How to Inspect Reed Valves on a Johnson 40hp Outboard

Reed Valve Inspection

Now that I have I have the carburetors off the engine and on the bench, it’s a good time to pull the manifold and reeds off. I want to perform a reed valve inspection. If necessary, replace any components that are not up to par.

The first step in this process is going to be to remove intake manifold. Disconnect the throttle and shift connector by removing the screw that connects it to the manifold. Remove all of the bolts holding the manifold on. There is one odd size bolt that goes into the lower left hand side, make a mental note of where this is located.

Once all the bolts are removed you can pull the manifold off. You may have to pry it off. The manifold and Reeds will come off all in one piece. Once you have them off, you can proceed to the next step.

Cleaning the Manifold and Reed Valve Body

I chose to do a thorough cleaning first. The service manual instructions not to use harsh chemicals or carb cleaner so I decided to use a foaming engine cleaner. I first  thoroughly coated the manifold and reeds with the cleaner and let it sit. I reapplied the foaming cleaner two more times and then scrubbed it with a nail brush and hot running water.

Using a fingernail brush I cleaned the body of the manifold and the Reed valve plates. I didn’t want to take any chance of damaging them the reed valves so I sprayed the whole thing down hot water to finish cleaning them off and get the cleaning foam out of the reeds.

Foaming Engine Cleaner on Reeds and Manifold
Final Cleaning and Drying with Compressed Air

After rinsing them I used 25 PSI compressed air to finish cleaning drying them off. The next step was to remove the reed valve bodies from the manifold plate. There are two screws holding the plates on. With these two screws from the back and the reed valve plate easily comes off. Per the service manual, do not disassemble the leaf plate assemblies unless you will be replacing any of their components! Inspect them first.

Once you have the reed plates off it’s time to remove the old gasket from the manifold. Use a putty knife scrape off the old gasket. In this case it was stuck on pretty good so it took a bit of effort. Take off any residual gasket material from the reed valve body that may be present. Be careful not to gouge or scratch any of the mating surfaces on the manifold leaf plates while doing this.

Removing Reed Plates from Manifold
Take Old Gasket off of Manifold

Inspecting the Manifold

The manifold should be clean and free of any nicks or burrs. It must also be flat to within .004″. I used a machinist  parallel bar for to check the gasket surface of the manifold. The spacer is straight to within .001″ so is a good tool for this. I found that the gasket surface was flat and there were no gaps between the parralel bar and the surface of the manifold.

Checking the Manifold with Straight Edge

Reed Valve Inspection

Now that everything is separated and the leaf valve plates are off it’s time to inspect them. Look at the edges of the reeds, they should have clean edges. Any cracks, chipping or uneven edges are a sign that the reed valves have failed. Look up at the reed valves through the bottom of the plate. there should be a minimal amount of light coming through the area where the reed valves meet the valve body. The Johnson service manual allows for some space between the reed valve and the body. If you see some light coming through, use a rubber tipped pencil to gently push the reed valve against the body.

Inspecting Reed Valves

 If it seats with light pressure it is acceptable, If it does not, you will need to inspect the body for any burrs or scratches (per the Johnson service manual). This will require you to disassemble the reed valve assembly and inspect the plates the reed valves seat against.  Replace any reeds that are frayed or cracked. The Service manual warns against turning used reed valves over for re-use as they may be prone to failure.

Inspecting The Reed Valve Plates and Reed Stops

The Reed Valve Plates need to be examined for flatness on the gasket surface. The leeway on this is a bit more liberal than for the manifold. The service manual on this 1988 Johnson 40 calls for +/-.003″. Of course, you also want to look for any distortion. You’ll also want to examine the leaf plate stops at this point. They should be straight and have no distortion in their shape nor should they be loose.

Replacing Leaf Valves

Should you have to replace the leaf valves or disassemble the Leaf Plate for any reason, keep the components together for each set of leaf valves. You will want to re-assemble the leaf plates with the same components that came off of each group. Assuming of course that all of the components are original and you are not replacing anything. The key thing to pay attention to when assembling a leaf plate is that the leaf valves are centered evenly over the port on the leaf plate. Once you have the leaf plates in place, place the leaf stop (and shim if required) on top of the leaf valve.

For this particular leaf plate, you would mount the leaf plate, shim and stop with the two center screws (Use Loctite Blue on these screws). They should be snug but not tightened. Then, check the alignment of the leaf valves. You can do this by gently opening each valve with an eraser tipped pencil. You are looking to make sure that the leaf plate leaves are centered over the ports. If they are, install the other two screws. Torque the screws to 25-35 in. lbs.

Reassemble Manifold and Leaf Valve Body

Attaching the leaf valve bodies bast to the manifold will be the next step. First, lay the new gasket over the manifold. You did buy a new gasket right? Lay the gasket on to the manifold. It only goes on one way so all you have to do is make sure all of the screw holes and ports line up correctly. Place one of the leaf valve plates on to the the gasket. The plate will actually hold the gasket in place when mounted.


Install Leaf Plates on Manifold

Make sure the leaf plate is mounted correctly as it will not fit into the engine if it’s orientation is reversed. Attach the leaf plate with the two screws you took off when disassembling. There are no torque specs listed for these so I would advise snug but not gorilla tight (somewhere in between). Repeat this for the second leaf plate and the entire manifold and leaf valve assembly is ready to be installed on the engine.

Installing Manifold and Leaf Valves Back on the Engine

This is simply the same processed used in removing the assembly in reverse. Place the manifold into the engine. Make sure the entire assembly is oriented correctly (it wont go in if it is not). Place the original screws back into their original position. All of the screws with the exception of one are the same. Finger tighten the screws. The service manual calls for 60-84 in. lbs. of torque for the screws. I started with the two in the middle and worked my way around in a figure eight pattern. Re-attache the throttle linkage and re-install the carburetors.

Installing Manifold and Reeds
Torquing Manifold and Reed Valve Screws

Lets Start it it up!

This engine hasn’t been run in over twenty years (according to the last owner). So I’m not quite sure what to expect. Actually, I’m hopeful that it will run. While I had the manifold off I inspected the crankshaft and what I could see of the cylinders and pistons. Nothing appeared that would tell me the engine was going south. I had also done a cold compression test. I got 90 PSI across both cylinders which tells me two things. There is a good compression balance across both cylinders, and I expect to get a real compression somewhat north of 100 PSI when I do a proper compression check.

Next Steps

Now that I know the engine runs I’ll do a compression check on it. I’ll do it properly with a warmed up engine and with a wide open throttle. Once I have that done I’ll switch my focus to the Boston Whaler itself. There’s a lot of de-lamination that I want to get sealed up. I’d like to get the bulk of the fiberglass repair done before winter sets in  so stay tuned!

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