Don't know much about rk970, but the advice he has given me has been pretty good....
Here is some info rk shared to help me. I will pass it along.
In my experience (especially with outboard engines) when the fuel supply cannot keep up with the demand is that the engine will "bog" and start loosing rpm under WOT. If jetting is to lean it will pick up a little as you close the throttle. If it were me.. Go gravity feed and remove the drain plug from the float bowl and time how long it takes to fill a 1qt cup (16 oz will be fine also). With that you can calculate how much power your fuel system can support. For gravity feed systems I try to get 30% more fuel flow then estimated power.
So lets use 15hp.. and .45 BSFC (little on the rich side)
GPH = .45x15hp/5.92
GPH for 15hp is 1.14
Take 1.14 x 8 = 9.12 pints per hour or 6.58 minutes per 16oz flow.
add bout 30% and look for 20oz flow in that same 6min 35sec time.
So according to this info, if gravity can fill a 16oz cup with fuel in 5 minutes 16 seconds, gravity can also support 15 HP.
Or 20HP is 1.52 gallon per hour, or 3.2 oz per min.
With the 30% margin described, 4.2 oz per min. If you can get 16 oz in 3 minutes 49 sec, then you are good for 20 HP.
Make sure the tank is full and it is venting enough. If it flows too slowly, try to loosen the gas cap and see if it flows faster.
The results of my testing were as follows.
I might have been using 1/4" fuel line. (stock is 3/16") Don't remember what I had. Probably 1/4". I also had another vent in the gas tank. It was a hole for a return line for a pump, then I made it a vent.
4oz 0:56
8oz 1:45
12oz 2:33
16oz 3:25
20oz 4:20
You can see that I was able to flow 16oz of fuel in 3:25 with gravity, so more than enough for 20HP. So at least on my set up, gravity would fail to provide adequate fuel flow at around 22-25HP.
