That's a lot of air that's wanting to move around

 

WHOA! I'd better do the vent mod !

 

Don't bother.

 

FWIW...

Don't know what the vent mod is for the 250. We with the 650 simply get the breather hose running up instead of down. The stock set up has the vent hose running down to the air box, all heavy air (high oil vapor content) goes to the air box to condense and drip on the ground, even if it condenses and runs down the inside of the breather hose.

Some will run a longer line up under the tank, above the engine then down. I ran a line up to a PVC tubing set up I made with a wad of scrub pad steel wool (not the fine stuff, the very coarse stuff that looks like machining shavings) inside the tube. It all goes up hill to the end of the tube, then a line goes down to the air box. The steel wool provides a baffle to collect and condense some of the oil mist and allow it to drain back.

Key proof it worked was that the unplugged air box drain tube would drip oil when parked. With the breather "box" it doesn't drip anymore - no big wad of oil being deposited in the air box.

I still blow off some oil from the engine, as most big singles do, but no drippage and a bit less rate of use.

 

The blow off mods have been tried all over the KLX250. I did it and the mod is still in place. But, I've yet to see one who really sees it as a benefit. If anything, just ream out the hole in the air box where the breather connects if it makes you feel better. Wish I hadn't bothered. I want to put it back to stock as the breather is in the way.

as always YRMV!

Cheers all!


p.s. certainly not to dispel the mod on the sixfiddy. Many reports of improvements there, but have yet to try that one myself. I don't beat the 650 nearly as hard as the 351!

 

So a 250 actually reads at the 500cc line; 88cfm when factoring the up and down stroke.

Probably not a big deal for trail work, but if you do a lot of high rpm then every little bit might help

Since I am on the street most of the time, I've taken out the oil fill plug and replaced it with a hose and small K&N that's lying on top of the air filter in front of the upper emissions outlet

I thought people seeing the bike would ask what that was all about but no one one's noticed it.

 

All my modification was done for was to cut some of the dripping of oil. There isn't really a way to avoid blowing off oil on most singles when run at a bit higher rpm. Just keep an eye on the oil level, that's all I do.

 

Of course the piston going up and down at high speed results in a lot of cfm in the crankcase. But if those cfm can't escape, it does not necessarily result in power loss. The energy taken in the down stroke to build crankcase pressure is mostly recovered in the up stroke. Air in the crankcase acts like a spring.

Some escape is necessary to avoid pressure buildup due to blow by gasses, which is the main reason for that hose to the airbox. Correct me if I'm wrong.

So, I am not really surprised by the lack of performance gain from enlarging the crankcase breather hole.

 

That's 88cfm at 5000rpm btw; 176cfm then for an engine at its' ignition shut off of 10,500rpm like KLXs



Once you've exhausted the normal mods and their possible gains, there may hp hiding in other places

 

Had this in mine.