Inside A Motorcycle Carburetor - Idle Circuit | MC Garage

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.

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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.

If you haven’t watched the first two videos in this series of carburetor series, go back and watch them first. Links are of course below.

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.

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.

[christiansanchez]

You’re carburetor videos have been very helpful when it comes to tuning and fixing my moped thanks man you’re definitely an angel to society

[bwxmoto]

I’m always amazed a carburetor works at all.

[ItsMaTOfficial]

This is an absolutely well explained video about carbs. I own a 1994 Suzuki RF600R and just from this video alone I've discovered my bike is running too rich! Exactly like he said, the bike would rev up normally but dip way below the normal idle. It's gonna take a day or too to get it right. Thank you Justin and MC Garage! You guys are life savers.

[radioaktivejay]

Great content and good presentation. But..miss you Ari Henning.

[Dkm]

in india usually lower segment bikes till 200 cc uses Carburator and some higher uptill 500cc too and i own a 125 cc Scooter which has the carburator with Electronic choke. Tuning that At the beginner level(me) was not perfect and i learned a lot. seeing this video i can compare and revise the procedure to get better understanding of the sound the engine produces. My other 110 CC scooter had a traditional, no electronic nonsense Carburator which in recent month giving me a problem where its overflow oriface at top and bottom one completely ejecting fuel whenever the engine is cranked. thats problem was a nightmare to solve untill i got a float needle valve to replace with the old one and the problem stopped. thats how amazing and nightmarish the Carburators are. Thanks for the Content.

[HpeAnLve]

This video is among the most helpful explainations on the internet.

[tuanoful]

Mine seems to be too rich, like you said, it's slugish and then it dips bellow idle, it does turn off some times.
Ugh.

Thanks for the video, amazing info.

[sizfhyb]

Great tips!...my bike is idling on the high side, so perfect timing with this vid.

[rodintoulouse]

Thank you. I was missing the bit on how rich and lean feel like. Good and simple explanation

[AndrewSzypula]

Justin looks like the middle of a "On a scale of Ari Henning to Lemmy from Revzilla"

[IrlDave]

Thanks. Great series. But can you drop the disco beat?

[punterash]

I have been waiting for this video since 1885

[joelomongos]

The best info videos on motorcycle, please make a video on throttle body cleaning on cbr

[zuhailishufller]

Carburetor are a work of art and I'm loving to work with it as it is simple and no electronics failure will happen to this bad boy. If you want more power from your bike just slap on a bigger carburetor with the right jetting and your bike are already good to go. Fuel injection on the other hand need a Dyno tuning which never be good on your wallet and all the upgrade parts are expensive as hell.

[henrryvilleta]

Will you post some video of how to tune multi-cylinnder carburetors?

[aggieforlife]

my two stroke ke100 idles fine possibly on the rich side. It doesn't really bog in neutral. It only bogs under load if I open the throttle more than 50% . If i gradually open it its fine.

[bc-mrsi]

hi thanks for the explanation. ive just changed my pilot jet and main jet .the fuel screws have been adjusted and are in the same position as before. is that ok 🤔i set the fuel screws that gave the fastest idle .

[Donaldsilverman]

I have a 2004 triumph speedmaster 790 that I ride regularly but it stumbles at 70mph+ in a strong headwind, like it may be lean. It has airbox/air injection delete and k&n filters.

[chrisrhoy]

Was there any difference in the lean and rich examples at 3:30-ish? Sounded the same to me.

[thedaydreamer]

Hi, I have both a Intruder VS1400 and Marauder vz800, , , I have the bikes running well and synced and carbs inspected and set to spec, , , However the idle on both of these bikes is a bit off, , , AND upon bliping or reving the throttle on both bikes the idle hangs for a sec before returning to steady idle, , , from my under standing when this happens its typically a lean condition, conversely when you blip a throttle and it dips, then returns to idle its a rich this issue solved by simply finding the correct mixture screw seting on both carbs? IF not do you have any other ideas for me to try? TY you have a great channel