This isn't really a how-to, but is an overview of how an external pump can be set up for the IMS 10. If one wanted to use a Mikuni vacuum-operated pump the process would be much the same.
Many thanks to GoMotor for getting my head screwed on straight. He suggested that the tank could be used as a gravity tank for the majority of the 10 gallons. Then the pump could be turned on and the right wing pumped dry. The 'On' position would handle the gravity feed and pumping out the right wing via a pick-up line from the right wing to the 'On' petcock pipe. Once the right wing is dry the petcock could be switched to 'Reserve' so that the left wing could be emptied via a pickup tube from the left wing to the 'Reserve' pipe.
Inside the tank
The tank is assembled with tubing similar to Tygon, but it will be hard and inflexible after prolonged exposure to fuel, so it will need to be replaced.
Two different diameters of tubing will be required. The tubing that comes up from the bottom of the wings will need to be 3/16” tubing, while the tubing that will attach to the petcock will be ¼”.
Two filters will be required, and the filter makes it easy to transition from 3/16” to ¼” tubing.
I used genuine Tygon tubing throughout the project. It is a bit spendy, but remains fairly flexible even after prolong exposure to fuel. The filters I used have a mesh screen similar to the mesh screen that is on the petcock. These screens will be removed in the petcock mod, so the inline filters replace those screens. The ones I chose are intended for gravity fed Briggs and Stratton engines.
The petcock will have the fuel pick-up lines attached directly, one side to the ‘Reserve’ port and the other to the ‘On’ port.
Since the ‘Reserve’ port has no pipe in it, the filter screen needs to be removed and a pipe made and installed.
Once the filter screen is pried out what is left is a hole that is a wee bit bigger than 5/16. Yet, we want to have a ¼” pipe to connect the ¼” tubing to. I made the pipe from a short piece of brass tubing that was soldered to a longer ¼” tube. The assembly was then fixed into the port using JB Weld. I made the ‘Reserve’ pipe a bit shorter than the ‘On’ pipe so that I could tell the difference by feel inside the tank.
My petcock is a Raptor 660; the stock would be modified in the same way, though details might be a bit different.
The Stock Pump
Simply leave the stock pump in place. If you remove it you will need to find a way to seal up the hole in the tank and it is unlikely you can do better than the stock seal.
The Pump Bracket
The pump bracket is made from 18 gauge sheet metal. The pattern was developed by using a heavy card stock; not too hard to do. This seems like a good location for the pump. Obviously it is close to the tanks’ petcock and to the carburetor inlet, but it is also a convenient location for either electrical connection or vacuum connection, should you elect to use a vacuum-operated pump.
This bracket only has three mounting points, but it seems to be plenty rigid.
The pump is a Facet-Purolator 60304, which delivers 2psi and 15gph.
Plumbing the Tank
Begin by removing all of the original tubing except for the cap vent. You will need to unscrew the brass fittings at the bottom of each win to remove the tubing from the fittings. Attach new tubing to the fittings and feed the tubing through the wings. This works best with the tank upside down and a bit of weight in the end of the tubing. I used a few .177 caliber pellets with the skirt deformed a bit so they’d stick in the tubing with no worry they’d fall out. It’s still a pain in the butt.
Use a piece of tubing that’s about three feet long on each side. Once you’ve got both of them in, trim them so that they both come up to about 3” below the filler hole. Install the filters and loosely zip tie the two lines together so that they rest on the tank’s hump.
Install the petcock. Cut two pieces of ¼” tubing long enough to connect the filters to the petcock pipes. Connect the ‘On’ pipe to the tubing coming up from the right side and the ‘Reserve’ pipe to the left side. The idea is that when the petcock is in the ‘On’ position you will use all of the gas that is above the hump and then empty the right wing by turning on the pump. Switching to ‘Reserve’will give you access to the fuel in the left wing.
The external plumbing is done with ¼” tubing. To accommodate the plumbing, four 90° fittings and two T fittings are needed. I used 5/16” fittings for a good, tight seal.
There will be two modes of use. First, gravity flow through a bypass line that goes around the pump and, second, electric pumping.
When the tank is full and operating on gravity flow, the fuel will come down to the pump and flow through the bypass, through the valve, and back to the pump outlet and then to the carburetor.
The bypass cannot simply be a straight piece of tubing that goes around the pump. In any hydraulic system the fluid will always take the path of least resistance. In this case, if the bypass is a straight shot around the pump, the gas will come out of the pump’s outlet, go through the bypass to the inlet, and simply keep looping. The pump will not lift any fuel out of the tank nor will it deliver any fuel to the carburetor.
What I really would have liked to have was a very low pressure (remember, this pump delivers a mere 2psi, so there’s not much to play with) normally open check valve. When the gas would try to go back up the bypass, the valve would close under light pressure. I couldn’t find what I was looking for with a $2 budget. The closest thing would have been a Tesla one-way, but nobody makes them and machining one without CNC would be a bear.
What I had to do was put a manual valve in the bypass line, which is just a simple petcock. This was a pain in the butt, as I had to fabricate a bracket for it, but fortunately I’d gone through that pain earlier in the year with one of my cockamamie schemes, so I just re-purposed that piece
When the fuel pump is running, fuel will enter the pump and be delivered to the carburetor. The closed valve will prevent the fuel from simply looping through the pump and bypass line.
My only concern was how much fuel pressure the inlet valve in the carburetor bowl could withstand. In normal operation it only sees the head of pressure from the gas tank.
A back of the envelope calculation says that the force exerted on the valve by the pump is 11 grams so, while the force exerted by the head of pressure in a stock tank is only about 2 grams, I decided to not worry about it. Problem solved.
It all seems to work OK. I just wish I could eliminate the manual valve.