Gas flow question...........

[Big_Steve]

Need some input from some of the mechanics here.......

My bike has a carb, and relies on gravity feed for gas. I'd like to install an Aux gas tank inside the body, back near the trunk. However, the tank would not be higher than the carb intake. So gravity feed probably isn't going to work. Couple of questions.........

If I connect the aux tank into the gas line between the carb and the Honda on/off/reserve valve, will a low pressure fuel pump work? Or will it overflow the carb? (I would have the Honda valve turned off.) I would have an on/off valve in the aux fuel line of course.

If I wanted to use it to just refill the main tank, and not run the bike directly, would this work? Connect the aux tank in the same place (after the Honda valve,) and install on/off valves in both the aux line (A) and again in between the Tee where the aux line would connect to the main line (B,) and the carb. I'm thinking to use, I'd pull over, cut the engine. Leave the Honda valve on either "on" or reserve" then close "B" (so that no fuel goes towards the carb) then open "A" and pump. My thinking is that fuel will then flow into the main tank through the bottom. But I don't know anything about the main valves. Like for some reason, will they flow only one way? Or is there some other reason why this is a bad idea? My valve is connected direct to the tank, and I really don't want to start having another fitting welded onto it. But if that's the only smart solution, I'll go that way I guess.

 

[1550vt]

What controls the flow of fuel into the carb bowl is the float valve and that valve is only held close by a very small amount of force from the float. (Sounds like I am talking in circles.). Anyway, if you put a pumped source in the carb input line from the tank...I think you will have a flooding problem when the pressure overcomes the float pressure on the float-valve. We're talking ounces here. You would have to go through some valving installations, which is going to result in a big plumbing mess and a "procedure" that you will have to remember each time you do this. I did a gravity-flow system for a guy on a Shadow once and he was forever forgetting to turn the right valve on at the right time. He would overflow the tank from the reserve tank, or forget to switch to the reserve tank and run out of gas.

It's a piece of cake on a Harley, because you have places in which you can inject pumped fuel right into the tank.

You can do what you said...put a valve between the carb input and the tank and shut it off as you pump-in new fuel from the reserve. The reserve fuel should back-flow back up into the tank, but you are really going to have a mess of valves and clamps.

I am not all that familiar with Honda tanks, but if you have no other method of getting fuel into the tank other than the petcock...I'd remove the tank and have a bung welded in so I could pump the fuel directly into the tank. If nothing else, I would modify the fuel fill or cap to accept a fitting that would allow the reserve source to pump into the tank.

You could install a check valve that would allow gravity flow and then close on higher pressure, but you still have to do something to keep the fuel you pumped into the tank through the petcock from draining back into the reserve tank...another check valve. Some fuel pumps have a check valve and some don't.

Now....some of you Honda guys prove me wrong here and help Big Steve out with a fix. Maybe you can do this with an aftermarket petcock, but you will have to keep the pressure from the pumped reserve from getting to your carb float or it will overflow and flood and you have to keep the fuel you pumped into the tank from flowing back into the reserve tank.

 

[Nauga Mok]

The way I plumbed mine was to put a low pressure 12V pump back by the aux tank to serve as a transfer pump. I put the fuel filter between the tank & pump so the filter stops the crud that eventually builds up in tanks from getting to the pump, and the rest of the system. Since my Harley has EFI, by just transfering to the main tank, there's no extra valving. I ran rubber fuel hose up to the rear of the main tank, then ran steel tubing under the main tank to the front. After the line got to the front of the tank, I bent it down & again ran rubber fuel hose & "T'd" into the main tank's cross over hose. I covered the steel tubing with a rubber hose so it didn't rub on the bottom of the main tank or frame. By doing so, I was only able to squeeze in a 3/16" line, but that's not a problem for a transfer line. By running the line on top of the frame's backbone, when I pump the aux tank dry, it creates an air bubble to prevent syphoning since the fuel level in the main tank after the transfer is below the highest point of the transfer line. Harley Ultras have an "auxillary switch" beside the ignition & cruse control switch that goes to a plug under the seat. I pulled power for the pump from this plug under the seat. Since it's already a fused cirfuit, adding extra fuses was unnecessary. When my fuel gauge gets below 1/4 tank, I turn the pump on & watch the fuel gauge come up instead of go down. When it quits rising, I turn the pump off. At the next fuel stop, I always turn the pump on & listen for air bubles from the transfer pump so I know the antisyphon air bubble is in place because the main tank is higher than the aux tank and when completely full, the fuel level is above the transfer line for about the first 10 or 15 miles. The top few inches of a Harley's main tank goes fast. True, the pump's check valves should prevent syphoning, but why take the chance of that happening, & overfilling the aux tank. The extra 2.8 gal in the auxillary tank + the 5 gal main tank gives me a range of 190 to 280 miles, depending on fuel economy. I'm getting 25 mpg in town, & 37 highway IF I stay at 70 mph or under. Here in Nevada, gas stations are often 100 miles or more apart. Don't count on your cell phone either! It's standard prociedure to loose cell service 20 miles out of one town & not get it again 'till 20 miles from the next -- 95 miles away. They've got the interstates covered, but I MUCH prefer running US 93 to I-15.

NM

 

[Big_Steve]

Thanks so much for the detailed responses. I knew there had to be more factors here then what I could think of. I think if I do this, I'll put a tour tank up high, and just Tee it into the carb (with a shutoff of course) and let gravity do its magic. If I tire of having it on the back, I'll see about having a connector added to the bottom of the tank, and switch over to a pumped transfer system while mounting the tank in the trike body.