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This may sound funny, but for a while, Yamaha put a steel bolt in the aluminum casing on the side of the motor. dissimilar metals caused extreme corrosion inside that affected operation. Check out Boating forums on Yamaha.
the serial number begins 2487558 and continues past 2491265 and as the beginning serial number denotes the year , your engine is 1969 model
it shows as a 2 stroke with auto oil injection
check the heads
if the plugs are in the flat of the head and there are no rocker covers it is 2 stroke
talk with a mercury /mariner service center
there is a pump for sale on e-bay --seller is western furniture (1918) used condition for $25.00 us 2 people watching it but it has a buy now tab
I found this by going google and typing in --fuel pump for mercury outboard motor 3.9 hp-- and the site is www.ebay.tv>sch>ebay-motors-
mariner is also a part of the mercury range so check for mariner 3.9 or 4 hp parts
With your lower unit off have someone shift the engine into forward while watching the gear shift select shaft's rotation,let gearshift remain in forward,now place your lower unit in forward and reinstall. In the future just put engine in forward when installing a water pump as there won't be a need to move gearshift