size of the fuel line and fittings and somehow that pressure changes once the bike is underway because the fuel will transfer when the bike is on the main stand and engine running. Does the fuel pressure regulator adjust the fuel pressure as the engine changes rpm? Is there a lower pressure at idle than say 3000 rpm?

Not sure, but I think pretty constant. But, something is working against the head pressure. Check the vent issue?
Should be pretty easy since you already have a tapped line there to put a pressure guage anywhere on the line and see what goes on.

[RICHO2006]

I just installed a different vent. I move it from the cap to a 1/4" tube I welded in the tank.

I did get my hand inside the tank and I can feel some pressure at the outlet orifice in the return fitting.

I drained most of the fuel from the main tank to the aux tank by blocking the outlet (forcing the fuel into the aux tank) to increase the difference in elevations. I will take the bike out for a spin and if the fuel doesn’t transfer under these conditions then I guess I need a transfer pump. I didn’t really want to install a pump because it makes it more complicated.

I might just scrap the idea and just resign myself to stopping every 200 miles.


Thanks for your help.



Richard


Commerce, Mi

[From MARS]

Why not return to the auxillary tank and let it gravity to the main tank?

[Bobmws]

I just installed a different vent. I move it from the cap to a 1/4" tube I welded in the tank.

I did get my hand inside the tank and I can feel some pressure at the outlet orifice in the return fitting.

I drained most of the fuel from the main tank to the aux tank by blocking the outlet (forcing the fuel into the aux tank) to increase the difference in elevations. I will take the bike out for a spin and if the fuel doesn’t transfer under these conditions then I guess I need a transfer pump. I didn’t really want to install a pump because it makes it more complicated.

I might just scrap the idea and just resign myself to stopping every 200 miles.


Thanks for your help.




Richard


Commerce, Mi

You will need a transfer pump if you are T'd into the return line. Where did you weld a 1/4" tube for a vent? If you install a tube in the bottom of the tank, you will be able to gravity feed. I did that on my K12RS, by installing a bulkhead fitting in the bottom of the plastic tank. 2.7 gallons transfers in about 20 minutes at speed. The bottom of your cell needs to be higher than the level fo fuel in your tank to fully transfer.

[RICHO2006]

You will need a transfer pump if you are T'd into the return line. Where did you weld a 1/4" tube for a vent? If you install a tube in the bottom of the tank, you will be able to gravity feed. I did that on my K12RS, by installing a bulkhead fitting in the bottom of the plastic tank. 2.7 gallons transfers in about 20 minutes at speed. The bottom of your cell needs to be higher than the level fo fuel in your tank to fully transfer.

I welded the vent in the top of the tank and the drain is in the bottom. I beleive there is too much pressure in the return line due to the restrictions caused by the return fitting in the tank for gravity feed. When the engine is off the aux tank will drain into the the main tank in about 10 minutes.


Thanks


Richard Flood
Commerce, Michigan

[Bobmws]

Your fuel system runs at 38-42 psi. The return line will always have more pressure than a gravity feed will over come. It has nothing to do with the venting.