99 yz400f

6 replies to this topic
  • dmcswain08

Posted January 03, 2015 - 02:47 PM


I have been having problems with a 99 yz400f just bought it and can't get it running has good spark now rebuilt the Carb and replaced the head gasket. Now it is blowing a lot of air out the breather tube on top of the cam cover. Would anyone know what that mite be or what I should check?

  • Mike Mike

Posted January 03, 2015 - 04:36 PM


Did you lift the cylinder to check rings, piston and bore? Low compression start poor.  Compression blow by will give extra crankcase pressure.  When do you feel the valve cover vent?   

  • grayracer513

Posted January 03, 2015 - 06:08 PM


It's supposed to blow air out of the breather.  Every time the piston goes down, there's 400cc of air under it that wants to go somewhere.

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  • Mike Mike

Posted January 03, 2015 - 09:45 PM


Yes, we all know that a crankcase needs to breath.  How long have drag bikes used a pump to remove pressure to greatly reduce engine drag.  Many dwarf cars in our neck of the woods ran GSXR 1100 and I wish the vacuum pumps where around then.  In a 15 lap race, 1/4 mile, these engines would blow around a quart of oil to a can in the rear (engine in front).  It makes sense to me that a cam chain lose enough to sound like it was hitting something might be mistaken for piston rattle on a engine that is burning oil and has enough crankcase pressure to leave a puddle in the air box. I have repaired many over lubricated air boxes with pistons rings and bore jobs.  Very lose cam chains often lead to bent valves since the head was off I would have removed the tensioner and upon reassembly have checked chain tension while timing the cams.

  I was curious to find out how someone might measure crankcase pressure to see if it might be within factory specs.  I have never come across these numbers in a manual.  It sounds a lot like the people that say there bike has good compression because it blows there thumb off the spark plug hole.

  • dmcswain08

Posted January 04, 2015 - 11:34 AM


Yes I did pull the cylinder up to check the piston rings and cylinder but there was no play in the piston so I never pulled the cylinder all the way off. So I haven't seen the rings. But the bore and piston were in grate condition. The blow by happens when I kick the motor over to the point that it will blow your finger off the hose. Dose this year bike have a automatic decompretion valve could it cause this problem.

  • Mike Mike

Posted January 04, 2015 - 01:55 PM


I believe there is only a manual decompressor.  If a valve was being held open by  any  type decompressor (I have only see exhaust valves) the air pushed up by the piston would blow out the exhaust not into crankcase.  Try a leak down test.

At top dead center on comporession stroke and with a good buddy using a breaker bar and socket to hold flywheel.  A leak down tester uses your air to pressurize the cylinder (through spark plug hole).  The leak down tester has a regulator and a gauge that usually shows percent of leakage.  Here is what I am getting at.  If a exhaust valve is open you will hear the air blow out the exhaust pipe.  Best to remove carb(s) throttle body to hear bad intake valves.  If rings are bad the air blowing sound will come out of crankcase.  A bit of a glitch in a leak down test, I have found, is that a tight valve adjust or slightly cupped valve faces can seat better with the air pressure and hide issues.  These issues, I have found, show up real good with a simple compression test.  Most engines need at least 100 lbs. compression to start.  I always like the dude that keeps on ridding when their bike only starts when towed down the road.  Stopping, avoiding the tow rope and keeping the bike running is an art.


I did not say this but in some cases I might have seen a leak down tester be substituted by a regulated (25psi start and go up from there)  air hose.

Be sure the guy on the breaker bar has a good grip and keeps his nose back.



  • dmcswain08

Posted January 04, 2015 - 02:21 PM


Where would i pick up a leak down test kit at and would you know around how much

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