Top End Talk

[JacobSlabach]

So I have rebuilt two top end's now and going through the process of a third.  I'm confused about how high the compression should be on these atv engines, and if low compression, what should be replaced or checked and how?  And as far as smoking, how to tell what is the issue there as well.  The first rebuild i did was on a kawasaki 300 and it turned out ok...  I realize that I adjusted the valve clearance on the exhaust stroke rather than the compression stroke which caused a tick.  nut the thing that got me was one both rebuilds, the compession reading didnt really change that much.  I've heard that engines should have over 100psi compression, yet have never seen 100psi reached on my tester by any engine.

this thread is open to any top end questions btw.

[06kfx440]

Every cylinder is different depending on stoke and piston size. Smoking is told by color of smoke. White or blue is oil or gas. You would have to smell it and see how it performs. Black is oil or rich. Again, smell it.

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Every cylinder is different depending on stoke and piston size. Smoking is told by color of smoke. White or blue is oil or gas. You would have to smell it and see how it performs. Black is oil or rich. Again, smell it.

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You would get your readings out of the manual or a simple search on Google.

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[JacobSlabach]

oh ok.  so it should give compression specs in the service manual?  Also have heard talk on 'torquing to spec'   I assume you find the spec in the manual as well but how do you measure the torque applied to a bolt?

[06kfx440]

oh ok.  so it should give compression specs in the service manual?  Also have heard talk on 'torquing to spec'   I assume you find the spec in the manual as well but how do you measure the torque applied to a bolt?

Yes sir. Everything you need to know is in the manual. You would need a torque wrench to achieve proper torque. Most have patterns to torque which can also be in the manual.

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[JacobSlabach]

oh ok.  I'm heard of a torque wrench, but never have used one.  I guess I've just been lucky enough not to have broken anything off yet lol

[06kfx440]

Well. The worst that can happen is bolts rattle loose and break things. But sometimes you can get lucky if you have a feel for ft lb strength.

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[JacobSlabach]

haha yea havnt had that happen- just usually torque them with a breaker bar down till they wont go anymore (the head bolts anyway)

[Frank Angerano]

Good thread @JacobSlabach 

Compression readings all vary on the size of the engine. (CC = cubic centimeters) which is a metric unit to measure the engine's capacity or its volume. So a 90 CC engine would generally have lower compression based on the size of the piston (volume) inside the cylinder head.  

A larger CC would produce more volume which creates a larger reading. 

Every engine has a CC size and a compression value in the manual that states what the acceptable range is for peak performance.  

Low compression stems from a few things. 

Bad valve seals. Oil in the head leaks down through the valve stem into the cylinder head and burns up upon the compression stroke leaving a blue smoke. 

bad piston/rings. Oil seeps up past the rings from pressure built up by the compression leaking into the engine past the rings and causes the same smoke condition but less smoke is seen because the vertical climb the oil has to make up into the cylinder head.  

Blown head gasket. This allows compression to be lost by escaping the cylinder.  Easy way to find it is a soapy spray all around the top end. You will see bubbles during the cranking if the engine and you can also hear it if you crank the engine by hand slowly. 

Warped or cracked head/cylinder  from excessive heat. The head and cylinder cannot meet flush and therefore a void allows air to escape.  Same type of test with the soapy water. 

 

[06kfx440]

That's a better explanation than I gave Frank. But same principal.

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[JacobSlabach]

awesome.  im picking up info on it bit by bit..  i guess this is kinda one of those things where only practice makes perfect...unless maybe you have that college degree- and still, theres nothing like hands on for this type of stuff.  i like how @Frank Angerano did it with his son i think it was- to take an old quad with a shot motor or one he was rebuilding and take it all apart and just get a good feel for what everything looks like and how it all works.  aint nothing like it👍