tristan18au

Member
Jun 3, 2001
186
0
my 1998 XR250 is leaking air (or compression) out of one of the valves i think, i took the tappet cap off the top end to do the tappets and as i was turning the bike over to TDC and held the compresion there the top end would hiss and slowly loose compression. when i put the cap back on it does not do it.
whats going on?? any help would be great!
Also its burning just a tad bit of oil. its got plenty of compresion
(when kicked over)
the hissing is coming from the front
:think
 

BigBore

Member
Jun 16, 1999
693
0
Has it always done this? The reson I ask is it sounds like it could be normal. I've noticed the same sound when turning the motor over with the tappet covers off. If its burning oil, it could be worn out rings, could be leaky valve stem seals.
 

smb_racing

Master of None
LIFETIME SPONSOR
Jul 31, 2000
2,085
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if your bike is burning some oil then you could have a little carbon buildup in your valve seats not letting the valves fully close. In this case you'd need to remove the offending carbon to let the valves fully seat.
 

Humai

LIFETIME SPONSOR
Feb 6, 2000
199
0
Tristan18au, what you are hearing when you rotate the engine to the top of the compression stroke with the tappet cover removed is compression blowing by the rings into the crankcase. This is perfectly normal, even on a fresh engine, and happens even with the cover on but is only evident to you when the cover is removed because removing the cover allows you to hear what is going on inside the crankcase/timing/cam/rocker areas better.

In short, do not worry about it. Just do your normal routine maintenance, monitor your oil consumption, ride as normal and the world will be a better place.
 

tristan18au

Member
Jun 3, 2001
186
0
Humai, it not my compresion blowing back into the crank caes the blowing (hissing) is coming from the exhuast valves or some where there abouts.
the valves have just been relaped and rebuilt top end.
:think
 

BigBore

Member
Jun 16, 1999
693
0
Is this the first time you've checked the valves on it? If not, have you noticed the sound before?

If your top end is fresh and you've just had a valve job done, I'd suspect this is normal. Every bike I've ever adjusted the valves on does this. I believe what you're hearing is called "leakdown." Yes, the sound does seems emanate from the valves.

If the bike runs great, doens't have any strange sounds coming from the top end, and has good compression, I wouldn't worry.
 

smb_racing

Master of None
LIFETIME SPONSOR
Jul 31, 2000
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are you sure you don't have your valve lash too tight? there should be some clearance between the rocker and the valve stem when the engine is rotated so that the particular valve should be closed.
 

Jon K.

~SPONSOR~
Mar 26, 2001
1,354
4
I am having trouble understanding how you could be hearing a valve leak by listening to the valve spring! Of course you can hear a valve leak, but you gotta put your ear to the port (with the pipe removed) or to the end of the muffler!
 
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