Jandre

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Aug 31, 2016
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Good day. I have a yz450f that i have bought 2 weeks ago. While i was riding i saw gas coming out of the breather tube. Can it be of the high compression of these 4 stroke machines or because of worn rings on the piston ? Will the bike not use oil when its rings are worn ? Because this bike is fantastic and still has a lot of power so must i be worried because of the gas that is coming out from the breather tube ? I also have a WR 450 '06 thats breather tube goes into the airbox but he doesnt breath as far as im concerned. Help me please i dont now if i must be worried or not.
 
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Rich Rohrich

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As the compression rings (the top 2) wear the amount of blowby into the crankcase will increase and you'll see an increase in the discharge from the breather tube.. This is a normal occurrence as an engine wears. If you have good compression readings and the engine is making good power it isn't a cause for concern. These engines make enough power that you can have fairly significant leakage and still have a great deal of fun.
 

Jandre

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Aug 31, 2016
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As the compression rings (the top 2) wear the amount of blowby into the crankcase will increase and you'll see an increase in the discharge from the breather tube.. This is a normal occurrence as an engine wears. If you have good compression readings and the engine is making good power it isn't a cause for concern. These engines make enough power that you can have fairly significant leakage and still have a great deal of fun.
As the compression rings (the top 2) wear the amount of blowby into the crankcase will increase and you'll see an increase in the discharge from the breather tube.. This is a normal occurrence as an engine wears. If you have good compression readings and the engine is making good power it isn't a cause for concern. These engines make enough power that you can have fairly significant leakage and still have a great deal of fun.
So i dont have to worry about it ? Or is it time to do a top end rebuilding ? If that is the case im not very impress with the guy i bought the bike at, because the bike was expensive.
 

Rich Rohrich

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You would need to do a compression test or a leak-down test to know for sure if it's time for rings. Does the bike have an hour meter on it?
 

Rich Rohrich

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If it it has 82 hours of MX on the engine it's could probably use rings at a minimum. I would suggest doing a compression test or better yet a leak down test so you know for sure. The service manual will give you guidance on acceptable readings.

If it were ridden on an MX track for 82 hours and it were mine, I'd rebuild it, but I like my engines to be tight and powerful. It is an incredibly easy engine to work on so don't hesitate to open it up if you think it needs it.
 

Jandre

Member
Aug 31, 2016
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Yes i know this engines well, i know its easy. No before i bought this bike it was been used for enduro by a old man, the bike still looks like a new bike and rides like a new bike with lots of power thats why i'm so amazed with that gas thats coming out of the breather tube. How does the compression test and leak down test work ? Sorry about my english im actually a south african boer so im trying my best.
 

Rich Rohrich

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The Yamaha service manual goes into the details of the procedure and tools you'll need to do a compression test. Basically you screw a compression gauge into the spark plug hole and kick the engine over 4-5 times with the throttle open. Because your bike has an auto-decompressor cam you'll need to do a couple of extra things to account for it. A leakdown test is far more accurate but it takes specialized equipment that is usually only available in a repair shop or a performance shop.
 

Jandre

Member
Aug 31, 2016
5
0
The Yamaha service manual goes into the details of the procedure and tools you'll need to do a compression test. Basically you screw a compression gauge into the spark plug hole and kick the engine over 4-5 times with the throttle open. Because your bike has an auto-decompressor cam you'll need to do a couple of extra things to account for it. A leakdown test is far more accurate but it takes specialized equipment that is usually only available in a repair shop or a performance shop.
Thank you very much.
 
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