Inside A Motorcycle Carburetor – Idle Circuit

Does your motorcycle idle funky or not at all? There can be several causes for this, but the most common and first thing you should troubleshoot or tune is your carburetor. Today on MC Garage we are going to talk about the idle circuit of a carburetor.

With modern fuel injection a stable idle, no matter the conditions, is a given, but if you have a carburetor in your ride, things could be less consistent. Environmental factors such as altitude and temperature can require you to make some adjustments to your carb.

And of course the darn thing could just be full of crud from sitting too long or from not being serviced for a long time. Ari did a killer video on how to clean a carb a few years ago, we’ve included the link to learn all the ins and outs of cleaning below. For this video we are continuing on proper adjustments of all the systems of the carburetor.

The pilot jet along with the mixture screw is responsible for how your bike idles. It also provides the fuel for about the first 15 to 20 percent throttle. The pilot jet is the smaller of the two jets under the main body of the carb in which fuel flows to mix and atomize with the incoming air on its way to the cylinder.

In your service manual the manufacturer will list the standard size pilot for your bike as well as give alternatives for ranges of altitudes and temperatures. However you can’t just change the pilot and be ready to rip. You might be close, but there are a few steps for tuning. This is where you are going to need to listen to your bike.

First your bike should be warmed up. When the bike is cold you are going to need to use the choke or enrichment circuit. A cold engine needs a richer (more lower air to fuel ratio) to run. Most flat slides will use a seperate enrichment circuit to supply additional fuel. It’s basically a plunger that opens and closes the circuit. Other carbs use a secondary butterfly to limit the airflow while the pilot provides the same metering of fuel, thereby creating that richer mixture. Once the engine is up to temp, turn off the choke.

We also need to be sure your air or fuel mixture screw is in the middle of the range to start this process. If the mixture screw is on the airbox side it’s an air screw. This one meters air into the pilot circuit. Turning it clockwise closes the opening, richening the mixture. Counterclockwise adds air and leans the mixture. Check your service manual, but a proper baseline setting should be around 2.5 turns out (clockwise) from fully closed or seated.

If the mixture screw is on the engine side, it meters fuel instead of air. Clockwise is less fuel and a leaner mixture, whereas counterclockwise is more fuel for a richer mixture. Just like the air screw, check your manual for the standard position or starting point, but a good rule of thumb is this should be around 1.5 turns out. So once you have those in a correct starting position we get down to tuning or jetting.

Let’s discuss what the idle behavior will be like when a warmed-up bike is properly jetted, too rich, and too lean. A proper idle should be consistent without any input from the throttle. It should also settle quickly after revving the engine.

If it’s too lean, you’ll find a hesitation or bog right off idle when you crack the throttle. Also, when revving the engine, the rpm will hang at a higher level than normal idle or will not settle into a consistent idle quickly.

A too-rich pilot setting will give you a sputter when the throttle is cracked. Response will just feel a bit heavy or sluggish. When the engine is revved, the rpm will drop quickly and dip below ideal idle before recovering; sometimes it will just die.

Depending on that behavior you should have a pretty good idea if you need more or less fuel. Is it rich or lean? From there begin with the mixture screws. Lean or richen the pilot circuit one-quarter turn at a time. Rev the engine, listen, feel, and repeat. If you get a nice, stable idle, you’re good to go. If you can’t get that nice idle without going to the extreme edges of the mixture screw adjustments, say one turn to a turn and a half from the starting point, you are going to have to go up or down a pilot jet size. When you change that jet make sure you put the mixture screws back to the standard 1.5 or 2.5 turns out before beginning the process again.

This should give you a great base to start with on your idle or pilot circuit. Some modern four-stroke carbs have an additional accelerator pump and jet. That’s an additional wrinkle that we will tackle in a later video. But just be sure the jet and pump circuit is clean before trying to set your idle and cracking that throttle. Later we will also jump into the art of syncing a bank of carbs on a multi-cylinder engine.

But that’s the basics and will get most of you with a single carburetor well on your way to fine-tuning your motorcycle idle circuit. In the next video we will move on to the needle jet and needle.


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