Fixing the Idle on a WW2 Jeep
G503 WO 539S Carburetor In Depth - The Idle Circuit
We have been having idle trouble with one of our WW2 jeeps - it won't idle below about 1,000 rpms without choke with about 600 rpm being ideal. After a bunch of diagnostics, it turned out that the original Carter WO 539S (that I rebuilt) was the culprit.
There are numerous reasons why a WW2 jeep engine might require choke to idle well. Importantly, not all are carburetor related. An intake manifold leak, poor timing, poor compression or distributor related problems are potential culprits that should be eliminated before you jump to the conclusion that you have a carb problem.
If you are satisfied that the idle problem is with your carburetor, know that idling is managed by a completely independent circuit on the Carter WO 539S called the low speed circuit - we will call it the idle circuit here for simplicity.
As long as there is fuel in the fuel bowl, at idle the idle circuit operates independently from all other functions of the WO carburetor and is 100% responsible for a smooth idle.
The good news is that you can pretty much ignore everything else on your carburetor until the idle circuit is working correctly and your jeep idles smoothly.
You will find it helpful to refer back to this WO low speed (idle) circuit diagram to identify parts as you read this writeup.
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One more quick theory of operation. The “so called” venturi of a carburetor is the main "tube" of a carb, the top of which is connected to the air cleaner and the bottom the manifold. In the case of the WO, the base, main body and top all make up parts of the venturi. Carburetors rely on vacuum from the engine to suck air through this venturi, collecting and vaporizing gasoline from small holes located along the way. This vaporized gasoline provides the explosive mixture that enables combustion. Look up Bernouilli’s principle if you want to understand more.
Description of Idle Circuit
The fuel bowl is filled by the fuel pump with gasoline until the float rises and shuts off flow. With a properly adjusted float, the gas level is very near the top of the fuel bowl.
At the bottom corner of the fuel bowl is a small hole. Inside of that hole you will find the idle well jet - a small brass screw with a hole in the middle of it (known as a jet) that allows fuel from the fuel bowl to flow into the idle well.
The idle well and low speed passageway are a part of the main body of the carburetor - these elements are contained in the "tube" in the casting between the fuel bowl and the venturi that rises up level with the fuel bowl.
Gravity forces gasoline from the fuel bowl through the idle well jet into the idle well and up the low speed passage until the fuel in the idle well is level with the fuel in the fuel bowl.
From the top of the the carburetor looking down the idle well, you are going to find the low speed jet. The low speed jet is another brass jet - but distinctive because it is a long thin tube that tapers at the tip. The low speed jet regulates the amount of fuel that can pass through the idle well and low speed passsageway and be delivered to the engine at both idle and in other operating ranges.
The idle well is connected to the venturi by a small passageway that continues down through the base of the carburetor to the idle port. The idle port is a tiny hole in the venturi, right below the throttle plate. See figure 29 above. Note that the idle port is a hole drilled at a pretty steep angle so it looks like a little slot where it enters the venturi.
An aside: At idle, the throttle plate covers almost all of the idle port- opening the throttle exposes more of this hole/slot and allows more fuel delivery when the engine is operated at higher speeds. This fact isn't important for idle, but will come into play as you tune the carburetor for higher speeds.
When servicing your WO, the idle port and this entire passageway must always be cleaned with a small wire (I use a B guitar string), carb cleaner and compressed air.
Note also the "economizer" in Figure 29 above. The economizer is just a marketing name for a restriction in the idle circuit passageway to make the carburetor more fuel efficient. The economizer does nothing at idle, but does serve to reduce the amount of fuel delivered through the idle circuit at higher operating speeds.
Compressed air blown into the idle port will usually clear this passageway, but you can remove the aluminum plugs in the carburetor (if your kit supplied new ones) and manually clean this area too. Carb cleaner usually works too so I don't generally recommend removing those plugs for regular maintenance.
Right below the idle port is the idle adjustment screw. The idle adjustment screw has another small hole, like the idle port (the idle adjustment screw hole - also drilled at an angle and forming a slot), that allows more gas into the venturi. The big difference is that the screw itself is a needle valve meaning we can adjust the amount of gas being fed into the engine by screwing the needle valve in and out. Note that the screw port is fed with gasoline through the same passageway as the idle port - this is all part of the same supply system.
Congrats if you have stuck with me this far.
At the risk of overstating the point, this idle circuit is a completely independent from all other circuits in the carburetor and is ALWAYS delivering gas into a running engine. Translation - you will never get the best performance from your L134 without getting this idle circuit functioning 100% within specifications.
Turning Theory into Practice
Let’s turn all of this theory into practice. When your jeep doesn't idle well due to a fault of the carburetor, it has to be because there isn't sufficient fuel being delivered through this idle circuit. (Before you pick on me about air leaks, lets assume that everything else on the carburetor is correct.)
What could be the causes?
Check your float level and make sure it is set correctly. Modestly incorrect fuels levels in the fuel bowl are unlikely to be an idle culprit, but let's get it right as this fuel level will matter at higher speeds.
The idle well jet could be clogged because we often open up these carburetors and find debris inside of the fuel bowl, even in rebuilt jeeps with correct fuel filters. The idle well jet hole is large though, so a clog here is not likely. To clear the idle well jet, you can remove the idle port cover and the low speed jet and run a wire into the fuel bowl or use compressed air to blow it out. The idle well jet does NOT meter gas at idle so as long as it is open and clear, you can eliminate that point as your idle problem.
Pro Tip: The idle well jet is small, sits deep and has generally been "glued in place" by dried fuel over the years. This jet is typically the most difficult jet to remove undamaged from the carburetor so I don't generally recommend removal. How to remove it is beyond the scope of this long post but we are going to post an entire WO rebuild video that will dive deeply into this and other Carter WO esoterica. The video will be available to subscribers on www.portrayal.tv
With the idle well jet clear and it and its related plug installed, you need to make sure that the low speed jet is clear. This jet is made with a tiny, very cloggable tube - use your fine wire, some carb cleaner and air to ensure it is completely free and open.
Finally, close the idle adjustment screw and use compressed air into the idle port to blow out the passageway and economizer all the way through the idle well jet. You can and should use carb cleaner too, but watch your eyes with flying solvents and other debris.
Reinstall the low speed jet and the related plug.
Finally, unscrew the idle adjustment screw, blow out the hole and clean and reinstall. The idle adjustment screw should be set 1 1/2 turns out.
Finishing reassembling the carburetor and reinstall it.
Your jeep should now idle beautifully. You can prove to yourself that things are working correctly by closing the idle adjustment screw. The engine should run really roughly or even quit. Open the screw back up 1 1/2 turns and then a little more to see if you get any changes. You will likely hear the rpm’s increase because more fuel is being delivered through the idle screw port. Know that there is only so much air available at idle (more or less a constant) so opening this screw past the point where you hear an rpm change means you are just making the mixture richer. Rich is generally better than lean, but can cause spark plug fouling and other issues - the target is smooth at 1 1/2 turns.
Set the idle adjustment on your throttle. These engines idle beautifully at around 600 rpm….see how low it will idle before sounds rough and then smooth it out.
Know that there is a connection between the throttle idle adjustment and the idle adjustment screw so play around and use your
ears. Remember that the carburetor was designed around the 1 1/2 turn
out setting so that should be your target with a smooth 600 rpm idle.
Happy jeep'in!
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