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Cracked boot, vacuum leak???

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Posted by: Winkel---------------------

On my 2005 KDX 220, the small rubber boot/diaphragm at the top of the carb where the cable goes through has a small crack in it. I know I need to replace it to keep dirt and water out, but would this also cause a vacuum leak. The bike runs great but the idle does seem to vary ever so slightly.
Thanks,
Winkel


Posted by: reepicheep---------------------

I can't imagine that boot would seal, even under the best of circumstances. And its on the "other side" of the slide, so it's not actually in the air flow anywhere.

For that matter, now that I think about it, when you whack that throttle open, that displaces a lot of air as the slide goes up and down... that air has to come from somewhere, it must be going through the cable port.


Posted by: julien_d---------------------

Yeah, no. If you have an airleak it's coming from somewhere else. That boot is simply a dust guard.


Posted by: mj4trax---------------------

Actually. .. I've had issues with that exact rubber boot before.

If it's bad enough, it can suck enough air to screw around with the jetting some. .. which I've personally seen. If it's cracked, get it replaced.


Posted by: julien_d---------------------

Maybe on a bike you had that was possible. On the KDX that boot does not create any type of seal at all. It's not even tight fitting enough to think about creating a seal.


Posted by: reepicheep---------------------

Easy to test on a good bike I suppose... warm up the bike, ride a favorite loop, reach down and pull up the boot, and re-ride the loop.

So somebody in a warm climate, drop everything, go out riding, and report back. Take one for the team..


Posted by: julien_d---------------------

LOL.

Wont' be me unfortunately. Foot of snow on the ground, rain this coming weekend, blah blah blah. It's starting to look like it'll be spring before I get to ride again!


Posted by: mj4trax---------------------

Quote:
Originally Posted by julien_d
Maybe on a bike you had that was possible. On the KDX that boot does not create any type of seal at all. It's not even tight fitting enough to think about creating a seal.


Okay, I don't have experience with the KDX, so I'll take your word.


Posted by: julien_d---------------------

You have me second guessing myself, lol. I'll get out in the garage later and double check. I'm 99% sure the cable seals into the cap without the assistance of the dust boot, but of course I could be mistaken.


Posted by: reepicheep---------------------

I don't think it seals with or without the dust boot. The boot goes around the outside of the cable, but the cable is effectively a long straw. So it's not sealing anything.

Secondly, the slide rises up into the body when you whack open the throttle. that air has to go somewhere. Probably a combination of out of the top of the carb, and to the intake side of the carb.

Thirdly, the slide is constantly being "sucked" towards the motor, so any play in the slide will be on the intake side of the carb, which is the "ambient pressure" side (actually at a light vacuum, the same as the airbox would be). I suspect the sealing surface is the motor side of the slide, and any "leak" on the intake side of the slide isnt a leak, just another port in the airbox.

Leaks would be leaks that are downstream of the slide, after the fuel has mixed.

(disclaimer... I like thinking about physics, but am not an expert on carbs or dirt bikes, so don't confuse my theory with fact... and go run a test before believing anything I say. )


Posted by: SVandal---------------------

How much is the part? Less than 5.00 then you might as well replace it and stop worrying about dirt/water or messed up jetting. If your idle is unstable it may also be the pilot circuit getting clogged. The biggest leak worry is the engine to carb boot. Mine was cracked on my 94. 40.00 later and a new gasket and I was good to go.


Posted by: Porkchop---------------------

Any where unfiltered air can enter, so can dirt. Dirt, BAD for every thing in your motor.
It wont be much of a vacuum leak, but carbs can be expensive to replace & dirt wears out carbs, & every thing else. You can epoxy the crack if it's not real bad, if it's cracked where the cable conduit enters it , a short length of vinyl tube bonded over it with a good 2 part epoxy, can be a good repair. A spot of thick grease at the cable entrance to the carb is always good insurance. How pricey is a new boot compared to a new motor? Good Luck, Porkchop...
P.S. I like & highly recommend Loctite 2 part repair epoxy for many types of fixes. It adheres to almost any thing, so long as you can get it clean & rough it up a bit with sand paper or a file.


Posted by: Joburble---------------------

You could stretch a cut length of an old bicycle inner tube over the top of the carb and pucker it up around the throttle cable and zip/cable tie it. Free and functional, but ugly.


Posted by: In come tacks---------------------

I would think that any obvious air leak that is not drawn through the air filter is going to mess with the air side of the A/F mixture (even though it's on the pre-mixed side). Just because the fuel volume is not changed, doesn't mean varying levels of air mixture won't vary the rpms. The air filter is not discussed much, but the filter and air intake need to be clean and tight. If you do seal the throttle cap, let us know if that was the leak.



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