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2-Stroke Outboard Carburetor Synchronization

In this video I synchronize the carburetors on the Yamaha 20HP outboard. This involves getting the throttle butterfly valves to be set evenly closed at idle and fully open at full throttle.

2-Stroke Outboard Carburetor Synchronization – Video Transcript

Today's video is on synchronizing two stroke carburetors and is proudly sponsored by Marine Engine.com.

For those of you that have been following along with this Yamaha 20 horsepower two-stroke motor problem, we've pretty much gone through and ruled out a whole lot of major things. It's not the order we would diagnose a problem like this in, but I want to do these videos anyway, and this seemed like a good excuse to do them. What we're going to do now is balance the carburetors on this Motor. As you saw at the end of the last week, if we ran this motor on one cylinder it ran well, if we ran it another cylinder it ran well, but if we ran it together, it didn't run so well. And, as it turns out these Yamahas in particular, are very susceptible to not having the carburetors balanced. They don't like to be run out of balance at all So we're going to balance the carburetors and see how we go, Hopefully that'll just solve this problem all together and be done. There are a few different things we can adjust to this balance. There is this screw here, that if we undo it, it allows us to change the effective length of the rod connecting the two carburetors. Each of them has their own top screw, here, and then there's another screw, here, that allows us to move this cam roller independent of this bottom bracket.

So, these are all the things we're going to be adjusting to get the carburetor butterfly valves fully closed at idle on both carburetors, evenly closed, and then fully open at full throttle. To start this process I'm just going to move all these settings all over the shop so we've got a bit of a bad setting to start with then we'll move towards the right setting. I'll wind a few of these too high, bottom one too low, roll away from the cam. Fine, so everything's set about as bad it can be. Okay, the idle is too high, still doing this sort of sneeze, backfire type thing. So, now let's see if we can get it all set right and see what it sounds like. Before we get started I'll show you what the butterfly valves look like. At the moment I'm not sure how well this is going to work to show it to you rather than trying to sort of point the camera inside the butterfly valves here where the microphone will only hit. I'm going to use this little inspection camera so I'll just put the camera inside the carburetor and then hopefully it'll show up on the screen here. So this is the bottom carburetor here, you can see there is a bit of a gap at the bottom there, it's not fully closed, and an equal gap at the top. And if I swap over to the top one you can see this top one's pretty closed in comparison. So they're clearly not even at idle. So to fix this I'm going to back off both the stopper screws.

What these screws do is there's a linkage here that opens and closes the butterfly valves that's sprung load, so when you let go, the valve will close. How far that butterfly valve can close is dictated by where this screw sets. So, you can see here as I screwed this screw in it's pushing down on that plate and the more it pushes down on that plate the more it opens the butterfly valve up. Hopefully you can see that, it's at a bit of an angle now. So what I'm going to do is back that screw off until the butterfly valves are completely closed. What happens eventually is it's pushing on a stopper and it pushes it open, If eventually you'll find that as you undo the screw it'll actually come away from that stopper and from that point onwards it's closed as far as it can. So I'm actually going to do it that far, I'm going to wind them till they've come right away from the stoppers and then I'm going to make sure that the linkage allows both butterfly valves to close completely. So what I'll do now is I'll back those stop screws all the way off and then we'll check the butterfly valves again. Wind it all the way back ... and then do the top one as well. Now if we look back in these two carburetors, let's go above the choke plate.

You can see this one's closed completely, but if you look in the bottom one, it's still open a bit. I'll show you why that is. What's happening now is this butterfly is resting on its stop but the length of this linkage here is too short so even though the stop is not preventing this butterfly valve from closing, the length of this linkage is. So, if I undo this cub screw here. it's allowed that now to pop up, so this automatically comed up, it's actually spring-loaded in here. So once again that's the top one still closed, now we can see here the bottom one is closed as well. What I'm going to do now ,is tighten this screw up. Now the other thing I'm gonna do is undo the screw for this little cam roller, and then just gently press it against this throttle gear. So this is the cam and this is the roller and with that screw undone this moves around, so I'm going to move that roller until it touches the cam. The throttle is all the way back in idle, and then I tighten that up as well. Okay that's come away a little bit then as I tightened it so I'm gonna put the camera down and just use one hand to hold that against while I tighten it here. So, just checking, throttle all the way back up. I'll undo it again. just hold the roll against the cam, and then nip that up.

These covers will now have these butterfly valves at a certain position, and now what I'm going to do is just wind these little stopper screws which you can kind of think as an idle speed screw really, just down until they gently, gently touch against this plate so there isn't an air gap at all. So now we've got both carburetors with the butterfly valves closed completely when the throttle is at idle, we've got the roller touching against the throttle cam when it's at idle, we've also got those two screws just touching onto the plates. Now, you can't really adjust these carburetors individually without separating the linkage that joins them. If I then move either of those stoppers it will open the throttle plates for that carburetor and the linkage between them will drag the throttle brake for the other carburetor open as well. So if you need to adjust them relative to each other, you need to loosen that sensor linkage first, get them both where you want them, then lock off that Center linkage. All right, what I'm going to do now is set the mixture screws to the factory setting for these, which is basically two and a half turns out. I always count the half turns, so I'm going to count five half turns to wind them out.

This is our mixture screw here where this red screw driver is going, so I'm going to wind it all the way in until it lightly seats, and then I'm going to count five half turns out. So it's one, two, three, four, five. Now I'm going to do the same thing up here for the top one, wind it all the way in so it's lightly seated, never really crank it in, you'll damage the tip of the needle, but then I lined it out the five half turns one, two, three, four, five. That's pretty much done now for a reasonable baseline sort of synchronization of these carburetors. So I'm going to take it outside, fire it up and then we'll just see whether we need to up the idle a little bit, see if it sounds good, see if this leans sneeze has gone away. This motor is now a little bit hard to start as it is, so I'm going to wind in the stop for the top carburetor to open both bottle plates just a little bit. Just going to wind it in another half turn. Actually possibly even a bit less, between a quarter and a half. I've gone up another quarter turn on the idle speed, so we'll try that now and if that gets us running consistently, we'll play with the mixture.

I'm pretty happy with that. Obviously we have to see what it's like on the water, get it warm, tune a bit more but, it's pretty much made the problem go away. So I'll shut it down, move back inside then we'll get the other camera out and have another look inside the carburetor and I'll show you where the butterfly valves are sitting. So here's what they look like now. This is the top butterfly valve just cracked a tiny amount, and then if we swap to the bottom one, it looks pretty much the same, just cracked a tiny amount open. So bottom one, top one. The next thing I'll check is whether we're getting full Throttle. The next kind I'm going to check is whether the butterflies are opening completely when we're at full throttle. So I'll crank the throttle open and we'll have a look what they're doing. So I don't know how clearly you can see That, but it's actually not opening all the Way, so I'll show you how we adjust that. To adjust it, I'm going to undo this screw here and then move the roller towards the cam here, which means throttle plates will get extended further open. Once we've done that, we'll make sure they're opening fully, then we'll double-check the idle again. And the idle setting still looks good too.

The final sort of test is going to be when fired up again but it's all looking pretty much spot-on now. Well, as you can see, all this outboard really needed was a carburetor synch balance sort of procedures on it. As I've sort of been saying all along I've really been using this outboard as an excuse to film a series of videos. It actually did have a leak from the power heads, I'm glad that's fixed, the timing was out some guide, so it's set now, but you really do want to look at things like carburetor cleans, good plugs, idle mixtures, balance on the carburetors, before you start taking power heads off things. So don't sort of do things in the order I've been doing them for the sake of these videos. So I've got to give a special thumbs up too and I wish it was a pronounceable user name but it's not it was a BSR KTM1 because he was pretty much on the money saying do a balance and it will be fine, and it was.

As a few people have said this particular outboard is very sensitive to being out of balance so you may find on a different model outboard you can have them a little bit un-synched and it'll still run okay but in this case obviously not. Another honorable mention has got to go to Justin Yeu, another user who commented that if it was a problem with the labyrinth seal, if you ran it on a 1-8 mixture of 30-weight oil, like quite thick oil, and the problem got better, then that's a good way of knowing that there probably is a problem with that labyrinth seal. So I think that's a good tip as well. There were quite a few comments about the fuel pump as well and one thing I didn't show, there was a clue to the fact that the fuel pump was actually fine, was that even after it had been running for a while, sneezing, doing all these things, I could just stop the motor and both carburetor bowls were full, so if the carburetor bowls are full your fuel pumps working. You know there's no real issue there. Well, thanks for following along with this one. I hope it helps you if you're having this problem. A few people did say "aah" this is a problem having with my motor, so I would definitely say if you haven't, just start by cleaning the carburetors, there's no point trying to adjust everything if one of the jets is blocked so, start with a carburetor clean, make sure the gasket behind the carburetor or the O-ring is good, it's bolted on firmly, then do your synchronization, do your mixture screws, see how it goes from there.

All right we'll take care and I'll catch you next week with a bit of a tear down on a Johnson 40. All right see ya.