Other Coolster 70cc

Okay this ones an odd ball, and before I start I'd like to say thank you to all of you who have commented and helped me out on my previous posts. I apologize if I don't respond... it's usually because the problem got fixed... anyway, my buddy and I just came up on a free coolster 70cc pit bike for his son. It runs good, shifts good, very happy with it especially for that price haha. What I'm still trying to understand is the weird air filter assembly... hopefully someone can shed light on this. Why the hell does the air filter have a hose coming out of it that leads directly back into the motor!?????!??!??

What is that for? Cause I was just planning on removing it, operator the bike with just a straight carb to air filter like 100% of the other pit bikes but I can imagine there is some reason it's there?


Staff member
Got any photos? Sounds like a crankcase breather.
Crankcase breather doesn't necessarily have to go back to the intake, but it is convenient if it does.
Either way it has to be vented somewhere. -BIG DAN:thumb:
Kinda hard to see. But it's a line that comes out of the air filter..... then to what looks to be an oil filter??? (Not to be confused with the fuel filter.) then into the top of the motor

My SDG pit bike for example has that same hole that goes into the top of the motor but it just connects to a line that hangs off the bottom for like an over flow or something. I just can't comprehend why anyone would want what might be an overflow port going into a damn air filter



Staff member
That is a crankcase breather.
As the piston goes up and down, the inside of the crankcase is subjected to pulsating pressurization. This has to be bled overboard, and also able to allow air back in the crankcase to make up for the opposite pressure.

Vaporized oil in the crankcase builds up and would be pumped overboard as well, which technically "pollutes." It's an emissions thing. -BIG DAN:thumb:
Okay so though it might not be the most environmentally friendly, can I do away with that weird design and just run a line straight from the little hole down toward the ground?

Then I'll just get a regular air filter to throw on that carb!