There are a LOT of things that can go wrong with carburetors, but more often than not running issues (like hard starting, bogging off idle, or a hanging idle) are caused by the same issue: the pilot jet. This video isn’t a comprehensive troubleshooting guide (that would be an hour-long episode), but it does address the single most common carb issue. Hopefully it helps you.
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A motorcycle carburetor is just a mechanical mixing device. It mixes gas from the float bowl with air passing through the throat in a precise ratio, and if that fuel/air ratio is off, you're going to have problems.
Far and away the most common problem carb problem is clogged pilot jets. The pilot jet, along with the mixture screw, are what make your bike run at idle on up until about quarter throttle, so that’s where you should look if your bike is hard to start or only runs with the choke on, bogs or stalls as you crack the throttle open, or has an idle that revs up on its own or “hangs” at a high idle before settling down.
All of these issues are indicative of a lean running condition, meaning that fuel/air ratio has too much air and not enough fuel.
More likely than not your pilot jets are jammed up with debris or gummy gas, and cleaning it means pulling the carb or carbs off your bike. Then you’re going to need to pop off the float bowl and clean the pilot jet, which is typically the smaller of the two brass jets visible on the bottom of the carb. Often it’s removable, so you unscrew it and blast it out with solvent and compressed air.
While you have the carb off your might as well give the thing a thorough cleaning, so click here to see our MC Garage video with step-by-step instructions for a complete carb cleaning.
So why is the pilot circuit such a common problem? Because it’s got the smallest passageways in the carburetor, so they’re the first to clog. Pump gas now has a bunch of ethanol in it and it starts to go bad and gum up in just a matter of weeks, so if you’re not riding your bike regularly you can save yourself a lot of trouble by adding a fuel stabilizer to keep the gas from degrading.
There is one other thing that can cause a lean condition at idle, and that’s an air leak. If your bike has rubber intake boots or relies on an o-ring or gasket to seal the carb to the flange or the flange to the cylinder head, that’s also something to check. But the majority of the time, it’s just an issue with the pilot circuit.
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