Finding a Vacuum Leak in an Outboard Motor
In this video I investigate whether the Yamaha 20 HP has a vacuum leak causing it to go lean. I do this by spraying LPG over the crank case and then also blocking off the exhaust and doing a pressure test on the crank case.
Finding a Vacuum Leak in an Outboard Motor – Video Transcript
Hey guys, Dangar Stu here. Today's video is on diagnosing this sort of misfire slash lean sneeze we've got with the Yamaha 20 and is proudly sponsored by MarineEngine.com.
Before we start going through how to look for a vacuum leak. What I'm going to do is go out start the motor up again and then go through a couple of techniques for how to check if a motors running lean the obvious thing to do there is just to add some fuel and if it starts running better it's probably running lean so we'll go out there we'll fire it up I'll spray a bit of starting fluid in each of the carburetors and then also just trying them with the choke partially on as well. That was just spraying a bit of starting fluid into the bottom cylinder which seems to smooth it out. To be honest with you, spraying into either cylinder seems to smooth it out. Because spraying that fluid seemed to smooth it Out, it does seem this is a lean condition so it certainly sounds like one. In order to really go on and test what's going on here now I'm pretty sure it's not a carburetor problem because they're pretty clean so I'm thinking now I'm going to start looking at crankcase seals, that kind of stuff. Even though this does seem very much like our lean condition causing this sort of sneeze. I'm going to go and just swap out those ignition coils.
By having a rummage around my spare parts box and I've got a couple of old Yamaha ignition coils here so I'm going to throw those on we'll see if it sounds exactly the same. If it does, it may still be something upstream, either the CDI unit or the coils under the flywheel. But I find these coils tend to fail you know more regular than the other parts of the system. So I'm going to put these on we'll test it. If it's no better we'll get the crankcase off we'll start doing some pressure testing. In case you're wondering too, the mixture screws on both these carburetors are two turns out. Which is pretty standard setting for them. So I'm not expecting that to be a problem either. All these coils are just a single bullet connector going to the CDI unit and then a 10 mil fastener holds both the coil on and acts as the ground for the unit. So there's a wing connector on here. So, as expected, pretty much unchanged again.
But it took four minutes to swap them out and check and rule out something kind of pretty simple. As it becomes increasingly obvious it's got some sort of lean condition what I'm going to do now is start checking for a vacuum leak. So a little bit like the initial cause, this is something that's really easy to do. To do this, I'm actually using this LPG cylinder that I normally use just for heating up parts. The way I do these vacuum leak tests is to take the torch like this from the LPG cylinder just crack the valve leave the torch unlit so you're not lighting it like you would normally you're just having the gas flowing and then just start directing that flow of gas over all the areas I suspect the vacuum leak might be. If there is a leak that's causing it to run lean, instead of just air getting sucked in and cause it to go lean, the LPG will get sucked in which will actually probably make it even richer. So you'll hear a really noticeable change in rpm when you've got the gas torch in the vicinity of a vacuum leak. Be careful when you do these vacuum leak tests though because fundamentally they're about spraying a flammable fluid or a flammable liquid onto the outside of the engine casing so if there is excess heat somewhere, any sort of spark, etc. so you know, it can catch fire. So you really need to have a fire extinguisher , and do it outdoors where there's just plenty of ventilation. Before we go to the bench and start seeing if we can find this vacuum leak. I will just sort of show you in concept what they are on the board just sort of makes sense what we're actually looking for. When the motors running normally the fresh air comes in it comes past this butterfly valve and the fuel gets picked up out of the carburetor bowl and it comes into the crankcase.
So that was what we looked at yesterday. We talked about the reed valves and how they stopped that flowing backwards towards the carburetor. Now, when we talk that vacuum in a motor, it's where a space is expanding or your Piston is going from top dead center down, and as it goes down the air sort of expands to fill that space or becomes the lower pressure, becomes a partial vacuum. That vacuum forms in the inlet manifold because the portal valve here stops the air from rushing in if the air can rush in really quickly you never really get much of a vacuum so diesel motors don't have a butterfly valve like that so they don't really generate any sort of vacuum but a petrol motor does because of that throttle plate. So when the throttle plates almost closed that's when you're kind of getting the greatest vacuum. If the vacuum that draws the air into the motor and then as the air goes through the carburetor the fuel gets added in the correct proportion and the motor runs nicely, when everything is working well. The carburetor is well tuned, there's no vacuum leaks, air and fuel come in in what they call this stoichiometric ratio which is 14.6 : 1 So this is your sort of fuel to air ratio that motor needs to run properly. If the carburetor wasn't well sealed against the crank case for example. so either the gasket wasn't there, o-ring wasn't there, the bolts were loose, rather than coming in through the carburetor and picking up fuel. Fresh air could come in through the gap and that would add extra air but without adding the extra fuel.
So it might come back up here and then end up here which means you end up running lean, you've got more air than you're expecting so you don't have the right air fuel ratio and this is essentially what a vacuum leak is. It might happen between the carburetor and the engine block. It might happen through this top crankcase seal, that might happen through this bottom crankcase seal. It might happen from a crack in the crankcase. all these kinds of things. When we do a little test with the gas torch, we're waving our gas torch around here, which means instead of just fresh air, air and LPG are getting sucked into the crankcase. So instead of being lean, we're adding a bit of extra fuel, and we'll hear that lean condition sort of disappear. So what we're doing by waving a torch around is trying to find this area and listening for the change in rpm. The other thing you will have noticed if you can listen to that motor when it's running, is it runs okay, it actually accelerates, accelerates, accelerates, does little sneeze goes back accelerate. So it has this fluctuating rpm. And that's a classic symptom of vacuum leak as well. We'll wave it around, see what happens. You see, once it stops requiring the gas It starts to lean again.
Now I'll put the gas again at the base of the power head. Next thing we'll do is pull the power head off so we can get it onto the bench and start to investigate a bit further. I won't go through the full details that's not really what this video is about, but needless to say, it's, you know, throttle linkages, gear linkages, fuel, the kill switch wires, all that kind of stuff disconnected. And then bolts down underneath the lower cowling here. These are the power heads from that exact Yamaha, it's only just still warm. Because the power head sort of sits this way up. This is the bottom here, and this is where the drive shaft comes from the propeller and goes into the crank shaft of the motor. What I'm going to do now, is take this little housing off here so we can have a look at the oil seal that goes around the end of the crank shaft. It was only 2 10 mil fasteners that hold this on but it's a little bit stuck so we'll...I like these screwdrivers they actually got a metal end and it's solid all the way through so you can't break handles, if you're screaming at me for using it as a chisel.
There's an o-ring around the outside. That's the static part that seals against the crankcase housing so that can also leak. On the other side we got a couple of oil seals. this one's deeper inside the motor and seals around the crankshaft. And then this one that's inside here is lower down and seals on the drive shaft. They don't look obviously bad but what it will do in the end is we'll put this back together and see if we can do some sort of pressure test on them. Now I'm just going to spin around and take the pull starter and the flywheel off so we can look at this seal at the top end of the crankshaft. The flywheel nut on this one's a 22 Millimeter. These puller sets I think are sold in auto stores as harmonic balancer pullers but they're really good for outboard flywheels. There's three Phillips head screws that hold this stator on, then we've got three ten millimeter fasteners and they can hold this plate on the top of the crank shaft here. You can see the top seal here. What I'm going to do now is try and take the exhaust off. It's one of the worst areas for corrosion and bolts breaking etc. But fingers crossed we'll get it off. Then I'm going to reinstall the lower seal and see if we can make a cover plate to block the exhaust off.
Just putting a little bit of rubber grease around this o-ring before we reinstall this lower seal. Now I'm going to do is get a little bit of metal, cut it, and drill some holes so that we can use it to block off the exhaust. Here now the plates drilled I'm just going to clean this gasket up. I we had another one of these things, but i don't. I think I'll clean up a little bit spray a bit of Hylomar on it, and just helped it seal a bit better and then we'll bolt the plate on. All right, we'll have a go at putting this place on. I'm slightly doubtful we're going to get a good enough seal but we'll give it a shot. As you can see, the huge difference in the sparkplugs the top one is quite clean with little combustion, the bottom one is quite sooty. So it looks like our problem is with cylinder number one. Because of that I'm going to start by putting some compressed air into cylinder number one, it's looking like the likely problem. Which means rather looking at the top reed valve or the top oil seal on the crankshaft. To get compressed air into the cylinder, I'm going to use this little leak down tester kit.
So it's essentially two gauges, a little regulator here. Compressed air comes into the gauges then the other ends the outlet where there's a set of adaptors and I'm using the adapter that matches this spark plug thread so very similar to a compression tester. The plate on the exhaust was originally leaking like a sieve. It just came straight out. so I actually took it off again, took that gasket away, put some Sikaflex on it and let it sit for about half an hour. Then put the plate on and tightened it up, let it set for a few more hours. And now it seems to be sealing quite nicely. So the exhausts pretty much blocked off now I'll quickly show you this leak down tester. But I think it's a bit of a cheaper nasty one. I'm having a little bit of trouble sort of getting it dialed in but I'll show you the basic idea with these testers, you have pressure coming in, and then this is a gauge that shows pressure after a really small aperture here and essentially what it's saying is that if air leaks through this hose it's a proportion of how big it is compared to a null and little aperture here. So you're really just saying is the leak better or worse than this particular little gap and it gives you an idea of whether it's considered a moderate or bad leak it's a very relative sort of instrument. With a glare there sorry. so the idea is you've got this regulator here, you pop it down, and then you wind it until this needle sits pretty much in this set zone, this yellow zone. More on zero.
The reason I say this one's a bit cheap and nasty is because it doesn't stay where you set it. it unplug, it and it's nowhere near goes back to zero. so I'm not super confident but what it will do is allow us to supply compressed air into the crank case and see if we can find any leaks. As soon as I hook that up the input pressure dropped pretty much to zero. So what I'm gonna do now is just dial that regulator up a little bit until we've got about five psi input pressure. Then you can start to hear it's flowing out. I'm going to pop the carburetors back on too because one of the air ports i forgot about that on this particular model, is at the front here is where the vacuum pulse goes to drive the fuel pump so I'll show you that so here there's a little opening here which comes in to this bottom carburetor here and then comes down and drives the diaphragm for the fuel pump so I'm going to pop these carburetors back on to seal all that off. Now, I've got the compressed air going to cylinder number one the only link I can actually hear is coming up through the cover itself and if I open the throttle gets a bit louder so it seems like it's pressure coming back through the read valves.
I'm kind of not surprised you can get a bit of compressed air coming through there. We're only running about five psi, it's not a huge amount but they're not a perfect seal on those Reed valves. I am now going to spray a little bit a soapy water around this top seal and see if anything's coming out from the crankshaft. As you can see there, it doesn't look like it's leaking at all. Not seeing any evidence of compressed air escaping from there. What I'm going to now is just swap the compressed air from cylinder number one to cylinder number two see if is any difference. A very comparable amount of noise coming out from the carburetor. No better no worse. now I'm just going to look on the bottom and see if there's anything coming out of that lower crankshaft seal. Once again, no sign of any leaks around there at all. Just to give you an idea the sort of thing I'm looking for with the soapy water this even with the Sikaflex, this isn't sealing particularly well on this edge because actually a tiny chip out of the aluminium here. But if we spray it, this is the sort of thing you see. Really clearly. bubbles being blown.
So other than that exhaust port which I've got covered up I'm not seeing that bubble effect appearing anywhere else on this crankcase, which is kind of weird given we had what seemed like a pretty obvious vacuum leak when I waved the LPG torch around. I'm going to change the compressed air back to cylinder one and just have another spray around once I've changed to cylinder one, I have to rotate the crank shaft so that cylinder one's sort of at the bottom of it's travel. so if the transfer port open and the compressed air can get past the top of the cylinder into the crankcase. I've just put this clamp on to pull that exhaust port cover a little bit tighter so it's not so noisy. Still squeaking, everything. Alright, I'm not finding any obvious leaks here. I'm now going to swap that second set of Reed valves I had onto this motor so I can compare the amount of noise I can hear coming out of the carburetors, and see if it's significantly better or worse or the same. Sounds pretty similar to be honest. I think that's about what you'd expect so their ability to feel such a constant pressure. The other seal the reason or 2 stroke is between the two halves of the crankcase because they do have to pressurize independently.
Somebody commented that because one was really black one was quite like clean that that's actually good sign that seals working because they'd be much, much more even if it wasn't. This is a crankshaft from an outboard, I can't remember but what we've got here is this was called the labyrinth seal and it's called that because it just forms a bit of a convoluted route that it's actually quite difficult for the gases to escape through quickly. Also if we understand the effectiveness of that seal is greater when the engine spinning because it sort of creates this laminar flow, sort of turbulent air that makes it even harder for the gases to get past. They're pretty solid things are there pieces of metal. So if this is failed, normally you would have bits of metal all through the through the outboard which we don't so I'm not really suspecting that to have failed in any significant way. This top seal we had the top of the crankshaft.
That's what it looks like, so it goes it's actually got a bearing inside then the oil seal on the top here just to give you a bit more of an idea what this is like disassembled. And then here on the base here is where the oil seals run on that surface as well. If you're looking behind the reed valves here, the crankcase, the cylinder number one is really dry and actually sort of partially rusty. Whereas still number two is quite clean and sort of moist with oil and fuel so it seems that this top cylinder really just isn't getting any fuel so even though we did the clean on the carburetors for both cylinders and the floats moving. the needle and seats free, compressed air can go through and all that kind of thing. There just seems to be a problem with getting fuel into cylinder number one. Because this crankcase is that dry I really do think we can consider this a fuel delivery problem, not a vacuum leak.
But I hope this video gives you an idea of what about what a vacuum leak is and how to go about starting to find one if it is a problem you have with your outboard. The waving the LPG torch around I mean because it's all such a confined space I think very quickly it was just making its way into the carburetor. So although it gave the effect you might have if you were to wave it over vacuum leak, I think the leak it was sort of going into it really was just through the front of carburetors in through the front door sort of thing. so I think they gave us a slightly misleading result. I put this motor back together now, nothing's changed, It runs pretty much the same, but I know you guys love to do the Scooby mystery, so I'll share with you one last clue to sort of ponder over the coming week. What we're going to do is fire it up, listen to it, then I'll just disconnect one spark plug at a time and you can have a listen to what happens. So you can see there it runs pretty well on one cylinder at a time, both the cylinders run pretty evenly, when they're isolated all the problems start when both cylinders are running at the same time. So, there you go something to think about.