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Mercury 115 fuel bleed system

P

PJL

New member
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.
 
Does your engine still have the original blue or black stator? They we're very problematic and there were upgrades made to a red stator with an adapter. Not sure how you arrived at a bleed system issue but your symptoms point to towards al least a stator check. My 1995's 115's were replaced under warranty with the same exact symptoms. You'll need a decent digital VOM and a DVA adapter to check the output, preferably when the engine won't start.
 
Racerone, my compression test dated 10/9/22 was cylinder #1 138 psi, cylinder #2 137 psi, cylinder #3 136 psi, and cylinder #4 130 psi. Fitz, the stator is new and is CDI electronics brand which I believe is a decent quality aftermarket product. I do have a DVA (also CDI brand) but not much experience using it. Maybe I should check the stator even though it only has a few hours on it. If I recall, some of the CDI troubleshoot specs are different then the Mercury specs on the same part, so it all gets a bit confusing to me and I end up feeling like my electrical tests are somewhat inconclusive. The more I do, the better I get, so I haven’t yet thrown in the towel.

I’m in NH so the motor probably has not been run since the compression test posted above. I replaced the exhaust plate and exhaust cover gaskets this winter, so I’m thinking I did that compression test as I was pondering whether to dig into the exhaust. I have never had this motor running perfectly in the two years I’ve owned it, and I noticed two of the plugs (new plugs) had rust on them late last season when I pulled them for one reason or another. Thus the exhaust project. The exhaust was relatively clean inside but did have an easily noticeable whiteish gel on the walls of the exhaust path in
one specific location. I’m hoping that is a sign of unwanted water that was getting in there. I’ve not tested the exhaust work, maybe that in itself would fix the occasional idle/stall problem. I had it in my pea sized brain that I was gonna do the exhaust plate work and check the bleed system this winter so that’s why I started blowing through tubes at my check valves last week. For whatever reason, I thought bleed system fuel might be flooding my motor at idle if the system wasn’t working properly. Maybe something I read on one of the forum sites last year. And for the record, I would definitely have put the boat in the water and tested the exhaust work improvements (if any) before pulling the powerhead to replace bleeder check valves. I guess I’m ahead of myself trying to learn about the bleed system. Thank you both for your responses and I will check back if you have any further advice at this time.
 
Not sure if I responded properly within the forum format last night as a new member that’s bad with electronic devices. Sorry for doubling up with info.. The stator is new as of last summer with limited hours on it, CDI electronics brand. I do have a DVA adapter, I will do my best to double check the stator readings.
 
racerone said:
The trouble shooting starts with a compression test.-----Post your actual numbers here.
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Not sure if I used the proper reply method last night, trying again this morning. The compression readings are cylinder #1 138 psi, #2 137 psi, #3 136 psi, #4 130 psi. My notes on that test are dated October 9 from last fall so they are current considering the boat has been out of the water for the winter months. Last nights post from me on this thread has some additional information if you choose to read it. Thanks for the responses and your patience as I try to figure out how the forum works.
 
Remove carburetors.----Remove reed valves.----See if you can then replace the bleed check valves.----Not that involved in my opinion.
 
racerone said:
Remove carburetors.----Remove reed valves.----See if you can then replace the bleed check valves.----Not that involved in my opinion.
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Your suggestion is correct if you are one of the lucky ones with what the service manual refers to as Design 3. Those motors have a check valve and carrier accessible behind the reed block and replaceable without splitting crankcase halves. Designs 1 and 2 both require removal of the powerhead and removal of the crankcase cover. The check valves (some models also have restrictors from the factory) need to be tapped out with a punch towards the crankshaft side of the crankcase cover. On replacement, the restrictor is seated first and then the check valve against the restrictor from the crankshaft side into the crankcase cover. There are no external hoses or tee’s from the valves, I assume there must be channels within the crank case that route to the lower bearing end cap. The only external bleed hose on my motor goes from the lower end cap to the top of the crankcase cover (there are also external check valves at each end of the bleed hose). The only difference between design 1 and design 2 is that design 1 had check valves only (it is suggested to add the restrictors if they ever need replacing) and design 2 had the restrictors included from the factory. The parts list for my motor at Marine engine.com breaks down the parts by serial number and the parts for mine (serial # OD122008) are the parts for what is called design 2 in the Mercury service manual which (if accurate) means I would need to pull the powerhead and split the crankcase. Which is why I was hoping there was a tried and true troubleshooting technique that would confirm one or more of those internal check valves to absolutely be bad before tearing that deep into the motor.
 
What is your in gear, idle rpm in the water? These engines idle down to 675 rpms and purr like a kitten when set up correctly, what is your idle timing? There are so many things to be looking at before you blame the bleed system. Does you idle speed change if you trim the motor up slightly from dead vertical? The 2+2's are quirky critters unless they're set up right. I have twin 115's that I bought new in 1995 and they run very well but it took me a while to dial them in. Idle timing ,speed and mixture on the top 2 carbs are super critical.
 
Why Racer? You of all people would go through system checks and basics before opening up the motor, even if just the carbs reed plate? Whats up?
 
The original poster is convinced that bleed valves are bad.---The original poster says that motor must be split to change them.----I say take a look and see.----Nothing wrong with what I am suggesting.----Might cost a few gaskets.
 
racerone said:
The original poster is convinced that bleed valves are bad.---The original poster says that motor must be split to change them.----I say take a look and see.----Nothing wrong with what I am suggesting.----Might cost a few gaskets.
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I’m not sure I’m qualified to convince myself the bleed valves are bad, but I do find it odd that I can put a barbed nipple in the lower bearing end cap with a length of 1/8” hose on it and blow through my motor. Seems to me if there are three check valves in there working right I would not be able to do that. I am in full agreement that removal of the reed block to look in there is a very smart first step if I decide to pursue check valve replacement. I’ve had the reed block out in the past (to inspect the reeds), but at the time had no idea that there were bleeder check valves in there that I should have scoped out. Maybe I get lucky and find I have the replaceable design 3 valves after all. Or maybe I find out the design 1and 2 valves can be replaced at the reed block, but that I think is a long shot as it sounds like they are pressed into the crankcase cover in between the cover and the 3 inner bearing races. But yes, need to look and know before doing something stupid. Thank you for your advice
 
fitz73222 said:
What is your in gear, idle rpm in the water? These engines idle down to 675 rpms and purr like a kitten when set up correctly, what is your idle timing? There are so many things to be looking at before you blame the bleed system. Does you idle speed change if you trim the motor up slightly from dead vertical? The 2+2's are quirky critters unless they're set up right. I have twin 115's that I bought new in 1995 and they run very well but it took me a while to dial them in. Idle timing ,speed and mixture on the top 2 carbs are super critical.
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The idle timing has been set during the link and sync procedure many times, always just in the driveway on muffs or I also have a tank that I run in occasionally. Always start out at 2 degrees BTDC. Not sure how Fitz 1995 motors are but my motor idle is set by adjusting the idle timing adjustment starting from the 2 degree setting I set with the timing light. It’s been since last summer, but I recall when I hook up the tack and take it all the way down to 650-700 rpm in gear it always makes me a little sketchy because it pulls me way off that 2 degree start-up setting. So I tend to set the idle a little high because of that. Is it ok to go way off the base setting to get the idle down to 650 rpm?. Going by bad memory, I think I was 4-6 degrees away from the 2 degree BTDC starting point in one direction or the other by the time I got the idle down that far. I think at the 2 degree BTDC timing it’s jumping around a bit between like 950-1050 rpm. My carb mixture tends to get set back to 1 1/2 turns out because I do the routine and if it doesn’t fix all my problems I get scared maybe it’s too lean so I set it back to 1 1/2 turns out. If it’s super critical I should get more confident on that also or at least spend more time on it. I’ve not got the boat in the water yet, but soon I hope. What will it tell me if the idle changes with adjustment to the trim angle?. Thanks for all your advice.
 
faztbullet said:
OP is going down a loooong rabbit hole. Need to take care of moisture on plug problem first.
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Faztbullet, it’s way too late to worry about the length of the rabbit hole. Fuel filters, carb kits and main jets, stator and trigger, regulator/rectifier, switch box, accelerator pump rebuild with check valves, fuel pump rebuild, then the exhaust gaskets project…I love bolting on those new parts. Hopefully my plugs will be dry, and maybe it will idle real good too. Be finding out real soon.
 
You are aware that there are 4 separate crankcases in this motor ?-----Are the other 3 check valves connected to that 1/8th external hose , yes , no or no idea ?-----The idle timing is the point where throttle plates OPEN.----Where the timing is when the engine is idling smoothly does not matter !!
 
racerone said:
You are aware that there are 4 separate crankcases in this motor ?-----Are the other 3 check valves connected to that 1/8th external hose , yes , no or no idea ?-----The idle timing is the point where throttle plates OPEN.----Where the timing is when the engine is idling smoothly does not matter !!
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I was not aware of that, and you make a very very valid point. The answer to your question is “no idea”, now that you ask. I had been assuming that all collected bleeder fuel was directed to that lower end cap, by gravity or otherwise, and then that the external bleed hose was under vacuum of some sort to get the fuel up to the upper bearing. Never once was my mind open enough to consider that the internal valves might get the fuel where it needs to go through a separate route. I do know some fuel collects at that lower cap, probably had a half ounce of drippings when the bleeder hose was first removed. Hopefully that’s normal and expected. Great to know about the idle setting also, my older motors (I have some old Chryslers that I love for their simplicity) all have idle screws on the tower shaft that I think also adjust the throttle shutter so setting idle on the timing screw always seemed odd to me as I try to learn more. I will bring the idle down to where it wants to be without concerning myself with where it ends up. Also at this point, because of all your input and infinite wisdom, I think it’s time to put the bleeder valve project on the back burner or cancel it all together. At least for now. Thanks so much for the insight.
 
When a 2 stroke is idling the throttle plates are closed.-----Most do have calibrated openings because a 2 stroke needs fuel that carries the oil , to go in.----Idle speed is controlled by retarding the timing.
 
racerone said:
When a 2 stroke is idling the throttle plates are closed.-----Most do have calibrated openings because a 2 stroke needs fuel that carries the oil , to go in.----Idle speed is controlled by retarding the timing.
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Well then you have schooled me on yet another misconception of mine. I knew that on my Mercury motor I was only changing the idle timing to adjust my idle, but for some reason I thought that was unique to the 2+2 design. I had thought on my other motors that the timing screw on the tower shaft was turning the distributor slightly (fine tuning the timing) but also opening or closing the throttle plates slightly when making the adjustment. And for some reason I had taken that misunderstanding a step further to assume that most other two stroke outboards did the same. Very much appreciating the shared knowledge from you and the others.
 
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