Blow By?

[MattBerghauser]

On a stock banshee with pipes, could this crazy thing called "blowby" cause a motor to surge up and down at idle and sometimes bog when running it? I'm just wondering because someone I talked to really believes this is my problem.

 

My elevation is 2400'

[mailman]

Blowby is a term used when the rings are worn to the point that they don't seal well enough against the cylinder and some of the exhaust gasses "blow by" the rings. This is evident on the pistons when the motor is torn down by black or brown residue seen on the piston. When the motor is running there really isn't a way to tell if there is blow by or not. It is just a sign of a worn motor. The more worn a motor gets, the more blow by it gets until compression starts to suffer. Don't be fooled into thinking a compression test will tell you about blow by; it won't. A really worn motor will lose compression but at that point the motor is pretty gone, performance wise.

 

Your erratic idle is most likely due to a lean condition caused by an air leak. Air leaks can show up in many different places like base gaskets, crank seals, intake gaskets, etc. etc.

Your bog could be caused by many different things. I would start by finding the air leak and repairing that or your top end could suffer damage. I would then try to address the bog. Most likely the bog is carb related but not always.

 

Hope this helps.

[MattBerghauser]

OK thank you for the great info. One other question. if there is too much blowby, is it possible that those gases could combust in the crankcase itself and still run? I know.....lol.....stupid question. please answer it though

[mailman]

No, it would be impossible. There is no ignition source and not enough compression for that to happen.

 

Keep in mind that blow by is the burnt mixture not the fresh mixture and the motor (rings actually) is not sealing with the cylinder. When it is sealing then all the burnt charge, or mixture, goes out the exhaust pipe as intended. Blow by does not mean all the burnt mixture is lost through the rings. It means some of the burnt mixture is lost. Most will still make it out the exhaust.

Edited September 18, 2010 by mailman

[06specialedition]

Kinda off topic but....I like your shee Mailman

[mailman]

Thanks man.