I have a 1981 Yamaha XS650. I recently put in K&N stock replacement filter (ie, they are in the airbox and conform to the size of the original), however they may be leading to a leaner mixture because they purportedly let through more air.

I want to tune my carbs (yes, I know that the B34s are notorious for being lean in the first place). I have attempted adjusting the pilot screw, which did alleviate the popping symptoms and gave me a much smoother ride at 25 mph.

In order to truly tune my carb, I want a better understanding of the symptoms of lean vs the symptoms of rich. Terms often used are: Stuttering Hesitating Running Rough etc etc

And worse, many claims of lean conditions and rich conditions are contradictory.

Please advise me on what the real symptoms of lean vs rich system is. Thanks.

4 Answers 4


Lean and Rich Symptoms in Motorcycle Carburetors

In your post you indicate that some of the symptoms you have read are contradictory. I will try and clarify between the two conditions and attempt to give you guidelines in troubleshooting between the two.


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 for a lean idle circuit or backfires in general.

  • Runs on choke - The vehicle may run on 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..."

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  • 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.

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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.


In doing two stroke tuning we would change settings, start the bike and run it at a particular throttle position as long as we could safely ride it that way then abruptly cut the ignition and hope we could coast back to where the tools were. Doing this allowed us to look at the plug at a particular throttle position and adjust accordingly.

I have not put any real information on how to resolve the various conditions and howto set a float consistently or when to use a different size main jet or raising and lowering your jet needle. The post would just turn into a short novel. I'm already going to get some deserved tl;dr on this one.

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A shop manual (particularly one with colour photos) will offer some spark plug diagrams / photos, to compare against your plugs, but they're not particularly useful.

As you said, claims are contradictory. The problem is, that a symptom of too lean/too rich is that it doesn't run as well as it should. That's both ways. So a too-lean symptom for one person is the same as a too-rich was for someone else. The best bet is to do one direction or the other and see if it gets better. Sometimes this can be accomplished by tricks (blowing in the vent tube to lower the gas level and lean it out (but just try to do that at full throttle!)) but more often it's going to be trial and error with part changes. This is why people pay money to dyno operators to tune the bike and do all the jet parts x throttle position x engine speed matrix testing at once. You can take the long approach and tune it yourself with ears, feel, butt-dyno and notes - lots of those! - over some time.

In order to tune your carbs you also need a better understanding of how the various circuits affect the mixture at different throttle openings.

I'd suggest starting with a read through the Mikuni VM Manual. The VM models are older, direct-action slides rather than the vacuum-operated BS34, but the same principles apply. In particular the last diagram "Functional Range Effectiveness of Tuning Parts in relation to the Throttle Valve Opening (Approximation)" illustrates which parts (mostly) affect which throttle positions. It's good that they put "(approximation)" in the title - because the diagram lies - everything seems to affect everything else, but at least it gives you an understanding to start from.

To tune your carbs, you need to either go through everything systematically, starting at the main jet and working down, or you can choose to just identify some problematic parts - particular throttle openings x engine speeds - and address those. Factory Pro has a CV carb tuning procedure for lower rpm engines that's an appropriate systematic procedure for your XS650.


A lean mixture in a motorcycle will symptomatically be hard to start - and you will see less/little fuel in the carburetor and if you remove the plug(s) immediatly after starting attempts, the plugs will be dry rather than an overly rich mixture where they might be "wet" with fuel.

If your bike runs rather bad/worse when cold, but better when warm it is likely running lean.


the easiest thing to do is to raise the needle a notch (or more) if it is too lean. Just don't loose the tiny "E" clip that locates the needle in the throttle slide. Sometimes they spring over a little so be ready for that.

Some carburetors do not have adjustable needles. You can enrichen them with a little washer under the E clip. I have used pop rivet washers for that.

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