FCR requires a filter for street use. Constant use of a velocity stack is not recommended.

FCR requires a filter for street use. Constant use of a velocity stack is not recommended.

This applies not only to FCR carburetors but to carburetors in general. Running filterless on the street is not advisable because the engine can suck in debris. Why should this be avoided? And what should you do instead? I’ll write about this from a column perspective.

Recently, I’ve received several inquiries, both via email and from acquaintances in my neighborhood, asking for advice on this matter.

First, understand that 'filterless' essentially means 'velocity stack.' This is when the engine draws air directly from the outside environment into the combustion chamber. I’ve posted about this topic several times before, so please use the site’s search function to find those posts.

Now, let’s get started!

Why is running filterless not advisable?

As mentioned at the beginning, the primary reason running filterless is not advisable is because it allows debris to be sucked into the engine. Since I am an FCR user, I tend to focus on FCR-related issues, so please consider applying this information to the carburetor you are using.

When running filterless, the engine draws in the air-fuel mixture without any resistance or obstructions all the way to the combustion chamber. This means that small debris, larger particles, and even insects can be sucked in along with the air. It’s frightening to think about larger stones or gravel hitting the cylinder head’s valves or the piston.

FCRs, in particular, have a weak point around the slide valve bearings. What happens is that the body of the FCR can wear down, rendering the carburetor unusable. Small debris acts like blasting beads, eroding the carburetor body.

To address these weaknesses in FCRs, you can use:

SEP Bearing Guides (small-scale sales in Japan)
Wide Rollers
Reversal Composition Kit (KEYSTAR)
My CBX1000 uses SEP Bearing Guides. OEM carburetors come with an air cleaner box that prevents debris from easily entering from the outside. This also serves as a measure against rain. These precautions are taken to avoid unnecessary troubles.

Running filterless increases power!

If the tuning is properly set, running filterless will definitely increase power. From the perspective that having no resistance is advantageous for producing power, it is certainly beneficial for the engine. However, this increase may only be a few horsepower or in a specific high-rev range.

I haven’t conducted a comparison myself, but I didn’t notice a significant difference in power between using a Foam Air Filter Sock and running filterless. However, the engine’s condition was somewhat better with the filterless setup. My CBX1000 is equipped with an air-fuel ratio gauge, and I didn’t notice any differences in the air-fuel ratio at low throttle openings.

Subjectively, the engine seemed to perform better without the filter, but the numerical changes were not very clear. Even if the Foam Air Filter Sock causes a slight decrease in power, I would still prefer to use an air filter because the difference felt minimal. A change equivalent to adjusting the clip position by one step would be quite concerning. If the power increase feels marginal, it is advisable to use the air filter.

Regarding the Use of Air Filters

If possible, I would like to use the air cleaner box. However, when tuning FCR carburetors, it significantly reduces maintainability. It is not recommended for motorcycles equipped with racing carburetors.

When comparing the Performance Air Filter and the Foam Air Filter Sock, the Foam Air Filter Sock offers much higher maintainability. While the Performance Air Filter also has a decent level of maintainability, it doesn't match the Foam Air Filter Sock.

The drawback of the Foam Air Filter Sock is its shorter lifespan. You can choose between the Performance Air Filter and the Foam Air Filter Sock based on your preference. The Performance Air Filter requires more maintenance, but there are no issues with its performance. Its lifespan is also longer than that of ram air filters.

Does the tea strainer type filter have any effect?

In the first place, a coarse metal mesh is designed primarily for backfire prevention, so it does not have dust-filtering effectiveness. It only prevents larger debris from entering. Filters that cannot be coated with filter oil are essentially ineffective.

If there are resistance elements around the entrance of the velocity stack, the air flow speed decreases. This is similar to the feeling of increased horsepower mentioned earlier, and it is hard to perceive the difference clearly.

I dislike the tea strainer type filter and believe it has almost no advantages.

You won’t notice a difference with just a slight increase in engine output

The other day, I watched a video on YouTube where they changed engine oil to see if there would be any change in horsepower. It was a channel run by a professional racer. They used a chassis dynamometer to measure the horsepower, and there was a difference of about 5 horsepower, going up and down. I have never heard of motorcycle oil causing a noticeable change in horsepower before. Even professional racers can only feel a difference of a few horsepower as just a sense of improved performance. They must have conducted the power check because they actually noticed a change.

Filter oil is essential.

Filter oil is essential. I use RAMAIR’s Foam Air Filter Sock. The use of filter oil has a significant impact on the lifespan of the filter. Without filter oil, you can start to notice degradation of the foam after about a year. This is due to issues such as fuel backfire. Filter oil serves two main purposes: fire prevention and dust protection. Additionally, it helps extend the product’s lifespan. I did not notice any performance difference between using filter oil and not using it. There was also no impact on the air-fuel ratio readings.

Disadvantages of Filter Oil

There is a drawback of oil dripping when storing the motorcycle. Additionally, when removing or storing the filter, you need to use vinyl gloves or plastic bags to manage the oil. Since you know from the start that the filter oil is applied, it might not be considered a significant drawback. The oil does not cause damage if it gets on the motorcycle. Sometimes, engine oil is used as a substitute.

FCR Beginners – Please Install an Air Filter.

Many riders who have just installed FCR carburetors are using them with red Velocity Stacks. If you are unfamiliar with tuning or disassembling racing carburetors, please install an air filter. My recommendation is the Foam Air Filter Sock. You may need to modify the Velocity Stack Cup, and aluminum Velocity Stack Cups are difficult to machine. They are also not cost-effective for modification. For conditions where an air filter is not required, such as circuit racing, you can go filterless. I use the Foam Air Filter Sock except when adjusting the air screw. Even when removed, it’s for a short time. Many beginners who install FCRs use new units. You will likely realize this when you start tuning the FCR yourself. You will feel sad when you discover damage caused by running without a filter.

FCR requires a filter for street use. Constant use of a velocity stack is not recommended.【summary】

This post is mostly written from my subjective perspective. However, it should not be incorrect from a street use standpoint. For circuit use, disassembling and maintaining the engine and carburetor is standard practice. If you want to shave off seconds from your lap times, running filterless is acceptable. What matters most is performance. For tuning FCR carburetors on motorcycles, I recommend the Foam Air Filter Sock. See you again!