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

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.

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

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.

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.

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..

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!

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

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.

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. )