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1995 115 fuel siphoning backwards when engine is off.

R

Roland J

Member
So i'm going to go through my fuel system and plan to rebuild the accelerator pump, and the main fuel pump. I believe my fuel pressure is on the low side, just because the way the engine is performing.

A few weeks ago i changed my in line spin-on fuel filter that is 2 feet before the engine. When i spun it off i noticed the fuel was blue. The color of the premixed fuel.

So i know the priming bulb that is between the spin-on filter and the fuel tank is not checking the fuel and allowing the fuel to back siphon. I did not think it was a big deal at the time but with the other fuel like symptoms the 115 is having i am planing on replacing that fuel priming bulb with a Yamaha fuel bulb. I'm told they are the best ones to get.

So my question is before i rebuild the pumps and replace the priming bulb is there any thing else that i should check or replace that may have contributed to fuel issues while im doing the other work?
 
You don't say how the engine is performing but in the fuel department condition of rubber lines is important, they can degrade on the inside and cause issues.
 
Swagonmaster said:
You don't say how the engine is performing but in the fuel department condition of rubber lines is important, they can degrade on the inside and cause issues.
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Cool thanks, the previous owner did say he replaced the fuel lines so that should be good.

It's hard to say how it's performing as it's a 2+2 and i have no 2+2 experience to reference, and i haven't had it that long so i'm not sure how smooth this transition should be. But i don't think it's been right since iv'e owned it.
I did just replace a coil that was cracked which was obvious when that trouble occurred. (this other issue is less obvious of a problem)
I only had it out once with friends trying to learn how to foil, so i didn't really focus enough on the details but this is what i remember.
1. It's a 2 +2 and transition is not very smooth like the accelerator pump is bad, kind of hesitate-ish and sputtery.
2. At WOT top end has a rare intermittent slow down of maybe 150 rpm during the the first few minutes at WOT but then goes away.
3. The Priming bulb seems to not have enough fuel in it when i checked it, requiring a few squeezes to get hard while it's running
4. WOT RPM's are a bit low 4850 but i did not spend enough time confirming this.
 
I had the same problem and assumed the check valve in my New $30 EPA mandated 3' fuel line with bulb from the engine to the built in tank wasn't checking. I got on ebay and bought an aluminum 5/16 valve (actually there were 2 in the pack for less than 10 bucks, free shipping). Put it in the line where the fuel line exits the boat proper, prior to the bulb...as the fuel flows. No more problems with dry bulb every time I wanted to use the boat. Just a couple of pumps to get the fuel level from running resistance, to starting resistance and engine started much faster.
 
There should be a check valve to prevent the oil from running back into the line. The primer ball being soft while the motor is running is perfectly normal. The.spin on filter might be your problem. Do.you have the correct manual for your motor?
 
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flyingscott said:
There should be a check valve to prevent the oil from running back into the line. The primer ball being soft while the motor is running is perfectly normal. The.spin on filter might be your problem. Do.you have the correct manual for your motor?
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There is a brass 2 psi check valve that is bad and allowing the oil from your oil tank to gravity feed oil back into your fuel. It's installed in a 2 outlet molded fuel line right behind where your fuel connector plugs into the engine. Every oil injected Merc I have owned needed that valve replaced at some point. I have twin 1995 2+2 Merc 115's on my offshore boat. The valve is inexpensive and easy to install. I am very familiar with these engines and their quirkiness but once they're set up correctly they run fine.
 
The part number for the valve is 22-818994 1 it's shown in the fuel pump section for your engine on this website. Also check the molded fuel hose it's connected to. I would replace that while you're at it since they tend to split at the mold line. I stock those and the check valves because I'm sure one day they will be NLA.
 
If you did much idling, or slow running on the 2 active cylinders, it takes a minute or two for the plugs to clean up and fire properly on 3 and 4. So, for OEM spec'd parts, including plugs that's normal. Tilting the engine forward slightly, any time you are below 2000 RPM will/will help to, keep unburned fuel...the fuel in 3 and 4 from puddling at the spark plug. This will improve the smoothness of low speed operation and solve if not greatly reduce the clean up time when advancing to WOT position.

The accelerator pump is just that....a fuel pump. If you look at your linkage, until you get in the 1800-2000 RPM general area the cam on the carb linkage does not touch the accelerator pump stem. As you pass through the transition region the cam moves from not touching to full depression of the plunger connected to the pump diaphragm squirting a slug of fuel into 3 and 4. By then the RPMs should be/need to be, high enough for 3 and 4 to light off on their own and stay lit.

If you want to dilly dally around in the throttle positions where they are lighting off you can feel them kick in and out as they are getting enough fuel to explode and generate HP......or you can advance your throttle to ¾ or higher, blast through that transition space and if desired, then cut the throttle back to cruising speed or whatever.....that's the way I run mine and I have no remorse.
 
fitz73222 said:
The part number for the valve is 22-818994 1 it's shown in the fuel pump section for your engine on this website. Also check the molded fuel hose it's connected to. I would replace that while you're at it since they tend to split at the mold line. I stock those and the check valves because I'm sure one day they will be NLA.
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Excellent info, Good idea i'll replace both, and save the old molded hose put in my "just in case" draw
 
Texasmark said:
I had the same problem and assumed the check valve in my New $30 EPA mandated 3' fuel line with bulb from the engine to the built in tank wasn't checking. I got on ebay and bought an aluminum 5/16 valve (actually there were 2 in the pack for less than 10 bucks, free shipping). Put it in the line where the fuel line exits the boat proper, prior to the bulb...as the fuel flows. No more problems with dry bulb every time I wanted to use the boat. Just a couple of pumps to get the fuel level from running resistance, to starting resistance and engine started much faster.
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Yep i'll get this check valve also. i don't see any down side to that check valve i think i'll put this at the gas tank in my center console.
 
Texasmark said:
If you did much idling, or slow running on the 2 active cylinders, it takes a minute or two for the plugs to clean up and fire properly on 3 and 4. So, for OEM spec'd parts, including plugs that's normal. Tilting the engine forward slightly, any time you are below 2000 RPM will/will help to, keep unburned fuel...the fuel in 3 and 4 from puddling at the spark plug. This will improve the smoothness of low speed operation and solve if not greatly reduce the clean up time when advancing to WOT position.

The accelerator pump is just that....a fuel pump. If you look at your linkage, until you get in the 1800-2000 RPM general area the cam on the carb linkage does not touch the accelerator pump stem. As you pass through the transition region the cam moves from not touching to full depression of the plunger connected to the pump diaphragm squirting a slug of fuel into 3 and 4. By then the RPMs should be/need to be, high enough for 3 and 4 to light off on their own and stay lit.

If you want to dilly dally around in the throttle positions where they are lighting off you can feel them kick in and out as they are getting enough fuel to explode and generate HP......or you can advance your throttle to ¾ or higher, blast through that transition space and if desired, then cut the throttle back to cruising speed or whatever.....that's the way I run mine and I have no remorse.
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You make a good point because would say i am dilly dallying more than most, but with a reason.

I spend most of my boat time pulling a hydro foiler. It's the main reason i bought the boat, but ignorant me had never heard of a 2+2 engine so that was my motor learning experience.
We are trying to learn how to hydro foil and get better at it, so we can apply the skills to wingsurfer foiling. So we are trying to keep the boat at 10-12 mph which is about 3000 RPM. With are trim on my 17' Cape Horn all the down we can keep it in the 10-12 MPH range just fine. But as you know we can't to back off hardly any at all, because below 28-2900 rpm it gets in that 2 cylinder or 4 cylinder zone. Or if i trim up any at all ill plane and speed up too much. I love to boat and it does do want it too so basically i just want it at top performance condition.

You said something that's seems different from my understanding of how these engine work.
Maybe i'm just unclear about what you're saying. But it seems like your suggesting that unburnt fuel can accumulate in the cylinder 3&4 at the below 1800 rpm zone. I thought 3&4 were firing spark all the time, and it was only the fuel that was "Off " at the lower RPM's. Maybe i'm wrong on how this 2+2 works. But if the plugs are firing all the time i can't see where the fuel you mention would be accumulating.
 
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The fuel will accumulate at the bottom of the crankcase in all 2 stroke outboards. The recamed oil is actually pumped from the crankcase pulses through a check valve up to the upper bearing under the flywheel. The engine has spark on all 4 cylinders but only enough fuel to fire 1 and 2. 3 and 4 are along for the ride below 1800 rpm but receive enough oil to lubricate the rotating mass without harm. There are no low speed jets on the number 3 and 4 carburetors.
 
You can realize your 10-12 MPH desires with a Turning Point Hustler if you are into aluminum, or if in a SS a ported SS prop. Fuel efficiency suffers, but whether or not that matters is up to you. On my current boat that I could spend time in that speed range, my rpms are over 2000 with a Merc. Laser II ported SS prop. Prop slip at 3000-3500 is 28% (probably more at aforementoned speed) yet when I want to run, WOT measures 8% slippage.

The Hustler is ported and the only aluminum of which I have used (3 over the years). It also has other attributes usually only found in SS props. Only thing it doesn't have that they do is the blades are thinner than most but not as thin as SS....but unless you are up in the 50+ MPH region I doubt that matters all that much.

On pooling fuel in the engine, and fouling plugs, the 1959 Mark 20 (and lower hp) is/are an example of an engine Mercury built for extended periods of trolling/low speed operation. The drive line is tilted forward 10-20 degrees forcing the rear of the crankcase higher than the front, and allowing the LU to slip over obstructions that fisherfolks need when fishing obstructed waters.
 
jaclark91 said:
My fuel siphons backwards when the engine is off too. I thought that was normal. Guess I'll go ahead and replace my primer bulb too.
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To be clear, it's not a primer bulb issue, it's the check valve in the engine of oil is the issue.
The main symptoms of the check valve being bad are:
1) excessive smoke when you first start the engine because of a super oil laiden fuel in your fuel line.
2) physical oil leaking out of the engine through the vents above the cavitation plate or through the propeller hub.
3) visible premix fuel in your fuel line when unplugging the fuel connector or visible premix in the sight bowl of your water separator if you use one.

This is from experience and not conjecture. I have had each on of these symptoms in various Merc powered boats the I have.
 
The squeeze bulb has to have a check valve such that when you squeeze the bulb, the fuel from the tank only has one way to go....to the engine. I never cut a bulb open so I have no idea what's used for the valve. In the metal valve, it's all metal....no plastic...not saying the bulb is using plastic, but wouldn't doubt it. I was really chapped when I bought this new line for this newly purchased old boat, paying the rediculous price for it...at wallyworld no less. Now problem solved.

On the oil check valve and your blue fuel (whatever is in the line) comment, since the carb bowls are open to the atmosphere via the carburetor's venturis....aka atmospheric pressure, and the fuel tank is below the carbs, not rocket science to see that the fuel in the carb bowl being blue from the 50:1 mix and if you are seeing something blue in the line back to the tank it isn't necessarily just oil. I'm not a mechanic so I don't see all the variables in the boating industry but I never had a problem with that valve. Seems to me to be a pretty benign environment, well lubricated, low pressure/volume control valve. The tubing into which it's inserted seems to be Tygon brand clear plastic tubing which is immune for the most part to fuel-oils.
 
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Texasmark said:
The squeeze bulb has to have a check valve such that when you squeeze the bulb, the fuel from the tank only has one way to go....to the engine. I never cut a bulb open so I have no idea what's used for the valve. In the metal valve, it's all metal....no plastic...not saying the bulb is using plastic, but wouldn't doubt it. I was really chapped when I bought this new line for this newly purchased old boat, paying the rediculous price for it...at wallyworld no less. Now problem solved.

On the oil check valve and your blue fuel (whatever is in the line) comment, since the carb bowls are open to the atmosphere via the carburetor's venturis....aka atmospheric pressure, and the fuel tank is below the carbs, not rocket science to see that the fuel in the carb bowl being blue from the 50:1 mix and if you are seeing something blue in the line back to the tank it isn't necessarily just oil. I'm not a mechanic so I don't see all the variables in the boating industry but I never had a problem with that valve. Seems to me to be a pretty benign environment, well lubricated, low pressure/volume control valve. The tubing into which it's inserted seems to be Tygon brand clear plastic tubing which is immune for the most part to fuel-oils.
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I'm not experienced with these outboard carbs ether. But if they are like other carbs the bowl can not siphon backwards, because the fuel inlet is above the fluid/fuel level. That what float valve do (assuming these carbs have float valve like other carbs.)

I believe like Fritz snuggest The blue fuel in the filter/ water separator is a mix from fuel lines and yes i do get the initial smokey start ups he talks about so yeah the 22-818994 1 check valve will help
 
The venturi tube sticks down into the carb bowl and the fuel. As the boat sits, fuel starts to run back a bit at a time until a vacuum is created on the fuel line which is the source for sucking fuel out of the bowl. It's like you siphoning fuel out of a tank with a hose and your mouth. Otherwise, how is it that with a known functioning check valve the bulb is still soft, as it is when the engine is running and refilling the carbs only takes a few pumps to get a firm bulb, whereas without the known functioning check valve the bulb is completely collapsed and it might take 20 pumps to get the feel of fuel in the bulb and more to fill the carbs till it gets firm.
 
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