I was telling my friend who used to race motocross about the crankcase mod, and he said that thats one mod that I sould leave well enough alone. He told me for the performance gain, I really wasn't going to tell a huge performance gain like if I just went ahear and rejetted it, change exhaust (or drill the baffle plate for me, I'm cheap) and drop the gearing. He was really skeptical about drilling the crank case air hole, and then I won't have to worry about getting oil in the air box. Whats your thoughts and opinions on this? Is it really worth the performance, what is the likley hood that if I drop my bike I will really mess it up by performing this mod. Thanks again for all your help guys!!

 

I saw a noticeable diffrence when I did ONLY the crankcase mod first. I later drilled out the stock exhaust, added the KX snorkel and rejetted. Finally I added the Muzzy exhaust and rejetted again. Each time there was a noticeable improvement in performance. THe Muzzy definitely made the biggest improvement.

As for oil leaking into the airbox. Couple things, 1) The bike would have to be on it's side and paritally upside down. IF oil did leak into hte airbox, there is a still a small restriction because you are only drilling to 3/8" not the full 1/2" of the opening. Finally, the airbox has a drain inside it. Most of the oil would flow out. If it did get into the engine, worst case you would foul the plug and might need to clean out the carbs. A small amount of oil would just burn up right with the fuel and cause no harm.

 

Hmm... Let's see...Hundreds of KLX300 owners who swear by this mod vs. one guy you thinks it might be a problem somehow...

 

This is one mod that i have done to my bike,The first thing i noticed is that the slight knock that is produced around the 4 to 5000 rpm is gone and you do notice a increase in throttle response. Think of it like this you put on new exh. to allow the exh gasses to flow better you install new jets, air filter and snorkel to get the air and fuel in faster or greater amount so now you need to let the other byproduct out (back pressure[:'(]). There is no way around it all combustion motors will produce a back pressure in the crankcase and this back pressure will slow down how the engine revs and produces power. I hope that this helps you out.

 

So is it a good or bad thing to drill out the vent hole??!! What if you keep the stock exhaust??

From previous posts:

I just spent some time comparing parts drawings of the '06 KLX250S cylinder head and airbox with those of the '06 KLX300R. Our bikes have an air injection system that's not present on the KLX300 or the older KLX250s. This includes a valve that draws air from the airbox into the exhaust valve area during the exhaust stroke to increase exhaust gas temperature. It's the first thing you rip off a modern Bonneville because it makes the pipes turn blue.

The purpose of the air injection system is only to help pass the EPA. There is also a catalytic converter in the exhaust pipe!

Removing the air injection system won't make a darn bit of difference in how the bike runs but it will lower the temperature of your exhaust pipe a bit if you care about that. Mine's already blued significantly.

Todd, are you sure there is not a CAT? The owners manual says it has one. If so, would the cooler exhaust tend to adversely effect the system.

Yes, you're correct. There is a CAT and lower temps would be bad for that. I'd not mess with the air injection unless you change exhaust systems.

I only bring this up so folks will understand why their pipe is bluing so fast underneath the heat shield and for future reference in case you decide to swap out the head and cylinder for those from a 300.

 

Drill away this will not effect the way the air injection system works at all. The injection system is there to help super heat the cat. so it can burn off any left over fuel that was not burnt during combustion.

 

You are correct. There is a CAT in the stock pipe. Removing the air injection system might have a small impact on the performance of that but it's main purpose is not to raise exhaust temp for the CAT but rather to burn un-burned hydrocarbons right as they exit the head. In general, removing this piece of junk does nothing but good. Less weight, less chance of a vacuum leak down the road, reduced header pipe temps.

 

Hahah, well I just so happened did that today, I was going up a sharp, uphill, off camber turn and my bike slid out from under me, and when it landed, the way the hill was, it was slightly upside down and off camber lol. I need to get some serious hand gaurds, I dropped my bike about 4 times today on some tuff trails.

 

Sort of... you have het right idea. On a engine with an even number of cylinders and an even firing sequence (90 degree V-twins, I-4, most V6's, V8) the backpressure generated from one piston will be offset by the vacuum generated by the other. In a single cylinder, the pressure generated in the crankcase will act as a spring and help push the piston upward, but the force requred to compress the air in the crankcase repeatedly takes away power and reduces the ability of the piston to accelerate quickly.

 

Thats funny! Ha, Ha!