Vent lines everywhere...


2 replies to this topic
  • rbreak

Posted May 10, 2006 - 07:11 AM

#1

Hello, I am working on a carb (FCR39) off of a 2004 WR450F. It has several vent lines. I'm going to block the ACV and was wondering if that would let me plug off some of the vent openings at the carb and clean the area up some. There are two vents directly above the ACV and two directly opposite on the other side of the carb, (I'm guessing vents for the hot start or choke maybe)? I know the fuel bowl needs to vent, but what are all the others for? I'm going to post this in the jetting forum also for better coverage. Thanks for any information.

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  • grayracer513

Posted May 10, 2006 - 07:53 AM

#2

The tube running from the float bowl connects to an overflow standpipe in the bowl which limits the maximum fuel level in the carb so that fuel will run out onto the ground before it runs into the engine.

The other tubes are atmospheric vents for the float bowl. They are redundant to some extent, but they wouldn't be there if they were totally unnecessary, either. Blocking any of them off might have an effect you didn't want.

It's important to understand that under normal circumstances, very little air actually moves through any of these in either direction. The void left by fuel leaving the float bowl through the jets is mostly made up for by new fuel coming in from the tank. The vents simply even out any pressure variations caused by temporary interruptions in flow, such as bouncing around, or forgetting to turn on the gas. They are also required so that fuel entering a dry bowl can displace air without having to push fuel through a jet to do it.

You could, I suppose, T them all into a single, larger hose. Some route the top two to the air box to reduce the possibility of water being drawn up and into them.

All of the circuits through which any significant amount of air actually does flow, like the hot start and choke, are fed filtered air from inside the carb, not through vents leading out into the dirt.

  • rbreak

Posted May 10, 2006 - 08:03 AM

#3

Thanks for the response. It seems like routing a vent line to the airbox would possibly put a small vacuum signal on the line, from the carb sucking air out of the airbox. It might not be enough to matter though. I am using this carb on an 4x4 ATV that sees deep water/mud on virtually every ride, so I have to route all vent lines down the frame and up to a high location (under the front headlight pod). I was hoping that maybe I could connect them as you mentioned into one line that would be much cleaner to run up to the headlight pod.




 
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