Carburetor swap, WMA9 for a WMA7

[Joel K]

I'm wondering if I can swap a WMA9 for a WMA7 carburetor? I have a 1994, 40 HP Mercury outboard with four cylinders and two original WMA9 carbs, but the lower one is bad, so I'd be mixing the two with the original 9 still on top and would be installing a 7 on the bottom. They appear identical with the exception of the fuel bowl. The fuel bowl for the 7 is roughly 3/8" shorter in depth than the 9.

Aside from not knowing yet if the boat will run with the mixed carb's, I'm wondering if trying to run it that way could cause engine damage.

Thanks,

Joel

 

[gm280]

Okay Joel K, see if I can understand what you area asking. First it seem you have two carbs on your Mercury 40. And you state one of then went bad. Exactly what went bad with the carb? Is there some physical damage? If not, why wouldn't a carb rebuild kit be the easier answer? If there is damage that can't be fixed with the carb, then you want to swap a WMA9 carb with a WMA7. While they can "look" alike, have you researched the internals for it? Could be different size jets and even the venturis. I would certainly look at an explodes view of both carbs first to verify they use the exact same parts. And if they do use the exact same part why would they make two different part numbers?

 

[Joel K]

The lower carb I'm replacing has had a nuisance leak for a few years, where it would trickle fuel out of the throat when you pumped the ball tight, but that's turned worse. Now when I pump up the ball, fuel will squirt out of the vent jet. It was dying at idle on occasion, but now it won't idle below about 1400 rpm.

I've rebuilt it several times, and replaced the needle along with checking the float again this season, but the seat is permanent and I'm positive the seat is bad. I've cleaned it every time I've had it apart.

The Clymer says Mercury put 3 different carbs on that engine. (Guess Mercury couldn't make up their mind) A WMA7, 9/10, and a 13. They say the difference is the 9/10 and 13 incorporate the main jet into the primer fitting instead of the carb body. However, my main concern is the amount of fuel the bowl will hold since the 7 I'm planning on trying has a bowl with less depth than the 9/10 I'm replacing.

I do know that they all use the same rebuild kit, so they can't be too far apart.

Appreciate it,

Joel

 

[gm280]

The lower carb I'm replacing has had a nuisance leak for a few years, where it would trickle fuel out of the throat when you pumped the ball tight, but that's turned worse. Now when I pump up the ball, fuel will squirt out of the vent jet. It was dying at idle on occasion, but now it won't idle below about 1400 rpm.

I've rebuilt it several times, and replaced the needle along with checking the float again this season, but the seat is permanent and I'm positive the seat is bad. I've cleaned it every time I've had it apart.

The Clymer says Mercury put 3 different carbs on that engine. (Guess Mercury couldn't make up their mind) A WMA7, 9/10, and a 13. They say the difference is the 9/10 and 13 incorporate the main jet into the primer fitting instead of the carb body. However, my main concern is the amount of fuel the bowl will hold since the 7 I'm planning on trying has a bowl with less depth than the 9/10 I'm replacing.

I do know that they all use the same rebuild kit, so they can't be too far apart.

Appreciate it,

Joel

If your only issue is the fuel cowl, why don't you swap the larger one with the smaller one? I do understand a bad seat. I had a huge issue with a Kohler 15HP engine that had the exact same problem. However, Kohler finally came out with a fix by issuing a new seat for that carb. That did fix the problem. I said all that to say this, maybe you could get the old seat out via a wood screw (that is how you removed the kohler seat and that screw came with the new one in their seat kit fix) and see if you can either find a new one or have one made for it. Those seats are really nothing to have a machine shop produce. Then you simple insert the new seat and you have all your carb problems fixed. Just a suggestion. :noidea:

 

[Joel K]

The bowls won't swap because of the length of the attachment point where the bottom screw is, is different. They are both as long as the fuel bowl for the individual carbs.
I've never thought of the machine shop idea. I may take the leaking carb to one and see if they can replace the seat from start to finish so I don't tear it up beyond any repair.

The boat is in a slip on a local lake, so I'm going this afternoon to try the swap it out just to see if it'll even run. I assume that with a smaller volume fuel bowl that the float will just work more often to keep up. My main concern (assuming it'll run at all) is whether or not it can keep up with the carb with the larger bowl fuel supply at full throttle. That's the part I can't quite understand. I don't want to starve the lower two cylinders and melt the pistons.

 

[gm280]

The bowls won't swap because of the length of the attachment point where the bottom screw is, is different. They are both as long as the fuel bowl for the individual carbs.
I've never thought of the machine shop idea. I may take the leaking carb to one and see if they can replace the seat from start to finish so I don't tear it up beyond any repair.

The boat is in a slip on a local lake, so I'm going this afternoon to try the swap it out just to see if it'll even run. I assume that with a smaller volume fuel bowl that the float will just work more often to keep up. My main concern (assuming it'll run at all) is whether or not it can keep up with the carb with the larger bowl fuel supply at full throttle. That's the part I can't quite understand. I don't want to starve the lower two cylinders and melt the pistons.

That is a real concern for certain. I can't tell you one way or the other because I have no idea the fuel pump flow capacity or the jet sizes that feed the cylinders. I guess you will find out though. :noidea:

 

[jimmbo]

Jets are only part of it, as mentioned earlier, venture size, plus air bleeds, throttle plates may have holes or holes of different sizes, venture booster may also be different. Rebuild kits are gaskets, needle and seat, core plugs, They do not permit change the calibration/fuel curve of a given carb, so one kit can do dozens of differently calibrated carbs.

On the original carb, are there any obvious scratches or damage to the seat? What is the float made of? Maybe it has absorbed fuel over time and can't close the needle at all.

 

[Joel K]

Update: I just got back from swapping them at the lake. The MWA7 carb I bought had the gasket to the manifold still on it, but upside down from the original, but after some thought I kept it that way.
No leaks when I pumped up the ball, so I fired it up, and it still wouldn't idle in neutral, but it was really close. So I adjusted the linkage for the idle stop a little which got it to idle well. Took it out on the lake for the first time in a couple months and it ran well through all ranges. Cavitated for a minute, but I got the wife to heard our over-sized dogs towards the rear which seemed to solve that Only ran it for a total of maybe 90 minutes, but it seemed to run better than prior to my troubles, so I'm going to assume for the moment that all is well.

The float'son the old carb is plastic and been changed once, and I can't see down into the seat well enough to determine anything.

I'll see if the book has any information at all on internal part differences I need to worry about. But unless I find something that really seems off, I'm hoping that it'll run as well next week and I'll consider it ready for the next new problem to arise.

Thank you'all very much for taking the time!

Joel