How To Check Your Spark Plug for the Correct Carburetor Setting

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.

Main Jet Sets are available for less than $10 and allow for adjusting the petrol to air ratio of the carb!

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.

Racing Air Filters are often the reason for too lean carb settings. Make sure to increase the main jet size when installing one of these!

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:

  1. Ride the scooter warm.
  2. Remove the spark plug and use a wire brush to completely clean the spark plug. You can also wipe spillage around the cylinder head.
  3. Reinstall the clean spark plug and ride the scooter for about 100-200m under complete load (full throttle), ideally up a hill.
  4. 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!

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