Aisan Carburetor Rebuild
A year or so ago, I bid on a couple of parts carbs and rebuild kit on the e-bay. Upon arrival, one of the carbs looked to be pretty much complete and the other was a complete rusted mess. The rebuild kit box was torn up pretty good, but the parts were still in sealed plastic bags. The whole works went into the “someday pile”.
In my continuing quest to learn, I recently rebuilt the carburetor. The starting point is a 1973 Aisan carburetor with a build date code of 2I4 (Sep. 4, 1972) on the air horn. The carb had a tag on it indicating it had previously (at least once) been rebuilt.
If you have the time to research, you will find there is very little about the rebuild process that has not been covered. I found the FSM to be a very useful guide on the teardown process. I also got some very good advice and tips from Martin (MDH33) who has done a few carb rebuilds. I took just under 250 photos during the process and took very detailed notes during the teardown. Even with the large number of photos, as I was doing this write up there are photos of steps/things that I missed. There are a large number of parts and pieces that need to be removed for cleaning. However do not fully disassemble every piece of the carb; butterfly valves and leave and linkage should be kept as intact as possible.
If the starting point is a working carb, chances are pretty good the rebuild process will be uneventful because all the parts are there. In my situation, I did encounter a couple of parts issues; missing a screw/bolt and the sight glass had a chip. Aside from that the carb I started with was pretty much intact.
Tools and Carb Kit
There are a few tools used in the rebuild that come in very handy and might not be in the common tool kit. Forceps and tweezers are very useful for the linkage pin clips and a clip at the bottom of the accelerator pump hole. A magnifying glass is needed for reading the size on the jets. Flat head screw drivers with heads wide and thick enough for the jets heads are a must have. It may take some minor grinder trimming on a couple of flat head screw drivers to get them perfect. A ¼” drive 9mm deep socket is needed to remove the power valve. A small measuring stick comes in handy for setting the level of the float. A Philips head screw driver, 17, 14, 12 and 10 mm wrenches and/or sockets should round out the tools needed. I came across this in the junk yard and could feel the cruiser karma..
UPDATE 12-3-2012: A small impact driver (Harbor Freight special) comes in handy for dealing with stubborn screws.
There are varying levels and types of carb kits. The most basic contain new gaskets and seals and the more robust include new components and jets. The kit I used was a Keyster and it came with all the gaskets and many new components; jets, power valve, needle valve, idle mixture screw, accelerator pump, and steel balls.
The objective of the teardown is to break down the carb into small enough pieces that they can be inspected and cleaned. When the teardown is completed there are a bunch of really small pieces (stuff in the egg carton picture above) and some miscellaneous larger pieces (stuff on the white towel picture above) and the three main carb pieces; Air Horn (upper left), Body (upper right) and Base (lower center) of this picture.
As mentioned above, do not remove and tear down the butterfly valves. The screws that hold the plates to the shaft will be destroyed during disassembly and they are not easy to find. Also, keep as much of the linkage attached as possible. This helps upon reassembly. If the carb needs work in these areas, it might be best to send it off to the professionals.
When progressing through the teardown, everything should be kept neatly organized and together. Taking lots of notes and pictures is appropriate. Some time will pass between teardown, cleaning and reassembly and it is very easy to forget what goes where.
Teardown – Separating the Air Horn from Body
The objective of this step is to remove the accelerator pump lever, remove idle solenoid, disconnect linkages, remove seven screws, remove the choke breaker, and separate the air horn from the body.
To remove the accelerator pump lever, a 12mm wrench (or socket) is used to loosen and remove the pump lever bolt. Next remove the retainer clip that holds the pump lever rod to the linkage at the base of the carb. Now remove the accelerator pump lever and rod from the carb assembly. It will just slide away to the side.
To remove the idle solenoid, remove the two screws that hold the idle solenoid in place. With the screws removed the assembly can be pulled off the carb. There is a gasket between the solenoid and the carb body that is sometimes missing. This gasket will be replaced with one from the rebuild kit.
Next remove the retainer clips and two linkages on the choke breaker.
Next remove the retainer clip and one linkage at the top of the choke arm.
With the linkages out of the way, remove the seven screws that hold the air horn to the body. In the left hand side of the first picture four are visible; two of these four hold down the choke breaker bracket and one is holding down a metal tag. In the second the three remaining screws are visible. Remove all of these screws out and set the choke breaker aside. An important note, the choke breaker should not be dipped. The choke breaker should be inspected by applying vacuum to it, if the arm moves in then it is ok.
Carefully pull the air horn up and off the carb body. There is a gasket between these two pieces that will sometimes tear. This gasket will be replaced with one from the rebuild kit. Also, there is a washer gasket that should stay on the base, but is often stuck to the gasket. This was the case with me and you can see the washer gasket at 10:00 from the pump.
My carb kit came with many replacement linkage retainer clips, fortunately I did not need to use any of them.
Teardown – Removing the tiny steel balls
There are two small steel balls in the carb; small ball goes in big hole, big ball goes in small hole – recite this often enough and it is easy to remember which ball goes where.
Let’s get the small ball, to do this remove the spring, clip and small ball from inside the accelerator pump hole. The spring should just lift up, the clip is best removed with forceps and the ball should just roll out. Be careful with the ball, it is tiny. On some carbs the ball is really stuck down in there because of old varnished fuel and/or oxidation. These are poor quality pictures, but in the first you can see the hole and the forceps have the clip. In the second you can see the ball sitting at the bottom – it is in there just squint your eyes.
Let’s get the big ball. This ball is hidden very well. To get it remove the gasket/washer, aluminum plug, and discharge weight first and then the ball. In the first picture, the pencil is pointing to the gasket/washer. Remember, sometimes this washer gasket gets stuck to the gasket between the air horn and carb body; that’s where mine was. I put it in its proper place for the photo. This washer gasket will be replaced with one in the rebuild kit.
The aluminum spacer should just fall out when the carb is turned upside down – but be careful because the discharge weight and ball will also just fall out. Here again these parts can really be stuck. When everything is out ok, here are the pieces and the order they go in.
The order and orientation of all these parts is important. Together with the accelerator pump they shoot a small jet of fuel into the primary barrel of the carb when you “pump the gas”. It is a really cool thing to see when bench testing the carb. My carb kit came with replacement steel balls.
Teardown – Removing Venturis
Inside the carb body are the primary and secondary venturis. These are held into the carb body with two screws each. The venturi need to be removed for cleaning. Before removing them, identify the primary and secondary. It is very important that these are not mixed up during reassembly. I got lucky and the venturi in my carb are exactly the same, still I kept these separate and made sure they were put back in the same place. In the photo below the primary venturi is being removed. The primary venturi is the one right next to the “big ball, little hole”.
Each venturi also has a gasket that will be replaced with one from the rebuild kit. Here are both venturi and you can see the gasket.
Teardown – Removing the Jets
There were 7 jets in this carb; 2 “slow” jets, 2 “regular” jets, 1 “power valve” jet and 1 “spare regular” jet and 1 “spare power valve” jet. All these jets should be removed and soaked in the carb dip. The jets are made out of soft metal and easily deformed. Make sure the best fitting flat head screw driver is used for each jet; failure to do so will likely result in destruction of the jet.
When removing the regular jets (primary and secondary), some grinding may be needed on the sides of the screw driver so that it fits through the plug hole.
The jets all look very similar, but each one is a different diameter. It is very important to keep track of each jet, its size and where it came from. Each jet should have the size stamped on it. A magnifying glass will be needed to read the size.
The “slow” jets are long and there is one for the primary and secondary. In the first photo, I am removing the “slow” jet for the secondary. In the second photo, I am removing the “slow” jet for the primary. Use the magnifying glass to read the size of each jet and write down the size and location.
There are “spare” jets that need to be removed. To get to them remove the plugs on the carb body. The plugs are just below the sight glass. Use a 12mm wrench/socket to remove each plug. In the first photo I am removing the primary plug. Each plug has a gasket and a spare jet in it. Separate the spare jet from the plug. Read the size of each jet and write down the size and location.
Next the primary jet and secondary jets are removed. I don’t have good photos of this, but basically run the screw driver through the plug hole to remove each jet. Each jet will have a gasket. As with all the jets so far, as soon as the jet is removed, read the size and write down the size and location. The plug holes are where the plugs were removed from.
Finally the power valve is removed. The 9mm deep socket is used to remove the power valve. The power valve jet is on the bottom of the power valve. I don’t have good photos of this step. The power valve is at the bottom of the fuel bowl in the v shaped slot on the back end of the first photo. The power valve (with the power valve jet still attached) is in the egg carton slot to the right of the sight glass pieces in the second photo.
My carb kit came with a replacement power valve in it. My carb kit also came with replacement jets, oddly though it only had one “slow” jet.
Teardown – Sight glass removal
Removing the sight glass is very straight forward. Just remove the two screws and push the pieces out from the inside with a finger. The sight glasses came in plastic and glass; there are specific gaskets for each and they are not interchangeable between the two. My carb had a glass and it had a chip.
Teardown – Separating the Body from the carb Base
The carb body is held on to the carb base with three screws. Two of the screws also have a head that will allow you to use a wrench/socket to remove.
The third screw is accessed from the bottom of the carb body. This screw has a hole going all the way through it. The first photo is what the screw looks like. In the second photo, you will find the screw in the hole that is between the primary and secondary bores.
Upon removing all three, the carb body can be separated from the carb base. There is a gasket between the two pieces. This gasket will be replaced with one from the rebuild kit.
The carb body will have a pivot arm for linkage on it. This can be left attached, but in my situation I removed it using a flat head screw driver. There screw holding it on to the carb body also has a washer
Teardown – Air Horn parts
The float, needle valve assembly, power valve piston assembly, accelerator pump, accelerator pump boot and fuel inlet assembly are removed from the Air Horn.
The float is held in place by a pin. Slide out to the side and remove the float. Inspect the float for damage or holes. If it is damaged, it will need to be replaced.
With the float removed the needle valve assembly pieces are visible.
The three needle valve pieces; needle, spring and pin in the needle valve can be pulled out. The float, float retaining pin and three needle valve pieces and their orientation are.
Remove the housing of the needle valve pieces. I did not have a flat head screw driver wide and enough, so I used a chisel. I did not take a picture. There is a gasket on the end of the housing. My carb kit came with all new parts for the needle valve assembly.
The power valve piston assembly is held in place by a retainer. A Phillips head screw driver is used to remove the screw holding the retainer to the Air Horn. The retainer, lock washer and screw can be removed from the air horn.
The two power valve piston pieces; piston and spring can be pulled out.
The accelerator pump is removed by pulling it out and the accelerator pump boot is removed using a flat edge.
The fuel inlet assembly is removed using a 14mm wrench/socket. Under the top nut is a gasket and screen.
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