I have a 1977 fl250, got it as a bunch of parts, had to put them all back together, was quite the learning experience as this is my first odyssey! I am having some issues with fuel delivery i think its an issue with the pump but i am not sure, when i do the flow test, it basically doesnt pump at all unless i give it some throttle, then its a little bit but not enough. When i open the tank, there is basically always at least a little fuel going through the return, usually alot. I have tried flipping the tee around and tried an extra one i got with the machine, tried with and without the fuel filter, tried main and reserve (in case pickup tube in one or the other was clogged. Its not, it flows great from both with no line on), tried a whole other pump, also tried rebuilding the pump with an oem rebuild kit and still nothing. The weird part is this, when i start it up, it runs pretty great, i take it for a very quick cruise, and check the carb afterwards, and its basically full. I am doing this by taking the line off at the tee and filling it with a squirt bottle of premix. It doesnt appear starved for fuel at all. It does have an exhaust leak, but going through it in my head i cant think of how that would cause the issues im having. I also replaced the crankcase bearings and seals so i don't think that's the issue? Any ideas on where to go from here? This thing is a blast, i would love to be able to ride it without worrying about blowing it up.

We have just been through this with fully but he has an FL350. Those units have a starter. Yours probably does not unless the previous owner installed a starter kit.
Therefore you will have to do the flow test by running the machine. CAUTION FIRE !
First off read fully's post: viewtopic.php?f=1&t=20560
Most of this stuff applies to you so check it.

Now we get to your flow test.
1) What I would do is start up your machine and lets it idle for a few moments. This is just so that the carb is full of fuel (or not).
2) Now shut it down and remove the fuel line from the carb and put it into a measuring container that has oz marked on it.
3) Next (have a stop watch handy) start your machine and the stop watch. Run it for 10 seconds and shut the machine down.
4) How much fuel do you have in the measuring container ?? If you don't got 4.5 oz approx then you're ded. You can most likely get away with 4 oz. Any less and you're gambling.
5) If you don't got the required amount then repeat test but pinch off return line. How much you got now ?? If you got the right amount now then the check valve is poo. See the fully thread above for making a check valve.

Your issue is: 1) a bad pump 2) loose or leaking vacuum line to pump 3) bad check valve 4) Engine does not pass leak down test and therefore your vacuum line to pump is not getting full pull.
CO