Post by Modelman on May 3, 2005 15:07:02 GMT
[glow=red,2,300]CARBURETOR AIR LEAKS
There are several carburetor designs, and each of them seems to have its own potential spots for air leaks, and then its own peculiar "fixes" for those leaky areas. We'll attack these by area, and try to mention particular brands as they apply.
The carb-crankcase connection:
OS and clones (ASP, Magnum, Royal, etc.), and many other brands use an "O" ring here. The problem is that the carb seats solidly at the bottom without putting much pressure on the O-ring, or the O ring is just too small or worn out. Use a new ring, one that is thicker, or double-ring it. You could also grind or file a bit off the bottom of the carb so that it will slide in lower and put more pressure on the O ring. Fox carbs are peculiar in that they are square, and are sealed entirely by silicone sealant. Use the high-temperature type.
The high-speed needle. An endemic problem, because most carb designs require that the threads be a little loose between the needle and its housing so that the needle can "float" a bit up near its point, to allow for manufacturing tolerances being slightly off. Fox, some K&B and OS FP are particularly susceptible - use some fuel tubing to make a "gasket" at the joint between the needle and its housing (see figure).
The more expensive OS and clone carbs use tiny O rings to provide a good seal and allow some movement. The problem here is usually with the clones - they don't use a good O ring, or the seats for the rings are too deep or too wide, and they leak! The OS O rings are available - if the O ring seats are not done correctly, though, you may be out of luck!
The low-speed needle. On Fox carbs the same applies as for the high speed needle. Most OS and clones use some type of packing inside the carb barrel to seal this needle - I don't know of a fix if its loose, other than perhaps some silicone sealant.
The fuel inlet nipple:
On Foxes, they just screw in; tighten or use some locktite, also. Many other brands such as OS and clones have a tiny gasket for the nipple - it can break or just be squeezed out, causing a bad leak. The same may apply for the nipple for muffler pressure. Buy or make a new one if this happens.
The idle stop screw:
Can easily be a problem, especially OS and clones that use a gasketed housing that screws into the carb body, and the screw then goes into this small housing. The little gasket can fail, or the screw - housing joint gets sloppy. Fox and some K&B carbs just use a little spring to keep the screw from backing out; with some vibration, these can really get sloppy. Sealant and maybe a locknut on the screw can cure this.
The barrel - housing joint: On the low-needle side, the joint between the carb body and the barrel is open. Supertigre and some other brands use a rubber cover to secure this joint. On other brands, if the fit between the barrel and the carb body gets too loose, it will leak, and theres not much you can do about it, except replace the carb!
TROUBLESHOOTING:
If you can't get fuel to the carb by choking the engine, suspect an air leak in the carb if your fuel line is in good shape. Smaller air leaks show up when your engine tends to lean out badly going uphill in the air.
The O ring between the carb and the body - remove the carb, and replace with a piece of paper between the O ring and the carb. The paper should be hard to remove, or even tear; if not, there just isn't enough pressure on it.
Other leaks - remove the carb, put some spare fuel tubing on the inlet nipple. Holding your thumb and fingers over the top and bottom air holes, insert the carb into water and blow, turning the carb over so various areas are on top (see figure). Wherever air comes OUT, air can get IN! This little exercise may amaze you - and with an older unit, blow your mind! In general, the larger the bubbles, the worse the leak. Be sure to oil the carb afterwards to prevent rust.[/glow]
There are several carburetor designs, and each of them seems to have its own potential spots for air leaks, and then its own peculiar "fixes" for those leaky areas. We'll attack these by area, and try to mention particular brands as they apply.
The carb-crankcase connection:
OS and clones (ASP, Magnum, Royal, etc.), and many other brands use an "O" ring here. The problem is that the carb seats solidly at the bottom without putting much pressure on the O-ring, or the O ring is just too small or worn out. Use a new ring, one that is thicker, or double-ring it. You could also grind or file a bit off the bottom of the carb so that it will slide in lower and put more pressure on the O ring. Fox carbs are peculiar in that they are square, and are sealed entirely by silicone sealant. Use the high-temperature type.
The high-speed needle. An endemic problem, because most carb designs require that the threads be a little loose between the needle and its housing so that the needle can "float" a bit up near its point, to allow for manufacturing tolerances being slightly off. Fox, some K&B and OS FP are particularly susceptible - use some fuel tubing to make a "gasket" at the joint between the needle and its housing (see figure).
The more expensive OS and clone carbs use tiny O rings to provide a good seal and allow some movement. The problem here is usually with the clones - they don't use a good O ring, or the seats for the rings are too deep or too wide, and they leak! The OS O rings are available - if the O ring seats are not done correctly, though, you may be out of luck!
The low-speed needle. On Fox carbs the same applies as for the high speed needle. Most OS and clones use some type of packing inside the carb barrel to seal this needle - I don't know of a fix if its loose, other than perhaps some silicone sealant.
The fuel inlet nipple:
On Foxes, they just screw in; tighten or use some locktite, also. Many other brands such as OS and clones have a tiny gasket for the nipple - it can break or just be squeezed out, causing a bad leak. The same may apply for the nipple for muffler pressure. Buy or make a new one if this happens.
The idle stop screw:
Can easily be a problem, especially OS and clones that use a gasketed housing that screws into the carb body, and the screw then goes into this small housing. The little gasket can fail, or the screw - housing joint gets sloppy. Fox and some K&B carbs just use a little spring to keep the screw from backing out; with some vibration, these can really get sloppy. Sealant and maybe a locknut on the screw can cure this.
The barrel - housing joint: On the low-needle side, the joint between the carb body and the barrel is open. Supertigre and some other brands use a rubber cover to secure this joint. On other brands, if the fit between the barrel and the carb body gets too loose, it will leak, and theres not much you can do about it, except replace the carb!
TROUBLESHOOTING:
If you can't get fuel to the carb by choking the engine, suspect an air leak in the carb if your fuel line is in good shape. Smaller air leaks show up when your engine tends to lean out badly going uphill in the air.
The O ring between the carb and the body - remove the carb, and replace with a piece of paper between the O ring and the carb. The paper should be hard to remove, or even tear; if not, there just isn't enough pressure on it.
Other leaks - remove the carb, put some spare fuel tubing on the inlet nipple. Holding your thumb and fingers over the top and bottom air holes, insert the carb into water and blow, turning the carb over so various areas are on top (see figure). Wherever air comes OUT, air can get IN! This little exercise may amaze you - and with an older unit, blow your mind! In general, the larger the bubbles, the worse the leak. Be sure to oil the carb afterwards to prevent rust.[/glow]
