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techtalk:ih:carb02a [2018/04/18 01:41]
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-====== IH: Carburetor, Intake Manifold & Exhaust Systems - Sub-02A====== 
-===== Finding and Fixing the Cause of the Degradation That Leads to Intake Leaks ===== 
-Article by Dr Dick of the XLFORUM ((http://​xlforum.net/​forums/​showthread.php?​p=3203642)) 
-  * Leaks will develop over time. While diagnosing, you're doing the seemingly most logical thing by focusing on the seals & clamps. But, you may not be seeing the obvious. If your tire kept leaking after you fixed the inner tube, time after time, you would start thinking that a bigger patch would work better than the last one you used as well as start looking for nails in the drive way. Same thing here. 
-  * The Big Picture: ​ 
-  * Finding and fixing the cause of the degradation that leads to the leaks: 
-    * The main factor is the pulses in the manifold that push & pull on the seals. If you put your hand over the throat of the carb with the bike running, you will feel how strong these forces are. When you're cruising, the throttle plate is barely open in the carb. So forces on this plate are just like what you feel with your hand over the throat. So, it's the seals that flex with each piston stroke. As they flex, fuel seeps into the interface between the seals and the head / manifold and acts like oil in there allowing more movement still until the rubber itself loses it integrity and fails. 
-    * The key here is to reduce the micro movement that the seals need to deal with. Consider what the factory did compared to what you have done. If the seals you're using 1st came on a 1979 or later, the air cleaner is big & beefy and it used a bracket system that held the manifold / carb / air cleaner solid. This removed the seals from having to restrain most of the forces in the manifold. Now the seals just had to seal and not support as much. Then the clamps they used were split in the middle, each acted like 2 skinny clamps (not one wide beefy one). So, when clamped, there was INDEPENDANT sealing pressure on the head spigot & on the manifold spigot, allowing the seal to flex somewhat in between while not moving in the clamped areas. Now, in addition to this when the motor gets warm, it grows taller. The manifold seals need to be able to deal with this. A taller motor means that the distance between head spigots grows. 
-    * So, in summation what the factory did that you probably don't have is mount / carb / manifold solid when the clamps are loose. Then the clamps just seal in 4 independent areas, so that area between clamp halves is free to flex without forcing any movement between rubber & metal. 
-    * Here's the bottom line. If, when all is assembled, you cant wiggle the carb / manifold / a/c in relation to the motor (solid mounted) then your problems are 80% over. If you go to split clamps also, then your as good as you can get with rubber band seals. 
-    * The O-ring set up is subject to the same principals. It's more robust in mechanical design than the band. So, it's even more reliable. But, it's more misunderstood in the way it goes together so it gets a bad rap. If you've got factory O-ring heads, then not using an O-ring is just shorting yourself. 
-  * Now that you have the problem areas defined, if you can get your mind wrapped around them, going to the O-ring will be more attractive. 
-    * The advantage of the O-ring is that the clamp can be used to help make a solid connection between manifold & head (no rubber in between). This totally removes the rubber / metal interface of the clamp-rubberband-spigot (the core of the degradation you are trying to rid yourself of). The reason most O-ring setups fail, is the commonly held idea that the closer the manifold and head get to 'no gap fit', the better the O-ring will seal. Totally wrong and the cause of 99% of O-ring sealing issues. When the manifold is set tight against the head, there is a groove created that the O-ring lives in. Common belief is the O-ring seals in this groove. It doesn'​t. In fact, this makes the groove smaller in volume than the O-ring volume. So, when the clamp compresses the O-ring on its way to getting tight to the spigots, the O-ring fills the groove and forces the manifold away from the head in order get enough volume to fit into. Now nothing aligns right and it leaks right from the start. 
-    * The correct action is leaving a gap of .030' -.060" (1/​32"​-1/​16"​) between the head and manifold at the skinny lip. This gap needs to be even all around on both heads at the same time. It's in this little gap that the O-ring seals when the clamp squeezes it. When setting heads & jugs for alignment, putting manifold & O-rings (but no clamps) in place, the inside of the manifold should match the inside of the intake port all by itself with no forcing. Don't worry if the gap seems to big. It will shrink when the heads & jugs get tightened. The last test is, when the clamps get tightened, the alignment is still on before you tighten the clamps. You will note that the O-rings are proud of the spigot diameters. When the clamps squeeze the  O-rings to the spigot diameter, the seal to gap occurs. During tightening, you may have to tap the manifold back into alignment until the clamps grab onto the spigots.