Cleaning Fixed Idle Jet Passage on an Outboard Carburetor

Sealed Carburetor Access Ports

After rebuilding these carburetors I still had a rough idle. I took the carbs back off and removed the access plug and carburetor lead shot to clean the idle circuit out completely. I also found a good alternative to the expensive “carburetor lead shot plug” from OMC Johnson/Evinrude.

This is my second go around with these carbs. I recently did a rebuild on them but did not open the idle jet passages. I had a replacement for the core plug that came with the carburetor rebuild kit. But the kit did not include a lead shot required to seal the idle passage back up after cleaning so hoped I could get by without that. As luck would have it, the engine idled poorly so I decided to go ahead with cleaning out the idle jet portion of these carbs.

Why Core Plugs and Carburetor Lead Shot?

This is probably one of the most commonly asked questions. Why do these carburetors have these openings in the first place. The simple answer is that they are needed for access during the machining process. The core plug is also commonly known as a welsh plug in the mechanical world.

If you look at this particular carb, there are two key areas that are covered by the plugs. The first is where the core plug disc is placed. This area servers as a final “mixing bowl” so to speak for the gas and air to mix, at the very front of this bowl there is a small orifice, which is where the fuel mixture will be injected into the carburetor throat during idle. The second area, where the carburetor lead shot is, has the tip of the emulsion pick up tube entering the idle circuit. This is where fuel is introduced in the idle air circuit. Presumably, the only way to machine these areas was externally, requiring a way to seal them up afterwards.

Opening up the idle Circuit

Removing the Idle Jet and Core Plug

I started by removing the fixed idle jet. I already had modified a screwdriver to remove both jets on this carb. Using that screwdriver i removed the jet and proceeded to the core plug. Removing the core plug was a bit difficult. In theory you should be able to poke a hole in it with a small punch and lever it out. I had to take a few good whacks at it before i could get a hole in it but finally got the punch in and levered the core plug out.

Removing the Carburetor Lead Shot

The carburetor lead shot proved to be a bit more challenging than I expected. You would think it was made of titanium! Getting a pick in there to pull it out proved to be next to impossible. I finally used a fine tipped screwdriver (from my bin of surplus over screwdrivers) to gouge out small pieces of the lead shot. Working my way around it I was able to finally get it move. I got most of it out but the remaining pieces fell into the passage way. I was able to shake it out. At this point I had the idle passages completely open

Removing Carburetor Core Plug
Removing Carburetor Core Plug
Removing the Carburetor Lead Shot Plug
Removing the Carburetor Lead Shot Plug

Drain the Carburetor

Before I went any further I wanted to drained the carburetor. This gave me access to the fast idle jet as well as the bottom end of the emulsion pick up tube in the float bowl. The emulsion pickup tube does exactly what it sounds like it does. It picks up fuel from the float bowl and delivers it either directly to the carburetor throat for high speed or to the idle circuit passage at the top of the carburetor. With the carburetor lead shot removed, you can see the very tip of the emulsion tube where fuel is introduced into the idle circuit passage.

Emulsion Pickup Tube
Emulsion Pickup Tube
Tip of Emulsion Pick Up Tube
Tip of Emulsion Pick Up Tube

Cleaning the Carburetor Idle Circuit

I used carburetor spray cleaner to thoroughly clean the idle circuit. I also gave the bowl a good cleaning. This would be a good idea since when I drained the carb, there was very little gas in there. More likely than not, most of it has evaporated leaving behind residue and potential blockage. Giving a few good squirts into the float bowl also lets the cleaner run into the emulsion pickup tube. This is a critical point in the fuel circuit, if it isn’t free to pass fuel through to the carburetor throat and idle circuit the engine will run poorly at best.

Next I sprayed carburetor cleaner through all of the ports in the idle jet circuit. I payed particular attention to the area below where the lead shot was. Cleaning the tip and inside of the emulsion pickup tube is critical to ensure a good feed of fuel into the idle circuit. I also made sure that the small port (or jet) at the end of the cirsuit was clean. This is where fuel is introduced into the throat of the carburetor when the engine is idling. If you look at the picture blow, you can see the cleaner coming through it in a fine mist. This is what it might look like when the engine is idling.

Cleaning Idle Emulsion Pickup Tube
Cleaning Idle Emulsion Pickup Tube
Carburetor Idle Jet Mist
Carburetor Idle Jet Mist

Sealing the Carburetor Idle Circuit

Sealing the carburetor up is relatively simple, as long as you have the right components. First I replaced the core plug, the convex silver disk that came with the rebuild kit. Place it convex side up, using a large punch, tap the center of the plug so that it expands into place. Next, replace the carburetor lead shot. Since the OEM units are so expensive, I bought a tin of .17 lead bb’s. These are a bit large but will do the job. This tin ran me about $6 on Amazon! Installing one is relatively simple. Place the bb over the small hole covering the tip of the emulsion pick up tube. Using a punch tap the bb into place. Lead is soft so it will expand into the hole (which has a concave indentation and seal that portion of the idle circuit.

Sealing Core Plug With Punch
Sealing Core Plug With Punch
Setting Carburetor Lead Shot with Punch
Setting Carburetor Lead Shot with Punch

Using Sealant

Technically, expanding the core plug in place and tapping the bb into the smaller hole should seal up the idle circuit enough. There is however much internet :wisdom” on using nail polish or another sealant to insure a good seal. I opted to use Loctite red. I figured it could only help and definitely wouldn’t hurt. Loctite red is impermeable to gasoline so is a good sealant to use in carburetor applications. I applied a small bead around the core plug and lead shot. If anything, this will insure there are no small air leaks into the idle circuit which could cause a lean condition.

Sealing Core Plug with Loctite
Sealing Core Plug with Loctite
Sealed Carburetor Access Ports
Sealed Carburetor Access Ports
Be sure to watch the YouTube video of this article

Read the Original Carburetor Rebuild Article Here:

1981 Johnson 40HP Carburetor Rebuild

What Next

I had already planned on removing the VRO2 oil/fuel pump on this engine and replacing it with a direct fuel pump. I’ve heard to many horror stories of these units failing and ruining an engine. A direct fuel pump runs about $30 and it’s relatively simple to replace. It will require using a pre-mix of 50:1 fuel, but I am comfortable with that. Having the carburetors off the engine makes this a little easier to do so, I’ll be replacing the fuel pump next and posting a detailed write up on how to do it, stay tuned!

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