I want to add an external auxillary fuel cell to my '99 Bird. I don't want to modify the fuel tank at all to accept fuel from an ouside source. Can I tee into the fuel injection return line between the fuel pressure regulator and the tank? How much back pressure and flow is there during normal operation? Would it cause problems if my system added additional back pressure to the tank side of the FPR?

I believe if you have fuel injection you need to add a fuel pump if you want to tee into the fuel line. I know you said you did not want to modify your main tank, but that really is the best way to do it. Using the gravity fed method has the least working parts and is the most reliable. That's the way the aux fuel setup is on mine and it works like a charm. Check out this site, it has a lot of good information on setting up aux fuel systems.

http://www.sampson-sporttouring.com/

If you do decide to go with a gravity fed system, make sure you use a uni-bit to drill the tank, and add a quick connect close to the tank so it's easy to r&r the tank in the future.

Most FI systems provide more pressure and volume to the injector rail than needed. The excess is returned to the tank.
To tee off the return will probably fill your Aux tank with the returned fuel. Not what you are going for. Plus you will be on the wrong side of the regulator to get the fuel into the tank.
How about buying a replacement fuel tank filler and modifying it? I bet you could come up with a cap that will still allow fast fills from
a station, with a extra quick disconnect hose coming into the top. With a little testing, you could figure out when to turn on a pump in the AUX tank and refill the main. You would have a hose coming across the tank, but it could be removed easily when not needed. Aircraft use this setup all the time for ferry tanks. Yes YOU have to manage the fuel flow, but modifications to the system are minimal.

I was thinking that I want to pump from my aux tank to my main tank. I have a few different styles of small form factor, high pressure (fuel injection) car fuel pumps that I have been collecting for this project. What I'm hoping is that I can find a pump that doesn't allow the back-pressure of the return line to go backwards into the aux tank, and also have a low enough pump volume that it won't effect the bike while it's running. I had a setup kinda like this on my Magna and it worked really well.

It sounds a little overly complicated, but I like it because I can hook the fuel pump switch to a ignition-controlled power source and nobody can mess with it. On my Magna I mounted the fuel pump under the seat towards the back, and it looks like there is room back there on a Bird too.

There is plenty of room under the seat for a fuel pump. If you want to go that way what is usually done is adding a tee to the return fuel line and filling the tank from the aux. Can I ask what your aversion is to using the gravity fed method and adding a bulkhead to the main tank?

Most of the setups I've seen just had a fuel line running from Aux tank to main tank. Open the valve and it fills the main tank, of course these were mounted on passenger seat.

Yeah, that's the most common and the setup I use (gravity fed). I have the tank on the rear Givi rack with a valve, which dumps into a 3/8 that goes into a bulkhead I added to the bottom of the stock stank. Along the way there is a quick-disconnect to make tank removal easy.

There are a couple of reasons I don't like gravity feed.

First, I don't like manual valves, because they can get bumped, accidentally left on, and they are bulky. Plus the lines to and from the valve are exposed to damage, and the valve itself has to be mounted somewhere where it's accessible while driving. It doesn't seem like it's something I can do and make it look really sexy.

Second, if I only tee into the return line than this setup can be 100% removed some day with no evidence. If I do ever sell the bike someday, I don't want to have to explain extra holes and brackets and stuff, I just want to bring it back to exactly stock.

Besides, I'm an electrical engineer, so I really want to put some cool wiring farkles on it somewhere.

So, back to the original question: can I tee into the fuel return line and pump gas into there without causing problems?

Heh, just asking..

And yes, you can tee right into the return line without any problems. Post up some pics of your finished product. I'm curious to see the final result. You have some good points above.

Sweet, thanks.

Since I'm in Michigan I have at least 4 months of down time in the winter to tinker with this sort of thing. I'm just doing the research now so I'm ready. I'll post up pictures when I'm done.