Mid 70's Mercury 115HP Outboard with slowly fading power on acceleration in water. Ideas?

Question

I have a mid 70's Mercury 115HP Outboard that was running good last week in the driveway. I took it out for water test got it almost on a plain and then power faded until stall. Engine would restart instantly, repeated numerous times. Then the engine would not start. Still cant get it started. Fuel line tattered but getting fuel to the engine. One of the chokes doesn't work on one of the carbs, others fine. Do these engines have fuel filters? Where should I start?

Answer 1

"fuel line tattered"    

that's a no-brainer.  Obviously enough fuel is not getting to the engine to keep up with demand at high RPMs.  New line with a new primer bulb is what, $20?

Also it might be masking other symptoms -- you don't know how it will run it with a good fuel line.

If the faulty choke plate is closing off, then you've got one cylinder right there not making power.  It's just a metal rod linking that carb to another.

Do a compression test and a spark test.  Not a *spark plug* test.  Spark should jump a 7/16" gap outside the cylinder.  Or if it's an old-school points/condenser/distributor type ignition, 1/4".  Compression should be about the same on all cylinders (within 5 or 8 PSI).

The non-start could be a number of things:  flooding due to sticky carb float valves, insufficient cranking RPMs due to fading battery,  water in the fuel, tattered fuel line sucking air & just getting worse, engine overheating due to neglected water pump impeller.

When you get it going again, with the new fuel line, and power starts fading, try squeezing the primer bulb;  if it collapses (refuses to open back up) then you've got a restriction upstream of the bulb (debris in the tank), or the tank vent is blocked.  If it causes it to run better, then rebuild the fuel pump.

If squeezing the bulb has no effect, then bump the choke button.  Choking a running engine should kill it;  if it makes it run better, rebuild the carburetors.  Soak 'em overnight and install new kits.

How well it runs in the driveway on the muffs is no indication of how it will run with backpressure on the underwater exhaust, once it's submerged.  

Don't rev it up on the muffs (except to warm up a cold engine).   If it goes into "thermal runaway" it'll race uncontrollably and you can't shut it off except by disconnecting the fuel and hope the carbs run dry before it tears itself apart.

Answer 2

Rebuild the carbs and replace the fuel filters.

Answer 3

I had this happen once, it ended up being the gas cap vent clogged.  It would run great, then die out.  I figured it out when I heard the whoosh when I opened the cap.

The fuel filter would be a another great place to start. There has to be one in there somewhere; also empty out the gas tank to get all the sediment out.  

Is there a temp gauge or alarm?  If the pump impeller is shot the engine will run great as it warms up then die as it severely overheats (not Good)

Good Luck