How To Fix A Motorcycle Running Rich: 6 Easy Ways


If your motorcycle is running rich, you’ll usually be able to smell a difference in the exhaust if you pay attention. Don’t stick your face down by the exhaust , just stand next to the motorcycle while it’s running and if it smells really strong of fuel then that means your engine is running rich and you need to adjust your air to fuel ratio. This is accomplished a few different ways.

The 6 main ways to fix a motorcycle running rich are to:

  • Adjust the air to fuel ratio
  • Check if the needle is sticking in the carburetor
  • Check for stuck open floats
  • Clean your air filter
  • Check for stuck open butterfly valves
  • Change your O2 sensors if your motorcycle has them

The most likely culprit to a running rich problem is the carburetor.  When motorcycles sit for extended periods of time the fuel starts to gum up and do weird things to a carburetor.  Parts start sticking together, holes get clogged, and the carb will need a thorough cleaning before it will start functioning properly again.

Adjust The Air/Fuel Screw

Every carburetor has a small screw located somewhere on the carb body called the fuel screw, idle screw, or air to fuel screw. Some manufacturers call it different things.  But its purpose is so to adjust the amount of fuel that gets sucked into the cylinders before it combusts. If your motorcycle is running rich, this should be the first thing to check.

Some older motorcycles vibrate pretty substantially while running, and over time this can cause that fuel screw to loosen out of the carb body and either supply more or less fuel.  Check your shop manual under the carburetor section and figure out whether to twist the screw in or back it out.

For example, on my 1969 Triumph the service manual says to turn the screw all the way into the carb body until it bottoms out and then twist it three full revolutions back out and that should give a good starting point for the air to fuel ratio.  So look through your service manual for how to adjust that screw on your motorcycle specifically.

There are other tools I like to have handy that I use frequently on my motorcycles. Click here to see my recommended list of tools.

Sticking Needle

The needle is one of the main components in a carburetor.  It’s a long skinny metal shaft that lifts up and down when you twist the throttle.  Many times if you take the air filter off of your motorcycle and look through the big center hole you can see the needle running right through the middle of the carburetor.

When you twist the throttle with your hand, it lifts up that needle and allows fuel to flow through.  If your carburetor hasn’t been cleaned in a while and you don’t ride it regularly enough then you might have issues with that needle sticking in the “up” position once you let your hand off of the throttle.  It can have a tendency to stick to the carburetor walls inside because they get gummed up after not using it.

There are a few easy fixes for this.  The best solution is to take off the air filter so you have a good view of the big carburetor hole.  Make sure the motorcycle is off and twist the throttle back and forth while you spray carburetor cleaner on the needle.

The cleaner will knock off a lot of gunk but if it’s really gummed up then you’ll probably have to take the carburetor all the way off the motorcycle and take it apart to clean it thoroughly.

Stuck Open Floats

Stuck open floats are also pretty easy to fix and will probably take you an hour or two to repair.  There are a few signs of stuck open floats including running rich, your motorcycle revs really high after you start it, and fuel overflowing out of the carburetor.  I’ve seen all three first hand on different motorcycles I’ve owned.

Floats are little bubbles that sit in the carburetor bowl at the bottom of the carburetor.  If you take off the bowl (usually 3 or 4 screws on the bottom of the carburetor) then the floats will usually dangle down a little bit.  It’s similar to looking inside any toilet tank. That big bubble that lifts up and down with the water level is a float. Your carburetor has one in each of the bowls.

Before you start taking anything apart, make sure you take pictures on your phone.  You think you’re going to remember how to put everything back together but I promise you won’t.  I’m speaking from years of experience rebuilding carburetors.

In order to clean the floats and make sure they aren’t sticking, just take off the bowl and there will be one pin in each float.  Using a small pointed object, push that pin out the side and the float will drop free.

There is a small piece connected to each float that looks like a tiny little missile.  That is called the stopper and it shuts off the fuel flow from the tank once the level gets high enough in the carburetor. It’s a cool little feature so your engine doesn’t flood constantly.

This is definitely a lot easier to do if the carburetor is completely off of the motorcycle but it doesn’t have to be.  After you have the float and stopper out, spray everything inside the carburetor really well with carburetor cleaner, especially the little hole where the stopper goes.  You can even use a small toothpick and try to clean off the inside walls where the stopper goes in. This can help it seat better.

Each section of carburetor has its own float and stopper, so make sure you clean all of them really well unless you specifically know which cylinder is the issue.  Clean the float, stopper, and pin really well before you put them back in. The pin should be really easy to push in and out. If it’s tight then you did something wrong and you need to take it back out again.

Make sure the float flops up and down really easily, it should be able to rotate up and down by just jiggling the carburetor.

Clean Your Air Filter

This is one that so many people overlook because they don’t think about an air filter causing a fuel problem.  In order for your motorcycle to run properly it needs 14.7 parts of air to every 1 part of fuel, that’s called the air to fuel ratio.  If you have a clogged air filter then that ratio will get reduced down to 10:1 or 8:1 and you’ll be running rich.

When an air filter is old, is has a lot of trapped dirt and debris inside the small foam pockets and it doesn’t let the air flow as smoothly through it. This causes air restrictions and can make your engine run poorly. They’re usually only a few dollars online, so go ahead and change yours if it has been a few years since you’ve done it.

Every motorcycle is a little different in how you change your air filter, but the manufacturers make it really easy most of the time because it’s something you should change at least every few years.  Some motorcycles even have two air filters, a pre and a post filter. Check your user’s manual to figure out how to change your specific filter. Check out this article of mine that discusses air boxes vs. pod filters.

Stuck Open Butterfly Valve

The butterfly valve is the round disc on every carburetor port that opens and closes as you twist the throttle with your hand.  These valves are controlled by really tightly wound springs on the carb body and if one of those springs breaks or gets loose then the valve won’t shut properly.

I was helping a friend once work on his motorcycle and he was having a problem where every time he would twist the throttle the motorcycle would stall.  I looked through the carburetor ports and realized that someone had put the springs on backwards. So every time he twisted the throttle it closed the butterfly valve and made the motorcycle stall out.

The same thing can happen to cause a running rich issue.  If that valve gets stuck open then too much fuel will be constantly spraying into the cylinder instead of getting throttle by the rotation of your hand.

In order to see if this is a problem with your motorcycle, you’ll have to take the air filter off and look through the carb hole as you twist the throttle cable.  If you open the throttle and the flapper opens at the same rate then it’s working properly. If not, then you know that’s what the issue is.

Change Your O2 Sensor If You Have One

Newer motorcycles are usually fuel injected.  If you have a fuel injected motorcycle then none of the previous points will apply to you, you have a different set of problems.

The most common running rich issues with a fuel injected motorcycle is with the O2 sensors.  O2 sensors get heat cycles a lot over their life. They get really hot when the engine is running and then really cold when it gets shut off, this causes them to break down over time.  If you have constant running rich issues then I would check into getting some new O2 sensors.

Not all fuel injected motorcycles have O2 sensors, but for those that do, it’s a common problem for that sensor to send misinformation to the ECU and tell the fuel injectors to spray more fuel into the cylinder on each rotation.

Related Questions

How often should I rebuild my carburetor?  If you ride your motorcycle regularly then you might not have to clean a carburetor for the entire time you own that motorcycle.  Carburetors get dirty and gummed up by not using them, so go riding as much as you can!

What are the best tools to clean a carburetor?  The tools I use most often when cleaning carburetors are a simple socket set, a pick with a 90 degree bend on the end, some small screwdrivers, and a mini drill bit set from a hobby store.  I use the drill bits to clean out jet ports, they work amazing.

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