May 30, 2018

Lean and Rich Symptoms in Motorcycle Carburetors


Most of us have had carb issues at one time or another. I think this is a helpful guide.



Motorcycle carbs have a few circuits that effect different throttle positions.


  • Idle Circuit - for idle and off idle

  • Pilot Circuit - small effect on idle. Higher effect at lower RPM's with decreasing effect to full throttle

  • Needle jet/jet needle - Mid range circuit

  • Main Jet - Wide open. Fuel is metered through the main by the jet needle at different throttle positions.

  • Choke circuit - Initiated by a valve or butterfly that increases the vacuum and opens the circuit up. Cold starting and warm up.

Symptoms of Lean and Rich

Here are some basic symptoms, some duplicate


Reduced Power - Sluggish at certain RPM's. Wide open throttle yields no

power. The engine may bog down until you reach a lower RPM and then

suddenly power returns.

  • Difficulty Starting

  • Spark plugs are clean - No residue. Insulator may be white.

  • Runs better at higher altitudes - AFR becomes normalized due to reduced atmospheric pressure allowing fuel into the venturi easier.

  • Backfires - Popping on deceleration choke when it's warm but stalls if the choke is turned off.

  • Idles poorly - Fluctuations in idle RPM, stalling

  • Engine runs hot - Due to more oxygen than fuel combustion temperatures are hotter reflecting on a temp gauge.

  • Hanging idle - The engine idles high and then drops and stalls. Typically a lean condition caused by an air leak between the butterfly and the head or a vacuum line that is not attached.

  • Sharp Odor - The exhaust smell may be sharp and burn your nose. This is NOx or Nitrogen Oxide. It's created by high temperatures in the combustion process between nitrogen and oxygen. Although NOx is present in all exhaust it can be more pronounced in higher combustion temperatures associated with lean conditions with more oxygen. More oxygen=higher burn temp


  • Reduced Power - It has less power but seems to be OK and runs.

  • Reduced Fuel Mileage - Your using more fuel per cycle so your mileage becomes worse.

  • Rough Idle - Combustion takes a just a bit longer with more fuel and at lower RPM's the engine can idle rough. Unlike a miss-fire on a spark plug this idle seems fine if the idle is turned up just a bit.

  • Spark Plugs Black - Carbon buildup on insulator and electrodes of plug. Wipes off your finger as sooty and back. Carbon is a conductor of electricity and getting considerable carbon buildup from a rich condition with ground the inner electrode down the positive insulator. When this condition occurs there will be no spark. You can clean the sparkplug with carburetor cleaner and re-use.

  • Exhaust Exit is Black and Sooty - Excessive carbon buildup in the exhaust is thick and overly grimy. A properly jetted carburetor will leave a nice grey scale color in the end of the exhaust pipe.

  • Odor of unburned fuel - In cases of a very, very rich mixture you may smell some unburned fuel coming out of your exhaust.This could be due to a grounded out sparkplug from too much carbon buildup preventing the combustion process in a cylinder or so much fuel that combustion is not occurring.

  • Runs better with more air - If you remove your air filter and the engine begins to run a bit better with more oxygen in the mix it is likely you have a rich condition.

  • Runs worse at altitude - If the bike starts to run worse at higher altitude with reduced oxygen, it's likely a rich condition.

  • Black Smoke - If you have black exhaust when you rev it, it's rich. You can get a light colored rag and place it over the exhaust lightly and rev it to see if you get carbon/black residue on the rag.

Troubleshooting Lean Conditions

There a few common issues that pop up with motorcycles that have carbs.

1. Rubber Intake Manifolds - Most modern motorcycles (Mid 1970's on) have rubber manifolds attached to the cylinder head. The carburetors fit into these manifolds and are clamped. Over time they become hard, brittle and cracked and suffer from dry rot due to excessive heat at an accelerated rate.


  • Cracking can create air leaks which create lean conditions.

  • Poor assembly accounts for another large portion of air leak created issues. Make sure the carbs are seated in the manifolds properly. When inserting a bank of 4 carbs into a 4 cylinder it can be very difficult to get them to seat right. Use a bit of oil in the inside diameter of the manifold to help them slip in easier. Tight the outer two clamps initially in stages before starting on 2 and 3.

  • Vacuum Lines Not Attached is very common. Leaving a petcock vacuum line off can create a lean condition on 1 cylinder but normal on the rest. You can get odd idle symptoms and a lazy idle condition where the engine RPM's drop slowly and it 'hangs'

You can test for air leaks by running the motorcycle and squirting carb cleaner at suspected cracks. The idle will reduce a bit if you hit a winner. As well, you smell a sharp odor from the exhaust as it is pulled through the motor and undergoes combustion. Don't breath much of that in and use fans while doing this. If you don't feel comfortable with carb cleaner you can use an oily thicker spray like WD40 that will clog the crack better temporarily and reduce the idle. I'm typically looking for the odor though, it's sharp and pronounced and unmistakable as opposed to, "did that idle just drop a bit, let's do it again..."






  • Sparkplugs Give you great breadcrumbs at to what is happening inside the combustion chamber. Lean conditions can create pre-ignition that melts electrodes or leaves the electrodes in a dusty white condition or simply white with no deposits.


Common Rich Condition Causes

Rich conditions are considerably less frequent than lean. Much of the issues lay in the fact the people are typically creating lean conditions upon reassembly of carburetors. Rich conditions can be created by people too but the condition is usually caused by:


  • Installing a jet that is too large

  • Setting a float level that is too high.

  • A Choke that is stuck or not turned off. Ensuring proper cable routing so that you can turn the handlebars all the way to the left and all the way to right without opening the choke when doing so is an essential test after installing the carbs and after reconnecting the choke cable. Ensure it doesn't bind up and pull the choke open.

  • A stuck float in the float bowl will allow fuel to overfill and then spill out the main jet into the venturi. If you have fuel in your airbox that's a good sign you have a stuck float and bad vacuum petcock

  • A stuck float with a standard petcock can create issues as the fuel will do what is stated above but when the bike is sitting. This fuel can also fill the cylinder and create hydrolock and destroy your engine when you start it. The piston cannot get to top dead center on the compression stroke and it bends your connecting rod. This is real. I've seen the results...AND of course if your petcock is stuck open it's overfilling all of your carbs and creating rich condition.

Sparkplugs Are Your Breadcrumbs

Again, refer to the sparkplugs. Look at the chart. See what you can see. If you have a multicylinder bike and multiple carburetors you will NEED to check ALL of your plugs. Each carburetor can have an individual issue.





All trademarks and copyrights held by respective owners.
Submitted and public relations material also owned by their respective owners.
All other content Copyright © Michael Cannon 2017/2018

©2017 by Kawasaki ZZR 1100 Riding a Legend. Proudly created with