New owner of a Mercury Mark 55e per serial number may be 1956 or about. Previous owner stated that he had "completely redone" motor which I was skeptical of since he couldn't provide a repair manual and now I see the lead sealing wire on case is intact. However, motor did start and run reasonably well in a barrel.
On first trial outing, with 40:1 fuel mix and new battery, discovered odd thing that shifter controls seemed to be reversed, ie. shifter lever needed to go aft in order to move forward, likewise for reverse, lever needed forward movement.
Also, 20 minutes into ride, when moving to an idle speed, engine stalled and would not start. After 20 minutes of trying (perhaps after cooling some, engine started and we were able to move almost to boat ramp area before it stalled again. My son ended up essentially towing boat to ramp from in the water.
With the engine back home, we rigged up adequate barrel arrangement and began troubleshooting. Compression is reasonably consistent across 4 cylinders (125). Plugs had good brown color. Tell Tale seems ok. After on-line inquiries, determined we needed to drop lower end to correct backwards shifting issue. With several hours fiddling with this, bottom end on and off, referring to a number of on-line posts, we managed to correct shifting process (we think) by finding the correct positioning of the shifter shaft coupling.
However, now engine will not start. Fuel appears to be pouring out of both carbs. We did not overdo bulb pumping. Previously, engine seemed to start reasonably easily. Where do we go with carbs? Do they need rebuild?
One note,,, while trying to correct shift issue, noticed "linkage" between shifter shaft and throttle control. ie. when looking below carbs can see what appears to be a notched piece that is "not quite matched up with" corresponding throttle link.interface. Someone said we need to use long screwdriver and correct this by lifting shifter shaft maybe? Did we miss something in correcting backwards shifting issue?
In respects to your flooding carburetors,...I recommend cleaning them, and replacing the needle & seat kit and a set of new floats.
Sierra Marine # 18-7057 for the needle & seat kits
Sierra Marine # !8-7208 for the floats.
Be sure and remove and clean the by-pass tubes.
Another thing to determine if the beast dies as it idles down is the condition of the magneto. An original coil from 1955 may not be as able to produce spark as it used to be, due to absorption of ambient moisture. New points aid in this struggle too.
Sierra Marine # 18-5148.
Currently Sierra Marine is in the middle of restructuring, so your best bet might be these part numbers searched on eBay.
There is quite a bit more to the proper placement of all the shift components, but if you have the cam in the lower unit in the correct position and right-side-up, and the Reverse Lock cams orientated correctly. then your much closer, and perhaps my attached picture will help with the final external adjustments.
i had that shifting backwards issue on a boat i bought about 40 years ago with an early 60's 35hp and if i remember correctly it was the wrong cam in the shift box. also if it still has the old fuel lines i would replace the with ethanol resistant fuel lines. that's my 2 cents