While determining what R1 said, keep your 50:1 premix and continue till you know the OP is adding oil to the fuel...you may smoke a little when the crossover occurs as your premix gets run out of the tank but its better than a froze up engine.
Pump is easy to find....follow the fuel line from where it enters the engine and within a few inches the fuel line makes a Y. The Y is where the oil pump outlet is connected...just follow the hose. My OP had 3 screws holding it and upon removing them and any hoses that keep you from pulling the pump, the pump comes straight out.
The geared shaft is right there and is probably missing...that is how I killed my oiler when I wanted to pre-mix on a 90 HP I had. If you pull your pump and the shafted gear is missing, just buy the shafted gear and any spacers needed...check dealer's parts list. Insert it into the pump and twist it ensuring it connects with the pump impeller. Install the pump but connect the output to a test hose and have a measurement container handy.
My 90 used 80:1 at idle up to 50:1 somewhere between idle and WOT. The easiest way to test is to hook your engine to a small tank of gas so that you can measure consumption. Fill the onboard oil tank. On the pump there should be a "bleed screw". Identification (without a picture and note) to me would be a screw that is easily accessible and not obviously used for any structural reason...like sitting in the center of a flat plate having perimeter screws for attachment.
Manual states, open it 4-5 turns and watch the hose coming into it from the tank and let it drain till no more bubbles....engine not running....solid flow of oil...close screw. When you get it all back together, after your testing is completed and you are ready to hook everything up. purging the air from the outlet hose of the pump is merely running the engine at idle till it is free of bubbles....then you can get off your pre-mix.
But, back to the test:
Connect engine to the 50:1 test tank
Connect cooling water to engine
Looking at the pump there is a rod from the throttle linkage that adjusts flow as a function of RPMs.....disconnect it so that you can manually control the movement of the lever on the fuel pump. Rotate the lever fully CCW which corresponds to WOT. If in doubt, before you disconnect, look at the lever on the pump and move the throttle to WOT...lever should revolve to the max CCW position.
Monitoring an accurate tach, for the test you will start the engine and set RPMs to 1500 for 10 minutes.
You need to adjust these numbers for your engine since they are for 75-90 HP 3 cyl engines and 100-125 HP 4 cyl engines.
3 cyl engine should pump 27.4 CC +/- 2.7 CC
4 cyl engine should pump 46.7 +/- 4.2
Comment is made that testing is to be done at 70*F for those numbers.....different temps mean different values. Manual cooler or warmer will supply less discharge....gotta do some interpolating here.....time to suck it up and take your best guess!!!!!
If I had a 40 hp engine and were going to do the test here is what I'd do:
75-90 mid point is 82.5 which is about 2x your engine hp so the number will be close to halve the above numbers for that ratio....2.06x actually.
100-125 is 112.5 so you are at 112.5/40= 2.8x your consumption
So the average of that is 2.4x your consumption so if it were my engine and I were doing it, I would divide the above consumption numbers by 2.4 (15.4 cc for 40 hp, my calculation) and if I got anywhere close I would be happy, or just forget the whole shebang and stay on pre-mix..
However, you could get yourself a manual and have all the correct info at your fingertips.
