I have a 1991 Mercury 115 (2+2) outboard that has symptoms that led me to attempt troubleshooting the fuel bleed system. My model, unfortunately, is one of a chosen few for that era that have the bleeder system check valves installed on the interior of the crankcase cover in the vicinity of the three inner main crank bearings. If you can believe it, you need to pull the engine block and split the crankcase to replace the check valves. There were also “restrictors” added to the system at some point. They apparently should be added to some units that came without them from the factory. If I understand my manual correctly, my serial number motor included the restrictors from the factory. The restrictor looks to be nothing more than a small orifice fitting installed beneath the check valves.
My motor has three internal check valves, and the collected fuel is passaged to a fourth (external) check valve located at the lower crank bearing end cap. There the collected fuel gets sent through a bleeder tube to a fifth (external) check valve at the upper crank case/bearing location where the fuel eventually gets recirculated for combustion.
In my attempt to troubleshoot this fuel bleed system, I first removed the exposed bleed tube running from the lower bearing to the upper bearing and easily checked the check valves at both ends of the bleed tube for proper operation. Both checked out fine.
I then thought, in theory, that the 3 check valves buried inside the crankcase would only work if they are within some type of closed loop (passages in the crankcase cover?) that channel the collected fuel to the lower bearing point where it exits the crankcase. So I replaced the check valve in the lower bearing end cap with a threaded hose barb nipple and put a hose on it with the idea that I should not be able to blow air into the hose if the inner check valves are working properly but should be able to suck air back in the direction of flow. I was able to blow into the hose and suck out of the hose, leading me to believe that one or more of the inner check valves is faulty.
So I am seeking advice or support on my theory, as I am far from a professional boat mechanic. Is it true that I should NOT be able to blow into my crankcase through my improvised barb fitting if my internal check valves are working properly?. I would sure hate to pull the powerhead and split the crankcase only to find perfectly good check valves. My service manual claims the check valves (by design) have nylon balls in them that are prone to melting from some types of misfires. My motor has about every new part imaginable, rebuilt carbs, good compression and runs good most of the time. If I sit at idle for a period, it decides to stall out and will not start back up unless I let it sit for 15-30 minutes. Then it’s good to go again like nothing is wrong. I’m sure there’s a million things that could cause the problem, but mostly am inquiring about the best way to troubleshoot those inner check valves before splitting the cases. And/or backup support that the test I performed isn’t flawed by something I am missing in my thought process.
My motor has three internal check valves, and the collected fuel is passaged to a fourth (external) check valve located at the lower crank bearing end cap. There the collected fuel gets sent through a bleeder tube to a fifth (external) check valve at the upper crank case/bearing location where the fuel eventually gets recirculated for combustion.
In my attempt to troubleshoot this fuel bleed system, I first removed the exposed bleed tube running from the lower bearing to the upper bearing and easily checked the check valves at both ends of the bleed tube for proper operation. Both checked out fine.
I then thought, in theory, that the 3 check valves buried inside the crankcase would only work if they are within some type of closed loop (passages in the crankcase cover?) that channel the collected fuel to the lower bearing point where it exits the crankcase. So I replaced the check valve in the lower bearing end cap with a threaded hose barb nipple and put a hose on it with the idea that I should not be able to blow air into the hose if the inner check valves are working properly but should be able to suck air back in the direction of flow. I was able to blow into the hose and suck out of the hose, leading me to believe that one or more of the inner check valves is faulty.
So I am seeking advice or support on my theory, as I am far from a professional boat mechanic. Is it true that I should NOT be able to blow into my crankcase through my improvised barb fitting if my internal check valves are working properly?. I would sure hate to pull the powerhead and split the crankcase only to find perfectly good check valves. My service manual claims the check valves (by design) have nylon balls in them that are prone to melting from some types of misfires. My motor has about every new part imaginable, rebuilt carbs, good compression and runs good most of the time. If I sit at idle for a period, it decides to stall out and will not start back up unless I let it sit for 15-30 minutes. Then it’s good to go again like nothing is wrong. I’m sure there’s a million things that could cause the problem, but mostly am inquiring about the best way to troubleshoot those inner check valves before splitting the cases. And/or backup support that the test I performed isn’t flawed by something I am missing in my thought process.