Folding Back the Air Ducts Can Eliminate the Hassle of Removing and Installing a 4-stroke Carburetor!

01-11.jpg How to

Many maintenance enthusiasts find the air duct connecting the air cleaner and carburetor to be a hindrance when attaching and detaching the carburetor for overhaul or setting. Forcing the carburetor out of the carburetor while deforming it is a hassle, but by simply folding the air duct back beforehand, it is a wonder! You can easily remove the carburetor by folding back the air duct.

Pushing, pulling, and prying, attaching, and detaching the 4-stroke carburetor is a big deal.

02-11.jpg As you'd expect from a 1970s motorcycle, the KZ900LTD has a generous layout around the carburetor and air cleaner, but even with this, if you try to remove the carburetor without folding back the air ducts, you'll have a tough time getting the ducts to work.

The stock intake system equipped with an air cleaner, whether carbureted or injected, incorporates rubber parts on the inlet side (air cleaner side) and outlet side (cylinder head side). The names of these parts vary depending on the motorcycle manufacturer.

Honda
 Between carburetor and air cleaner box: connecting tube
 Between carburetor and cylinder head: Carburetor insulator

Yamaha
 Between carburetor and air cleaner box: Air cleaner joint
 Between carburetor and cylinder head: Carburetor joint

Japanese sea bass
 Between carburetor and air cleaner box: Outlet tube
 Between carburetor and cylinder head: intake pipe

Kawasaki (city)
 Between carburetor and air cleaner box: air duct
 Between carburetor and cylinder head: Carburetor holder

It is called as above by each company. There are some differences depending on the age and model, and also between the carburetor era and the injection era, but even if the function is the same, there is a unique character among manufacturers.

In the carburetor era, the air duct (hereinafter referred to as the duct) and the carburetor holder (hereinafter referred to as the holder) were not just joints. If you have ever tried to install or remove a carburetor, you will know that the air duct and the carburetor holder are not only joints, but they also play an important role in absorbing engine vibration while positioning the carburetor.

This means that it takes considerable effort to attach and remove the carburetor, which is sandwiched between the duct and holder. On a supersport model with a twin-spar frame and a forward-leaning engine, the downdraft carburetor can be accessed by removing the fuel tank and pulling the air cleaner box upwards. However, with a double-cradle frame and an engine with upright cylinders, the carburetor must be pulled out to either side with the duct and holder attached. Since the duct is more flexible than the holder and duct, you have to pull the carburetor backward to remove it from the holder and then drag it out while crushing the duct.

This is quite a hard job, and it is rare that it comes off straightforwardly even if you remove the clamp fittings, and it is a common pattern to pull it out somehow or other while shaking up and down or prying left and right. If the duct is hardened by aging, it is even more difficult. In the meantime, the throttle cable interferes with the frame and there is no way out, and some people may have lost their way.

POINT
  • Point 1: The carburetor is sandwiched between the carburetor holder and the air duct for positioning and vibration absorption.
  • Point 2: Ducts are not easily dislodged due to fall prevention, and they are not easily dislodged during maintenance because they are in the way

Just remove the clamp and fold back the air duct to make a gap.

One thing you can try in such cases is to fold back the air duct. If the clamp fitting is in the way, remove it beforehand and turn the part that is inserted into the carburetor over to the air cleaner box side. The duct, which is hardened like plastic, can be temporarily softened by heating it with a hairdryer or heat gun, so turn it over when it is warm.

Depending on the distance from the air cleaner box to the carburetor, some models may not have enough duct length to fold back, but most horizontal carburetors have enough duct length to fold back.

03-11.jpg If the air duct has hardened over time, you can soften it by heating it with a hairdryer or heat gun. If you heat a part of it rapidly, it may be deformed, so heat it gradually and evenly while checking with your fingers.

04-11.jpg Even though the duct was hardened to the extent that it could not be bent at normal temperature, it was softened by the power of heat and could be bent back. This part absorbs the negative pressure change and vibration of the air cleaner box, so if it's hard like plastic, you might as well give up and replace it.

05-10.jpg Even though the four carburetors are tightly connected by a plate, it is not good to pry with excessive force when installing and removing them. By folding back the air duct, the carburetor can be moved closer to the air cleaner box, and there is more room on the carburetor holder side, making it easier to set the carburetor on the engine.

The negative pressure inside the air cleaner box fluctuates greatly depending on the throttle operation, and when the throttle is opened suddenly, the air inside the box is sucked in at once, and the box itself is pulled toward the carburetor side. Depending on the model, the box itself may be bolted to the frame, in which case the duct will contract to absorb the pressure change. In any case, since the duct is the part that receives the negative pressure change, it is often secured with a certain length.

By folding back the part that is covered by the carburetor, you'll be lucky if there is even a small gap between the carburetor and the duct. At the very least, the removal of the part stuck in the carburetor will make it easier to pull out of the holder in the cylinder head and give you much more freedom in the left and right directions. It's not uncommon for the holder's clamp hardware to be deformed if you pull left or right with the duct stuck in the carburetor, but if you fold the duct back and pull the carburetor through the gap created, it will come off the duct and the clamp won't twist.

To fold back the ducts, you have to put your fingertips in the narrow gap between the carburetor and the air cleaner box, and it may not be easy. If this is the case, folding back the ducts at both ends (No. 1 and No. 4) will reduce the resistance to pulling the carburetor out by half. In the case of the KZ, the stock ducts had hardened so we replaced them with new ones, along with the holders, and the carburetor went on and off very smoothly.

As mentioned earlier, the holder and duct are also important parts for positioning the carburetor itself. When the air cleaner box is removed and the power filter is installed, the carburetor is held in place only by the holder on the cylinder head side, so the load is concentrated on the clamps and the carburetor itself may come off easily. In addition, if the flexibility of the holder and duct is reduced, engine vibration will be easily transmitted to the carburetor, which may affect the stability of the oil level. Folding back the ducting makes it easier to put on and take off the carburetor, which is a big advantage in terms of maintenance, but holding the carburetor in place with soft rubber is also important for stable performance. So it may be better overall to replace the ducting that has hardened like plastic with a new part that is flexible enough to fold back.

POINT
  • Point 1: Just fold back the air duct to create a gap between it and the carburetor, making it easier to remove.

Utilize a long shaft screwdriver to place the folded duct over the carburetor.

06-11.jpg By replacing the stock Mikuni VM with a Keihin CVK that is shorter front to rear, there is more room on the air duct side and a clear gap by folding it back. This eliminates the need to struggle when installing and removing the carburetor for setup.

07-11.jpg The air ducts are new and flexible, so it's easy to put the folded over part over the carburetor.

If the duct is folded back, the carburetor can be installed smoothly without applying any extra force when restoring the carburetor after cleaning and setting. If you push the carburetor in without folding back the duct opening, the side that plugs into the air cleaner box may shift or come off, causing you to have to remove the carburetor from the holder twice.

After attaching the carburetor to the holder, place the folded duct over the carburetor while stretching it. If the duct is flexible, you can easily put it back into the carburetor with your fingertips and tighten the clamp to finish the installation process. In the case of the KZ1000J, the duct clamp is not a band but a spring, and in order to fit it in place, it is convenient to have a metal rod with a long shaft that has been modified like a pick tool with the tip bent to 90 degrees. A metal rod with a long shaft, bent 90° at the end and modified to look like a pick tool, is useful to hold it in place. The bent end of the rod will make it easier to get the folded duct over the cab.

Folding back the ducts is just a small step, but it will dramatically ease the tedious process of attaching and removing the carburetor, so why not try folding back the ducts at your next carburetor maintenance?

POINT
  • Point 1 - If you need a tool to put back the folded air duct, use a long shaft screwdriver to cover the carburetor little by little.
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