Gas leaking from throttle cable side of carb after releasing throttle


8 replies to this topic
  • MTS II

Posted May 18, 2009 - 09:07 AM

#1

Just noticed this after washing my bike. When I open up the throttle and then let go, I see some fuel coming down the side of the carb. Is this normal if the bowl is full? Have never noticed before.

Thanks.

  • grayracer513

Posted May 18, 2009 - 09:32 AM

#2

You shouldn't be opening the throttle with the engine not running, simply because there is an accelerator pump in the carb that squirts a load of gas into the engine each time you do that, and if the engine is not alive to burn it, it can do no good to pump it in there. In fact, there are a couple of bad things that will happen because of it.

Having said that, it is also abnormal for any of the fuel discharged by the pump to leak externally in any way. You need to try to isolate the exact spot from which the fuel leaks so you can correct it the 3 possibilities that come to my mind quickly are:
  • The pump diaphragm, which would leak from around the actuating rod
  • The float bowl gasket. The AP squirt passes across that joint.
  • The carb mounting sleeve (isolator).


  • dankman

Posted May 18, 2009 - 11:54 AM

#3

You shouldn't be opening the throttle with the engine not running, simply because there is an accelerator pump in the carb that squirts a load of gas into the engine each time you do that, and if the engine is not alive to burn it, it can do no good to pump it in there. In fact, there are a couple of bad things that will happen because of it.

[/LIST]


just wondering, what can happen?

  • grayracer513

Posted May 18, 2009 - 12:26 PM

#4

It will flood the engine, foul the plug, and contaminate the oil with fuel.

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  • MTS II

Posted May 19, 2009 - 07:50 AM

#5

Thanks. Best way to check for the leak? From what I can tell it's behind the rubber cover where the throttle cables attach. I can pull the carb, but how I can locate the specific leak?

  • grayracer513

Posted May 19, 2009 - 09:13 AM

#6

Remove the cover and operate the throttle. If it's behind the cover, it's most likely a bad pump diaphragm, which is the more likely source of the 3 I listed in any case.

If necessary, you can remove the carb and still make it leak as it does in place. Just needs gas in the bowl.

  • MTS II

Posted May 19, 2009 - 08:40 PM

#7

Remove the cover and operate the throttle. If it's behind the cover, it's most likely a bad pump diaphragm, which is the more likely source of the 3 I listed in any case.

If necessary, you can remove the carb and still make it leak as it does in place. Just needs gas in the bowl.


Took the carb out. Turned out to be the diaphragm as you described. The rubber was all worn and peeling. Gas must have been going past it.

Ordering a new one tomorrow. Cleaned up the carb and installed new hoses while it was out, along with a zipty fuel screw.

Thanks for the advice!

  • MTS II

Posted May 20, 2009 - 07:35 AM

#8

Any specific diaphragm setup recommended? I tried to get my head around all the options in rivet size, but get confused when I hear that the '02 doesn't need as much adjustment. I do have bog as described. (2002 YZ426)

  • grayracer513

Posted May 20, 2009 - 08:03 AM

#9

If your diaphragm was leaking, you were getting less than the prescribed shot of fuel from the AP as it was, so I don't know that I would increase the size from standard.

The BK Mod comes highly recommended for the 426, and that's where I'd go with it, along with a standard diaphragm.

If you want, review the links in the Common Threads Sticky under fuel system for accelerator pump diaphragm info. The shorter the rivet, the longer the stroke, the more fuel dumped.





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