Vintage Motorcycle Carbs – Lean vs. Rich?

Carb Adjustment, Lean or Rich

Experienced wrenches sometimes forget that we operate with a set of understandings that becomes so ingrained as to become second nature. I don’t normally rely on gizmos like the K&N Air Fuel Monitor shown on my supercharged GL1000 (above). But since things happen so quickly on that bike, it’s a useful and entertaining tuning aid.

Normally I can tell immediately if a bike is running rich or lean by clues that are obvious …at least to me. I forget (sometimes), that others operate with different levels of experience on such matters.

When you are chasing carb gremlins, rich vs. lean is a very important diagnostic determination that can help send you down the correct path to a solution. This purpose of this tech tip is to help readers make this determination with certainty.

What are we taking about here? Briefly, rich vs. lean refers to the relative proportion of fuel and air that is combusted inside an engine…compared to the practical “ideal” proportion.

The simplistic ideal proportion for our purposes is 14.7:1 This is known by scientists as stoichiometric combustion – the ideal combustion process during which a fuel is burned completely with no pesky leftovers. This means 14.7 parts of air for each 1 part of fuel. Ignore for now complexities like air density, altitude, barometric pressure, fuel vaporization, relative humidity, ambient air temperature, warm-up requirements, how much “non-gasoline” is in gasoline, how we measure “parts” of air and fuel, etc., etc.

Slightly confusing – but the scientific ratio (e.g. 14.7:1) is always expressed as parts of air to 1 part of fuel. 

Described from the point of view of AIR:

When you have more air relative to fuel than ideal (ratios greater 14.7:1)…that is operation in the lean range.

When you have less air relative to fuel than ideal (ratios less than 14.7:1)…that is operation in the rich range.

or…

Described from the point of view of FUEL:

When you have more fuel relative to air than ideal (ratios less than 14.7:1)…that is operation in the rich range.

When you have less fuel relative to air than ideal (ratios more than 14.7:1)…that is operation in the lean range.

In the real world, practical considerations like ease of starting, engine heat management, fuel economy, throttle response, drivability, max power output, emissions control, etc. necessitate departures from theoretically ideal mixture ratios. Carbureted engines usually have a range of fuel and air circuits and other strategies to vary the proportions of fuel and air throughout a wide variety of engine speed and load conditions. Electronic fuel injection is of course more capable and clever, but carburetor engineers have developed some very elegant solutions that work quite well!

A good example is the choke (or starting circuits on GL1200 carbs). The choke provides a temporary super rich condition (approaching a ratio of 10:1 or so) which promotes easy starting of a cold engine. The richness is necessary because cold fuel in a cold engine does not vaporize as readily …so the effective richness of a given mixture is reduced.

Incidentally, chokes work not by cutting off air as commonly explained. Rather, they reduce the effective size of the main carb venturi. This generates an increased vacuum signal which can draw more fuel through the applicable fuel circuits.

Here’s a chart that can give you some clues of Rich vs. Lean:

Rich Symptoms:

  • Poor power output
  • Starts too easily when cold – requiring little or no choke. Responds readily to the throttle when cold. Runs worse as it warms up
  • Hard starting when hot
  • Ragged idle …due to “loading up” of unburnt fuel
  • Idle that tends toward lower rpms than expected based on the setting of the curb idle screw
  • Easily settles to idle, but has tendency to dip below set idle speed then recover …sometimes stalls
  • Strong pungent odor of unburned fuel
  • Buildup of black, dry, sooty carbon deposits on spark plugs. Bad cases of this buildup will foul the plug completely and kill that cylinder.
  • Excessive buildup of dry sooty deposits in the exhaust system
  • Responds to throttle, but sluggish
  • When fully warmed up, runs much worse when choke is applied.
  • Poor fuel mileage
  • Black “puffy” smoke during hard acceleration
  • Temporarily removing air filter element makes the engine run better.
  • Runs worse as you climb to higher elevations
  • Fresh engine oil quickly turns black from excess fuel dilution

Lean Symptoms:

  • Poor power output
  • Hard to start when cold …requires excessive choking.
  • Lengthy warm-up required.
  • Runs better (but not good as it warms up)
  • Spark plugs overly clean …with no deposits or slight glazed appearance
  • Rough, erratic idle that drifts toward higher rpms than expected based on the setting of the curb idle screw. Sometimes a lean condition will cause a “hanging” idle that is slow to settle down to set idle speed.
  • Backfiring
  • Sluggish …hesitates when the throttle is opened, then recovers (often accompanied by a slight backfire)
  • Vague throttle response
  • Surging at steady throttle cruise operation
  • When fully warmed up, runs better when choke is applied.
  • Engine runs hotter than normal. Headers can turn cherry red in extreme cases!
  • Temporarily removing air filter element makes the engine run worse.
  • Unusual “sucking” noises in the intake area.
  • Runs better as you climb to higher elevations
  • Slight back-firing on deceleration. This normal tendency is controlled by the air cut-off valve(s) operation. If the air cutoff valve(s) is working properly and you still have backfiring on deceleration, something is causing a lean condition.

Note: Excessive “bluing” of exhaust pipes/mufflers is usually a symptom of inferior chrome plating compounded by excessive leanness which causes higher than normal exhaust temperatures.

Notice that the first item on each list above is “poor power output.” Any calibration other than ideal …rich or lean will reduce the efficiency and power!

Once you’re on the right trail, then you need to verify the rpm range where the problem occurs: idle, transition off-idle, mid-range, full power, or deceleration. This will point you to the circuit(s) or issues which are causing the problem.

Remember:

  • Rich conditions can be caused by too much fuel and/or too little air.
  • Lean conditions can be caused by too little fuel and/or too much air.
  • So, it is imperative to verify both fuel and air circuits (including emulsion tubes).
  • On early ‘Wings, each cylinder is individually carbureted. Thus, you can have 1 cylinder that is over-rich for one reason and another cylinder that is over-lean for another reason!

One example: a very simple and common problem is dislocated or pinched intake runner-to-cylinder head o-rings. This causes a big vacuum leak that permits extra “false air” to enter that cylinder resulting in a very lean condition.

Here’s a list of the typical reasons for lean and rich situations.

Keep in mind that ignition problems can mimic certain carb problems. Always verify ignition function thoroughly before delving into carburetion!

Comments

  1. charles L russell says:

    thanks for your excellent info. I am restoring a ’75 gl1000 and when I get far enough to need it that will be very helpful. I have the’75 and also ’76 & a ’77 all with good engines.

  2. Great work buddy this is the easiest to follow explanation ive found on the net,and ive been searching.Alot of people are knowledgeable about their particular field but few can translate that to us novices successfully,cheers mate.

  3. Seth Deaver says:

    I have a 2002 zx9r. Idle specifications are 1050-1150 but if I set it to that range the idle surges abbot and the bike almost stalls out, however if I set it to 1500 rpm it idles fine but runs choppy at steady throttle below 3k rpm. Bike runs perfectly fine as long as I drive it above 3k rpm. Is this a rich condition?

    • Not much expertise with that model. “Surges” have many explanation including ignition anomalies and “lean” condition from vacuum leak, etc. I would investigate for clogged idle circuits as a likely culprit.

  4. Great article, I love the symptoms charts. I am trying to revive a bone stock 1984 Kawasaki GPz 550. The jets are correct per the shop manual. It’s running waaaaay to rich; even with the air filter removed. It fouls the plugs in about 4 -5 minutes of running. All four plugs look essentially identical (sooty black), so it seems to be a problem with the whole system rather than just one or two of the (4) carbs. It sat for 10-11 years. The carbs have have been thoroughly cleaned. I think they are TK’s

    • Norton Muzzone says:

      We suggest to move forward with your troubleshooting. Excellent job on the carbs of course. You did ensure your fuel enrichner circuit (choke plunger) is to specifications right? If so, you may want to check top end compression on all four cylinders. You may be experiencing piston ring blow by which would cause excessive crankcase pressure. There may be a top end job in your future. Another cause of fouling is worn and tired valve guide seals. Let us know if we can be of further assistance.
      Norton
      Randakk Customer Service Team

      • RCST – thanks for the quick reply. The compression readings for all four cylinders were within 10lbs of each other right around 150. The bike only has 78xx miles – still pretty fresh. I suspected the enrichment circuit as well, so I pulled the carb assembly from the bike. Everything mechanically appears to be functioning correctly, but I don’t know how to check or make adjustments the circuit. On the TK carbs, though, each carb has an actuator, but it appears that there is one primary enrichment circuit control on carb 3. Discovered that using BikeBandit Parts diagrams.
        Thank you for the assistance!

    • Some possible causes:
      1. Clogged idle jets (can you see daylight when removed?)
      2. Clogged idle circuit passage (can you see/ feel compressed air being blown through?)
      3. Someone drilled out stock idle jets (had it happen to me on what I thought was a bone stock Honda CB500)
      4. Defective electronic ignition
      5. Clogged exhaust system

      • John,
        1. The caburetors were very gummed up, but were thoroughly cleaned.
        2. Idle circuit? Are you referring to the air suction system? I have not tested that system.
        3. I purchased the bike from a close friend. He says he did not have any carb work done.
        4. I did replace the stator after testing indicated that it was out of spec. I have thought about the coils, but testing indicates they are good.
        5. The exhaust will be easy enough to check.

        I am using a Clymer manual as my guide. Thanks for the extra check points. I am looking forward to getting my GPZ running.

  5. Anthony James says:

    Great article. 800 marauder here. Needs choke at every start up cold warm or hot. Once warmed up about 2 or 3 minutes of idle, sounds great. Slow response on throttle. Backfires only during throttle use accel or deccel. No backfiring during idle. Also no backfiring if choke is opened about a quarter way. Any ideas?

    • Probably need a carb rebuild with special attention to vacuum leaks.

    • Allen Busch says:

      Dude idk your name but you hands down have the best explanation of all this and the way its laid out really made it easy to figure out a problem ive been battling and researching forever and i cant thank you enough bro whoever you are i want you to know this information made my bike very thankful too.I did a lot of research too man im embarrased to say how long so i wish every problem i researched clicked with me the the way this one did and i dont even reply to these things but this was a life changing article for someone out there and thought u should know it rt now at 1:07 am on a thu in june of 17 and my bike is tuned like jimi hendrixs guitar and there may even be a little purple haze in the backround .peace and thank you again!

  6. For this vintage motorcycle newbie (’82 Yamaha XJ650R Seca) this has been the most helpful write-up explaining tuning my carbs. Thanks for investing into my education.

  7. Interesting.

    I have an 03 Honda Shadow 750

    Usually needs choke to start in cold conditions, occassionally in warmer .

    Whats weird to me though is that it smells like unburnt fuel but it also backfires slightly in idle and regularly backfires on the decel.

    Suggestions?

  8. Excellent article. I have a polaris sportsman 700 atv that sat for 3 years. Carbs were rebuilt. Starts up easy but has a rough idle and very smokey at idle. Wide open it hauls ass. But once back to idle, smokes and runs rough. I’m thinking too rich? Readjust the float?

    • Not likely that it would need float adjustment unless it was out of whack before. But, float valves can “leak” and that can cause richness or flooding.

      Most likley – you have a problem with idle fuel jets or air bleeds that are affecting idle performance.

  9. Excellent article. I already knew that I was runing lean due to the grey spark plugs however your writeup also confirmed all the other simptoms. What I mean by that is ALL THE SYMPTOMS THAT YOU LISTED. Hoping for a remedy with a single new carb that is closely matched to engine demand. Going from kehin twin to mikuni single. It is a CB400T in a 350kg bugy that had a shatered piston and stood idle for 5 years before I coaxed back to life. Thanks for the education.

  10. A very helpful article. Found out that my 2 stroke carb’s mixture was rich and I lessen the fuel intake and follow the manual’s 1 3/4 counterclockwise turn from seated position. But i still have one problem… It is still hard to start the engine especially in the morning, and sometimes even if the engine is still warm I need to rev a little bit to make it start and sometimes idle is not stable too. If the engine is cold it sound like waving but when its warm it sound normally at 1500 rpm. My spark plug reads optimal with light brown rust on the porcelain and light black and brown on the base ring. Any diagnosis on this. Thank you

    • Try raising the curb idle 100 rpms. Also, always “crack the throttle” when starting …especially if you do not have a choke or enrichment device.

  11. 79 gl 1000 timing good . I believe that cylndar #3 is not firing pulled plugs all looked normal but # 3 plug looks like it just came out of box !!! Bike starts good drives ok bad gas mileage rough idle. Can I take just one carb off ? Float stuck ? Unscrew bowl drain no gas came out. Help.

    • Lee: I believe you’ve diagnosed the problem. The dry, clean plug and no gas in the bowl at that cylinder points to:

      1. Float is binding and “hung” in the “off” (up) position thereby preventing fuel from entering that carb bowl causing it be be fuel-starved.

      2. The screen beneath the float valve seat is blocked with debris.

      Most likely, the carbs will need to be removed and throughly cleaned and rebuilt.

      But, this is worth a try. Whack the problem carb with a hard rubber mallet. Sometimes – that will unstick a binding float and allow gas to flow. That’s usually a “fix,” but sometimes the flow now won’t self-regulate and that introduces a flooding problem.

      Good luck.

  12. Excellent post Randakk. Incredibly helpful to this shadetree knucklehead. Thanks for this.

  13. Thank you very much! This article was VERY helpful!
    I was looking for the symptoms of lean and rich condition. I know basics, but a lot of these were new to me.
    I am currently working on a fuel table for my Honda which has a highly modified exhaust, thus greatly changing the optimum ratio. Knowing these symptoms helps me make adjustments throughout the rev range as there are signs of both depending on throttle/revs.
    Thanks again!! Much appreciated!

  14. John Schrecker says:

    I race a Porsche 356 (racecar) 1620 cc four cylinder engine. I run 12.5/1 compression. My Solex carbs (a 2 barrel for each two cylinders) run 160 main jets and 160 air correction jets. I must be close to 14.7/1 fuel air mixture because the car runs good, the plugs look good, the exhaust looks good. I’m thinking about experimenting with 112 octane Exxon Oxygenated fuel and I don’t have access to a dyno. If the fuel has 10 percent oxygen I assume I would need 10 percent larger main jets like maybe 175 mains or I guess I could use smaller air correction jets. The fuel I use now is 110 octane race gas non oxygenated.
    How should I determine that correct 14.7/1 mixture.

    • John:

      The first question would be why use anything less than real race fuel in a race car? If the car runs fine now, it will certainly make less power on the oxygenated fuel …simple physics.

      The “flow” of jets is not linear!

      So your statement: “If the fuel has 10 percent oxygen I assume I would need 10 percent larger main jets ….” – is not true!

      And the flow of fuel is very different compared to the flow of air.

      If you persist on this path, there are these methods to dial-in the air /fuel mixture:

      1. Trial and error. I.E. observing the plugs carefully after a hard pull.
      2. Install a K&N air /fuel meter (or similar) on the dash and observe under load. You would want to do “before and after” testing with real gas …then the gasohol stuff.

      Good luck.

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