Did a flow test on my brothers machine today.
https://www.youtube.com/watch?v=DBUzaVCFdS0

If anyone is interested, you can look close at that return line and you can see my homemade check valve in the line. These work great and have had no issues with them for years.
CO

I have installed a restrictor on the return line on modified FL250's that required more fuel.

Cool video.

I see in the video you pump location. How long is that line?
Also keep in mind you use aviation fuel. It's Caustic properties can have an effect on the pump. No big deal just keep it maintained.

Is there any data for flow at given pulse rate for these pumps. I also see a check valve. With a restriction at that point and at idol the carb float valve closing sooner the pump will reach equilibrium and reduce volume on out put. I would say on the test remove check valve and install line equal in size to out put port on the pump with a open tee. This way the out put line size will flow at it's max. See if that changes the flow rate. To recap the port line length, shorter the better. I would check and see if there is a max length rating.
I would also set idol rpm at 1300 to 1400 after the Engine is warmed up. I would say this would provide a base line fl350 pump volume flow rate. I would also try to match the factory lengths and location for base line than compare to yours. Then a guide can be made. Then guys with twins can change to a pump that supports the twin and get a proper flow rate. The same equilibrium in the pump can happen with a twin if a check valve and improper return line size is not in check when tested.

Cool video more please.

Looking at the pump mounting.
Is there an up arrow or position?
Can they be mounted any any position?
Not familiar with that pump. Link please.
Any installation instructions.
Looking at the pump it should put out a ton of fuel.
Keep playing.
Does mikuni have a site to read up on pumps.
I want to educate my self.

Your bothers Engine sounds strong.

These are good ideas and I was thinking the same thing about the restriction in the return line at the check valve. I have other things to do but will get back to this. At least now I can edit videos. The card in my dinosaur phone is a bit small so I make several small vids and edit.
CO

Edit:
1) No UP arrow on the pump that I remember seeing. These are pulse pumps so I can't see that being an issue. These would work at any angle.
2) Flow rate for pulse rate: I have never been able to find any info on that -- ever -- anywhere. The closest info you will ever find on that is the Pilot manual.
The FL400 manual states 4.7 oz at idle. So I guess it would be what ever idle is. So that's all we got.
3) Right now I think we should be testing our pumps at machine idle. It was obvious to me during this test that the pump puts out a lot more when the machine is running. So it's possible that all those pumps I thought were shyte may be still good. I did measure starter rpm on an oddy one time and I think it was 450 rpm. So for those people who were having trouble getting 4.5 oz in 10 sec flow, that may be the reason. If you got a weak starter or a low battery or poor electrical connection or high compression, then the starter method may be a problem.
CO

you starter spins way faster than mine does..
I think this is flawed a bit especially with it running. your using the return line which is a reduced to force flow to the carb.
I think the carb bowl would hold enough fuel for a 10 second idle to use the carb feed line..

I agree but just didn't want to do it that way because of fire hazard.
CO

with cold Engine I don't think it would be a problem..

I'm going to revisit this but not right now.
I agree with you that flow rate could be a bit impaired using the return line when the machine is running. I'm planning on pulling the carb line and pinching off the return line. Or just disconnect the whole mess and run a line from the pump direct to the measuring container.
Fire is a big thing for me. I've been on fire in a race car and have been involved in two really big fires at a job site years ago. We almost lost the plant twice. These fires were caused by someone not doing their job and on the race car the line going to the carb snapped off and the electric pump pizzed fuel all over. What you didn't see in the vid was two fire extinguishers sitting where I can get at them from a safe distance.
CO

I agree 100%
BE SAFE

I have done some of this doing vacume testing setting the air screw on the carb.
It can be done in a safe manner.
Looking at the flash point of gas or fuel/gas mix you simple remove all ignition/fuel sources. Plenty of air movement is a must for vapors using a closed /vented container set up.
You can control the vented gases back into the tank thru the return line. So pump into a container with a vent line hooked to the return line of the tank with the cap on. This with keep pressure build up to a minimum. A fuel supply bottle can be used with a natural air inlet at the top. It does not take much fuel to run our engines at idol for 10 seconds.
Be sure there no static electricity potentials and do on a dry air day. When in doubt ground it out.

There is always a risk when work with a fuel supply. Do it as safe as possible remove potential as to a safe level. Just like storing you gas cans outside and not In your garage next to the gas water heater with no make up air supply. I even like to keep my tanks full of gas on the bikes when stored. Plus it keep the tanks in good shape.

If you do not feel safe doing it, "DON'T do it". It's not worth it. "Never do it alone"

I actually witnessed this happen at PIR (Portland International Raceway).
A car pulled off the track into the hot pit to fuel up. Car off, no smoking blah blah -- it was the rules. All of a sudden during refueling it lit up. It happened as soon as he touched the car with the gas can. It just seems strange because the fuel cans are plastic but that's what I saw. All anyone could think of was static electricity. No real damage because we are fully equipped at the race track.
It happens to me when I go to Vegas certain times of the year. The static is real bad in the casinos. Every time you touch a hand rail or slot machine you get hammered. I used to walk around with a penny in my hand. I would touch the hand rails with it constantly and the penny would get the shock not me. You could see the spark jump a mile to the penny LOL. Seemed real bad at Ceasar's for some reason.
CO

Ok I have redone this flow test, but this time I did NOT use the return line.
The line was removed from the carb and the return line was also out of the loop.
Basically what I've learned is that we must test our fuel pump flow direct from the pump and the machine MUST be at idle. I've been doing these tests using the starter all of these years and it's wrong. You get the wrong reading. Honda was correct when they say "at idle".

In the video below you will see in the first test that I had rev'd the Engine at start a bit and the test actually went 14 sec and not 10 sec.
In the second test it was actually 10 sec at idle so this reading was spot on.
Based on this video we can clearly see that the Engine rpm makes a HUGE difference in the flow rate. At higher rpm these pumps put out a lot more. Even if I only bump the rpm for a second I got 2 oz more flow. Clearly the starter test is not accurate because of the spin rate.
I now think that all of those pumps I thought were shot may still be good. I'll have to retest each one.
https://www.youtube.com/watch?v=PkAjyV6-SS8
CO

Great job. Any idea the idol rpm?
Since you mentioned the throttle bump and increased flow.
Don't have to worry about carb running out of fuel. Now you have a good base line to test other pumps.

Thanks for the video

No I did not have a tach hooked up but I can do that another day.
The idle was set to the very minimum -- just above stall.
All my machines are set like this so that the transmission shiv don't turn and you can put it in gear without grinding the box.
So idle is what ever the stall rpm is. If I remember right, the last time I had the tach hooked up it was 1400 rpm.
CO