Spark Plug Replacement – The Idling of Carbureted Engines Smells Like Gasoline

Spark Plug Replacement – The Idling of Carbureted Engines Smells Like Gasoline

I had been noticing a gasoline smell during idling for a while. I thought it might be a tuning issue, so I tried adjusting the screws before riding to see if it made a difference. If you don’t mind the smell, you can leave it as it is without any problem. In my case, I didn’t rush to address it. The reason was that I frequently tune the engine, so I planned to adjust it to a lean setting at low throttle and observe any changes.

One day, I didn’t notice the gasoline smell during idling. Upon reflection, I realized that I had changed the spark plugs.

I used NGK spark plugs. The part numbers are DR8E for the standard type and DR8EIX for iridium. I also adjusted the gap.

In this article, I’ll mainly talk about spark plug replacement. The premise for FCR tuning is that everything except the carburetor is in perfect condition. The motorcycles you ride should basically be in good condition. The problem is that they ‘should be’ in good condition. It’s often hard to tell if something is wrong. If it’s broken, you’ll know immediately. But if the symptoms appear intermittently, it can be hard to diagnose. Sometimes, something might be broken without you realizing it.

The motorcycles that typically undergo FCR tuning are often carbureted bikes from the 1990s or older models from the 1970s. Even older vintage bikes are often enjoyed without installing FCRs, so they are not the focus here.

Symptoms with low reproducibility often relate to voltage or temperature. Hard-to-detect issues can include secondary air intake from deteriorated insulators, oil rising, or oil leaks. Basically, all of these are difficult to notice. Tuning assumes that the motorcycle is in good condition. Preconceived notions can make it even harder to notice issues.

In this article, I’ll discuss how changing the spark plugs might have reduced the gasoline smell during idling. Spark plugs are a regular replacement part, but they aren’t something you change frequently. If you notice an issue and it's close to the time for replacement, you can go ahead and change them to see if it helps.

This also includes some DIY tips for beginners. If you’ve never replaced a spark plug before, I hope this serves as a guide. So, let’s get started!

Can Replacing Spark Plugs Reduce the Gasoline Smell During Idling?

Deterioration of spark plugs is a potential inspection point if you notice a gasoline smell during idling. It could also be related to tuning issues. After replacing the spark plugs, I believe the gasoline smell subsided. A gasoline smell during idling indicates that the fuel isn't being fully combusted, meaning unburned gasoline is being expelled through the exhaust. This suggests possible ignition problems or a tuning issue where the mixture is too rich. It's also possible that the spark plugs were loose. Multiple factors could be at play. If replacing the spark plugs doesn't change the symptoms, you can likely rule them out as the cause of your motorcycle's poor performance. In my case, the gasoline smell seemed to go away after replacing the spark plugs. If you notice a gasoline smell, it might be worth considering the spark plugs as a possible culprit and inspecting them.

It might be a good idea to check the spark plugs occasionally and replace them promptly if anything seems off.

I'm writing a blog post titled 'Spark Plug Replacement.' If you notice anything unusual, it's a good idea to replace the spark plugs immediately or clean them by removing carbon deposits. When running in a rich condition, black soot-like deposits can accumulate. If your spark plugs are completely black, use a cloth to wipe off the soot. You can also try heating them or using a wire brush, but do so at your own risk. It doesn’t have to be perfectly clean; a rough cleaning is sufficient. However, sometimes it’s better to replace the spark plugs with new ones rather than just cleaning them.

If the spark plugs are black, it means the tuning is too rich. However, I advise against relying solely on the color of the spark plugs to determine tuning. Checking spark plugs for tuning accuracy is not a highly reliable method. The color of the spark plugs gives an overall average, so it should only be used as a reference.

FCR tuning is done meticulously at each throttle opening. You shouldn’t rely on the spark plugs to determine whether the tuning is rich or lean at any specific throttle position.

The photo above shows the spark plug removed from the 6th cylinder of my CBX1000. The threads are a bit black, possibly due to the use of thread compound. A new spark plug has a completely white insulator around the electrode. With use, a standard plug tends to take on color more easily, while iridium plugs tend to become sooty and blackened. In some cases, they might even become extremely white, almost like ash. Sometimes, except for the slightly exposed electrode, the entire plug can turn completely black.

You shouldn’t rely on the color of the spark plugs for tuning. If you remove all the spark plugs and want to determine if there’s an issue with any specific cylinder, you’ll need to check the color balance of the plugs. In my case, based on my tuning tendencies, the color of the spark plugs suggests that the tuning is on the richer side.

Precautions When Replacing Spark Plugs

Here are some absolute precautions to take when replacing spark plugs:

1. Avoid Dropping Objects: Never drop washers or other objects into the spark plug hole. If you do, starting the engine could cause the object to get lodged in the piston, potentially damaging the engine.

2. Don’t Force the Spark Plug: Do not force the spark plug into the cylinder head. The cylinder head is made of aluminum and has threads specifically for the spark plug. Inserting the spark plug at an angle or forcing it can lead to serious damage.

3. Avoid Over-Tightening: Do not over-tighten the spark plug. Also, if you are working with a 2-valve engine, make sure to align the plug’s tabs properly.

4. Be Cautious of Dropped Items: Always be careful to check for any dropped bolts or nuts.

5. Take Your Time: Replace spark plugs calmly and take your time.

6. Avoid Removing Plugs in the Field: In the past, I would check the spark plug condition while out on the road, but now I avoid removing plugs in such situations. It’s very disappointing to end up with an immobilized bike just to check the spark plug color.

Please ensure to follow these precautions to avoid potential issues and keep your engine in good condition.

FCR Beginners’ Guide – Tuning Based on Spark Plug Color

As mentioned earlier, tuning based on spark plug color is not advisable. While the "fox color" often referred to is a common concept, it’s less noticeable with iridium plugs. Using spark plug color to tune is more of a myth or a concept seen in comics rather than a precise method. It only provides a general indication of the air-fuel ratio trend, but it is not reliable for precise adjustments.

In my experience with the CBX1000, I generally do not check the spark plug color when tuning.

Spark Plug Replacement – The Idling of Carbureted Engines Smells Like Gasoline【summary】

If you feel that the idling is slightly off, replacing the spark plugs and then re-tuning is an option to consider. Tuning involves various interconnected factors, and the issue with idling might not necessarily be the spark plugs. However, replacing the spark plugs can eliminate one potential concern.

Some might believe that replacing the spark plugs is not worth it, but if it’s only slightly early for a change, it’s still worth replacing them.

See you again!

Photo including spark plugs

I will be including photos related to spark plugs and the area around the cylinder head from past posts. Since I don’t focus on the color of the spark plugs, I haven’t taken many photos of them.