the gravity tank on top of the engine is just a transfer tank, the bypass he did, didn't do much. The part that gives issue with Merc oil injection system is still working(you hope). The way the stock system worked, crank case pulses pressurized the oil tank in the boat, that forced oil to the transfer tank on top of the engine(two hoses, one pressure, one oil feed). That part was pretty reliable and hardly gave any issue as long as you kept the top of the oil reservoir tight. The small reservoir on top of the engine gravity fed the oil pump on the port side of the engine. The oil pump was driven by a worm gear that ran off the crankshaft. That's where the problem is, the gear on the crankshaft was made of plastic. If there was any issue with the pump, the gear would strip off and not pump any oil. Merc was smart enough(seemed to be at the time) to put a sensor on the oil pump drive shaft that would set an alarm if the shaft stopped spinning. That system worked pretty good, good enough for Yamaha to copy some of it and actually use an almost direct copy of the oil pump(same manufacture), Yamaha was smart enough to use a bronze gear instead of plastic. After years of trying to figure out what was causing the gear failure, Merc finally found the coupling they put in there for the sensor to work would swell up and lock up the pump drive, causing the gear to strip out. Had Merc used a bronze gear, there system would probably have been just as reliable if not more so than Yamaha's system.
Easiest way to remove oil injection on a merc(maybe not the best, but the easiest). Remove the reservoir inside the boat, remove the oil feed and pressure hose(two black hoses made together, one with a blue stripe). Follow the hoses to the engine. You will see a fitting under the starter on the starboard side, that's where the crank pulse come from. Remove that fitting from the block, replace it with a pipe plug(NPT) the same size, throw the fitting away. Remove the engine reservoir from the engine, the oil lines will go to the oil pump, its a clear hose that has probably turned green from the oil. Check and see if the hose is in good shape, If it is, use a section of the hose and loop it from the oil pump inlet to the oil pump outlet, leave some oil in the hose so it keeps the pump lubricated. Follow the pump outlet hoses down to where it tees into the fuel line. Remove the Tee, good time to replace the fuel line with new, so just replace that section. The wires from the oil pump sensor and the oil tank lid go to a black box(warning module) on top of the engine, unhook the wires, remove the box and pitch it. The box ties into a terminal on the starboard side that has tan/blue stripe wires running into it, make sure to only remove the wire from the module. You can test the alarm system later by grounding that terminal with the key on, it should sound the alarm. The only thing left to do is remove the linkage from the oil pump to the carburetor, it just snaps on, it may take a little prying to get it off, remove it from the pump and carb. The next time you start the engine up, make sure you premix with 2 stroke before starting it up.
There is a better way to deal with the oil pump, but it requires ordering some parts and doing a little more work, this is the easiest, but not necessarily the best way
good luck
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