Hi Guys,
This is my 1st post. I just got a stock 06 klx250.
I've been reading about the free mods at planet klx, and I don't understand the purpose of rerouting the crank vent.Could someone explain?
thanks
Marc

 

Marc,
As the piston goes up and down, there is positive and negative pressure built in the crank case. If significant enough, this can reportedly cause resistance to piston movement and rob power. The KLX250 already vents to the airbox. Some bikes benefit from more venting. It seems to be general consensus that this isn't such a big deal on our twofiddys and the stock venting is adequate, no need to "improve" on it, although many did in the early days trying to get a little more performance out of her. I did it before researching enough and wish I hadn't.

 

I think this mod is overrated for the KLX. There are a couple of theories about this mod. Probably the primary is that in deep water crossings where the airbox could fill up with water, that water will more easily go into the crankcase via the breather tube. The KLX airbox inlet is quite high and somewhat protected by the frame rails and saddle pan to stop anything but complete immersion in water...which brings on a whole other set of major problems.

Another idea behind the mod is one of fuel getting in the airbox from the bike lying on its side or even a badly stuck float while the bike is on the side stand. I've experienced the stuck float with gas getting into the crankcase on a KLR650, but our KLX airbox has a better positioned drain hose and crankcase inlet to prevent this IMO. I also had my KLX get blown over by the wind while parked out on a trail while I hiked over to an overlook. A lot of gas ran out of the tank vent hose and carb vent hose but none went into the crankcase via the crankcase hose. Again, it looks like our airbox has better positioning for both the crankcase vent hose and drain hose to prevent problems in this area. Laying on its side for awhile can douse a good bit of fuel onto your air filter, however, if the bike is laying in the right position.

There is a third idea behind modded crankcase vent hoses, but I don't think there's any application to the KLX. Crankcase pressures in big singles can sometimes be an issue. The KLR650 is one such bike. Mounting an external exit crankcase vent and filter can actually help the KLR, or at least opening up the existing breather hole in the airbox to allow easier flow. There's even the application of an automotive PCV valve in one theory that may help the KLR, but that's a whole other debate. The KLX doesn't appear to have any excessive crankcase pressure issues, even with the bigger bore mods. The absolute most that one might want to do is to drill the airbox vent hole a little larger just for peace of mind. You then just use the stock hose, so it's an easy mod.

 

When the engine is running the negative and positive carnks pulses are happening so fast that there is pretty much a canceling out effect in that hose. There is a very very small outward movement of any combustion gasses that get by the rings, sure nothing requiring a big hose opening unless your rings and valve seals are toast., the effect is also aided by a slight vacuum from the airbox.

where there is more flow in the vent line is when starting the motor, then the slow moving stroke makes the suck and blow pulse is long, that's the only time bigger venting would help IMO, and as long as the bikes starts you wouldn't care. Also why you shouldn't try to restart a bike that is obviously submerged in water. If the bike is still running very little water will intrude, kick it over and your pulling water on the upstroke.

A pvc system also uses another vent opening in the case or valve cover somewhere else so there is actual flow in and out, pvc keeping it going the right way.The klx doesn't have a second opening to do this so it's kind of pointless. On another bike, klr, I bought into the hype and found a faster acting valve off a smog pump, the only thing I accomplished was sucking the counter shaft seal into the case to a point it leaked. Popped it back out, removed the valve and never had another problem.

 

While the crankcase mod might not be effective, I think the carb breather mod is worth the effort

 

Well said.

At running speed an equilibrium is reached. The energy used to compress the air in the crankcase on the down stroke is returned on the up stroke. There is no net energy loss or gain. Blow by past the rings upsets this balance slightly and exits out the breather hose.

Ron

 

This crankcase pressure equalization issue can still be an issue on something like a 4-stroke engine and especially a single and especially on a high rpm single. Multicylinder engines tend to smooth this phenomenon out more effectively, but even they can reach a point where it can be a problem issue at the high end of engine performance. Now, in reality it's a small almost unnoticeable issue on something like our KLX. It's probably been more of an issue on the big KLRs because they seem to have some blowby issues even when everything is correct and within spec. And there are effects to just about any 4-stroke engine where crankcase pressure can affect ring sealing, gasket leaks, and other attendant issues. This becomes more of an issue as the engine is stressed with more power-adders like supercharging, turbocharging, high rpm, etc, etc. Some fuel dragsters even have the application of pump driven crankcase evacuation systems. Again, obviously we're talking about the extreme ends of the performance spectrum here and hardly something to be concerned about with most street driven vehicles, much less our lowly KLX. Still there is a cycle point in engine operation where there is crankcase pressure that is not beneficial to performance...but...it's such a small factor for most street engines that it's not worth the pursuit of money and/or hardware to pursue for any noticeable gain. It exists...just not at a level for concern.

 

on-off topic, A friend recently bought a new 450exc that the dealer desmogged for him. On the 1st ride he's about 1/2 way up the mt. and there's a big bang! the valve cover gaskets cam end piece blew out. The dealer had the case venting blocked off somehow when he was desmogged it (my theory). They "fixed it". Rode with him today and noticed the valve cover vent went right back in the case??????????????? They told him it equalized or something, waited all day for the big bang but never happened. That case is still not vented IMO.

 

leave you vent alone the only thing you need to worry about with it is if it stars pukeing oil or oil water(coolant,ect.) mix, keep it in your air box to keep it from picking dirt or other stuff up bigest thing to do to a smoged motor is egr type stuff and air pump type stuff

 

I didn't read everything in detail, but here's the bit on the KLX650 -

The vent on the 650 runs DOWN to the air box, so any condensation ends up in the airbox, dripping out. I rerouted the tube up, added a PVC breather box I made up (stuffed with coarse steel wool scrub pad material) to condense out some oil to run back down into the cam cover, then ran the tube to the air box. Others simply wrapped more tubing UP around the area under the tank and down to the air box or just down around the swingarm.

I eliminated all the drippage, but the big single still blows off oil, especially on higher speed continuous road runs. It always has since I got it with 1670 miles. Others with XR650Ls did the same.

I've read a bit about adding a PCV valve (they used some Mazda unit I think) on the KLRs to deal with the breather issues. I may look again some time when I start working on the 650 to refurbish it a bit... If I can get myself to take a running bike apart.