Lost Compression on a 400

[rbsdsu]

1st off I am a newbie to this site and I would like to say hello, I am Ryan and I live in So Cal and I ride as much as possible!

That being said, I have a buddy who has a YZF 400 that has lost almost all compression, I can not find TDC and when you do kick it over you get nothing or maybe a backfire... So we tore it down the other night to find that the cams and valves were all fine upon going deeper I found what I hope to be the problem! The cylinder wall which has that plating on it has been completely worn of in 2 distinct areas and I can feel some pretty good gouges with my finger nails. My initial impression was that these are extremely high compression engines and that must be the cause.... I hope! Anyways I was just hoping to get some back up on my diagnosis. Does anyone know where I can have the plating re-done, 2nd would it be a good idea to have a new piston and ring set to install???

I appreciate all of the info you guys offer

Thanks...

 

[Rich Rohrich]

Is the wear on the exhaust side of the cylinder?

 

[rbsdsu]

Is the wear on the exhaust side of the cylinder?

I'd have to go back and look to see what side the wear was on. Would a particular side denote a specific problem????

 

[Rich Rohrich]

Short skirt slipper pistons like this tend to show the majority of their wear on the exhaust side thrust face of the cylinder.

 

[SFO]

If it is a well ridden 400 it is a prime candidate for a full rebuild.
A good valve job with new springs and seals along with a piston and a replate would be a bare minimum.
I would seriously consider rod replacement and cam chain as well.
Early thrashed 400s have the habit of shedding the small end of the rod.
This ruins everything.

 

[Rich Rohrich]

Early thrashed 400s have the habit of shedding the small end of the rod.
This ruins everything.

... and they end up looking like this :ohmy:

 

[SFO]

Just think, parts from that very motor went on to be part of a bike that won an AMA national event last year!

 

[Rich Rohrich]

Just think, parts from that very motor went on to be part of a bike that won an AMA national event last year!

The cosmic circle of racing life :aj: :yeehaw: :aj: