Running a tuned scooter requires attention to details. Especially the carburetor needs to be adjusted when increasing the performance by installing a new cylinder kit, or exhaust pipe. The spark plug is a great indicator, to see if the carburetor settings are correct. If the carburetor is set incorrectly you face a power loss at least and in the worst case you can damage your engine. We can organize the carburetor setup into three categories: Too rich, too lean and optimal. Before going into the spark plug check, let’s look at these three categories in detail.
Too Rich Carburetor Setting
If your carburetor setting is too rich, this means that the ratio of petrol to air is too high. Whilst you can not damage your engine with a too rich setting, you face a loss of power and increased petrol consumption. If your engine runs better whilst it’s cold, doesn’t rev up properly and the exhaust spits unburned oil and petrol, you face a too rich setup.
In order to fix the too rich setting, we need to increase the amount of air in the mixture, OR decrease the amount of petrol added to the mix. Increasing the amount of air is usually an easy thing as we can simply apply a racing air filter, or modifiy the stock air box to allow for a better air flow. Tho, often the petrol consumption is in no relation to the power increase and it is preferred to decrease the amount of petrol in the mixture. This is done by decreasing the size of the main jet.
Too Lean Carburetor Setting
The opposite of the too rich setting is the too lean setting. This means there is too much air in the mixture which causes the explosion within the cylinder getting too hot. Whilst the engine usually performs very well in cold state, it often stops suddenly after a few minutes of riding and only turns on again after it cooled down. The worst thing to happen is a seizure of the piston due to the high heat which usually renders the cylinder kit unusable. The too lean setting is especially dangerous, since a carburetor is automatically too lean when engine modifications are applied, but the carburetor is not adjusted accordingly.
Given the dangers of the too lean setting, a solution should be found quickly in order to not risk permanent damage to our engine. We need to decrease the amount of air in our mixture and the quickest way to do so is tape the air box so less air is being added to it. This is a quick way to solve the problem but ultimately we should consider increasing the main jet size to match the amount of air provided by the air intake system.
Optimal Carburetor Setting
The optimal carburetor setting is found when the engine performs well in cold state, when the choke is applied and revs up evenly in warm state. There should be no spillage of oil and petrol from the exhaust and the engine should basically perform as new.
How To Check Your Spark Plug
Now that we know what to look for, let’s use the spark plug as the main indicator for our carburetor setting. With the explanations above for its extremes, the spark plug is a good indicator for smaller settings issues which then become less obvious. In order to perform the check we should follow the following steps:
- Ride the scooter warm.
- Remove the spark plug and use a wire brush to completely clean the spark plug. You can also wipe spillage around the cylinder head.
- Reinstall the clean spark plug and ride the scooter for about 100-200m under complete load (full throttle), ideally up a hill.
- Immediately stop and remove the spark plug (be careful, it’s hot).
Now we compare the looks of the spark plug with our reference image.
If your carb setting is too rich you will notice oil spillage on the tip of the plug. Also, the spark plug might be slightly wet on it’s thread, which causes the spillage around the cylinder head. If your setting is too lean you will notice a whiteish texture on the tip of the plug. The plug will also be extremely dry. In the worst cases the tip of the plug starts melting away. In the optimal setting the spark plug tip will be slightly wet but clean. The tip should be grey to brownish and generally clean.
Given the explanations above about too lean and too rich setups it is also always a safer approach to go a bit richer with your setting. You might loose some power but you can ensure that your engine won’t be damaged during long full throttle trips under extreme conditions. Also remember that air tends to get denser at sea level. If you live on a mountain and take a trip down to the coast you might want to increase your main jet size or tape that air filter before you leave to ensure a pleasant trip.
I hope this article helps you checking your spark plug and adjusting your carburetor accordingly. There is a lot more to carburetor adjustment than what I’ve just explained here, tho it should give you a solid start, when tuning tuning your scooter and hopefully helps you to understand the topic a bit better. Let me know in the comments if you have any questions. Peace, Daniel!
you need to read this !
Thanks Jamie! Whilst I agree with PP that it’s not easy, I disagree that its not possible. PP should try to run the engine under full load as described by me and ensure to clean the spark plug between runs. This should help to get more precise results. Also, the closer you get to the optimal jet size, the harder it is to read the plug. Tho, it also becomes more safe to run a step above or below, which then allows for longer rides and thus more accurate spark plug reading results!
Hope this gets my point over a bit better!
Hi daniel, i had a Shitty spark plug on my polini Sport, equivalent with the NGK B9ES, the color of the spark plug was chocolate brown.
Now, ive got a NGK BR9EIX iridium spark plug, and looks like it’s running too lean… WTF?
Btw, the iridium spark plug will ensure better performance than standard spark plugs? More hp , more torque? Or just no difference?
The manufacturers always try to sell their products at a higher price :) Whilst it is probably true that you see a performance difference in an F1 engine, it is unlikely that there is any measurable or neven noticable difference on a scooter engine :)
For how long did you ride the new plug? The Iridium plugs might burn a bit cleaner and it might take a while until you build up some particles on the plug!
I ride the new spark for 5kms and now its chocolate brown, how good it is?
5kms should usually be pretty good to get an idea. I’d suggest keep an eye on it. If the engine performs well you should be save.
I was wondering if i change my air filter with K&N Cone with its Cotton based filter, will it make my engine too lean?
Hello Daniel , I have a Piaggio typhoon air cooled ,183 highly modified with pm Tuning expansion chamber 55mm crank , 34mm carb rd 350 reed valve , horse power ignition kit ,cylinder has also been heavily modified, ….. Setting the engine up !! I used the guide from your previous post about carb main jet settings , 34×5=170 … Still feels flat needle setting on the top clip , idle is also rich 50 pilot … My 1st thought is the main jet being rich is causing low end issues do you have a guide to doing idle jet aswell , I’ve turned the mixture screw all the way in and engine is still on does this mean main jet is too big or am I wrong
Also I got a guide with the carb when I bought it and the settings they have give are
42 slow jet
142 main jet
Jet needle setting 3
The carb is a pwk34mm flatslide power jet but I’ve blocked it off
I found this article looking through search of spark plug conditions. I don’t have a scooter I have a 8 cylinder truck and you are the only one I could understand in determining my plugs. Mine, by your description, were running lean. Anyway thanks for the article.
Glad it helped :)
you got your spark plug reading all wrong. please review. lean is optimal. vice versa.
Yeah dude your picture is wrong…. your lean plug is supposed to be optimal.. optimal is brown not white.. you are going to be responsible for blowing up peoples top ends you dick
Absolutely correct !!
Pretty sure your plug labels are incorrect; the rich one is right but the lean and optimal should be swapped round… a lean mixture increases the heat of combustion and tends to glaze the ceramic of the spark plug and burn off any deposits that may have accumulated, leading to the very white, clean insulator on the right of the picture. The left plug looks to be optimal with a grey brown colour from the light deposits of a correctly adjusted mixture.
yup pictures are the wrong way around,anything looking white is lean or hot,hopefully not too many people go by the pics
It’s actually correct. The spark plug on the left shows a burned ceramic insulator, which tells you the ignition is too hot. The optimal one shows a clean insulator, but not burned. I agree, the picture is not optimal, but it’s not wrong.
How about this photo. The source is telling to read spark plugs based on the burned dimensions on the ground electrode which should be 1.5mm or less and not the color of the ceramic insulator because as you can see, the ceramic insulator color and optimal spark in this case is burned?
Photo link: http://prntscr.com/douhjc
Hey Timothy Seward
Anyone can make a mistake so don’t be so objectionable by call the author a DICK.
He shares his knowledge to us free of charge. Ok sometimes he gets it wrong but don’t be such a “plonker” about it. Suggest you shove your head back up your arse where it belongs.
Hi just a quick one? My scooter is running to leaning running it on a 24mm carb 108 main jet it’s fine on tickover but when I take it for a run it looses power top end I’ve put a 150cc big bore on it any suggestions where I’m going wrong?
Also there is a spec,setting,say from .06 mm.-.07 mm.or 24-28 in.I tried in so .either to rich or lean.What is best?
Providing you have the correct heat range plug, base of insulator deep inside is read to check main jet. Tip of electrodes checks idle mixture adjustment and pilot jet with air screw 1 3/4 to 2 turns out from closed. Make runs also at 1/2 throttle to check needle position.
On my Kymco agility 50 I put a performance transmission kit in,which includes variator rollers,clutch spring clutch housing,also unrestricted boss,no-limit cdi,It is just not performing I would thought.Spark pug it at 0.26in. new air filter and it is a 2017.I gonna try a 4 gm roller weight.If you have some suggestion,please.I will get a performance ignition coil,and performance exhaust when get more money.Thanks David.it is hard to find performance parts for the kymco agility 50,most are GY6
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Thanks for sharing such an informative article. The spark plug is the essential thing of a vehicle. You described clearly about the maintenance system of it.it will be helpful for us.keep posting like this. :)
I only clicked in here to see the comments about the incorrect spark plug picture
According to Holley and other sites your plug pictures are wrong. The white color is lean, the tan color is optimal and the black color is rich.
I have been building engines for cars, high performance, trucks, semi trucks, motorcycles ect. for over thirty five years. Your plug pics are wrong.