YZ250F - I think I got a bad valve job...

06 YZ250F....problems.


Motor was recently fully rebuilt.  Cases split....all new bearings and wear parts.  I did that.   I sent the crank out for new bearings (My press is set up for karting motors...dont have one for MX cranks.).  The problem is not the bottom end.  It is cherry.


New piston and rings.  Had the cylinder re-lined (top re-plating shop...where the stuff goes) to match the piston (I sent in the piston..and they fit it right to the tight end of the YZ250F spec)


Full cyl head job (new valves, grinding, new guides, new springs)...also by a top shop...where the pros send their heads...


So...I have ridden in 4 times now.  Yesterday...the 4th time...it acting like it was running rich...just not right.


Today...it would not start....even though I rode it yesterday for 2.5 hours.  The only way I could get it to start...was to drain the float bowl, and bump start it.  Even then...it would not stay running.  The plug was BLACK....so it is burning oil.  My plugs always look right...the bike is jetted correctly...  So rather than risk breaking my neck at the MX track..I packed it in.


Got home...ruled out ignition.  Ruled out fuel.  This left me with head problems, cylinder or lower end (siezure in progress)  Pulled the head cover...checked valve clearance.  The shop that did the head set the valves up a little loose.  They were all about .001" over the loosest Yahama setting when I did the rebuild.  Today...they are still all the same...with the exception of the middle intake...which has tightened up to spec.


Pulled the head...there was oil all over the top of the piston.  Probably 1/4 TSP....a lot.  The cylinder walls look perfect.  Pulled the cylinder.  The oil ring was correct...but all rings seemed to have rotated from their installed position....  Overall...I do not think the issue is blown rings.  That leaves the head...and the lower end.  The lower end spun free as can be...so that is not it.


Checked all valves for seal by putting gas on the port side.  They all sealed perfectly.  Pulled the shims and buckets. I do not do heads...so I have no valve compressor.  I put a 1/2" wood dowel in a vise...so could carefully lean into it...and compress the springs one at a time.  I then tried to move the valves on the piston side to see if there was any play.  They were flopping all over the place.  ALL 5 valves look to have very excessive stem to guide clearance.  I am guessing .004-.008"....  So...the valves are leaking oil into the cylinder.....what else can it be?


I AM PISSED.  I spent a ton of money having THE BEST shop do the head...and it looks like the blew the most basic part of a head rebuild.  I am not naming names till we see if they step up and support their work.  My plan is to take it to a local Yamaha dealer...and have them measure the clearance to make sure my conclusion is correct.  Then I will call the shop that did my head job and see of they step up.


Could this be anything else?  Have I missed anything...?  There are only two ways oil can get into the combustion area..correct?  At least I caught it before it grenaded....

Edited by Blutarsky

Are you sure it was oil on the top of the piston? It could have been a 1/4 TSP of coolant mixed with carbon, and your symptoms sound like it could have been a blown head gasket. There isn't really a way that a valve job could cause oil to get into the combustion chamber unless it was going past the valve seals. Look in the intake and exhaust port, if the top side of the valves are dry (inside the ports) then it likely is not the valve seals. I don't think the shop that did your valves screwed you over, I think you have something else going on.

It was OIL.  Clean oil. Lots of oil.  No coolant.  No blown head to cylinder seal.  The exhaust port is black with burned oil, including the exposed guide OD and exposed valve stem.  The plug was also black with burned oil.  Valve faces are covered with burned oil.


There are only two paths for oil to get there...through the piston rings, or by valves.  The valves have visible movment in the guides.  How can that be right?  The spec is .0005 - .0015 on the intake side, and about double that on exhaust.  That is not enought to see...yet I can seel them move side to side when I compress the valves from the back side, and wiggle them.


I am travelling on business...but will be back Friday..and will try to post some pics.

Edited by Blutarsky

was it overfilled and came in through the airbox? I know this can happen if your ccv is routed WR style...but when you say you can wiggle the valves, that's generally not so hot.  I would call the shop that did the head works and see what they think.  

Sounds like the guides are reamed to large.  You can pop the keepers by hitting the top of the valve retainer with a appropriate socket and a hammer. a sharp blow should release the keepers.  Than you will know for sure you probably only have to do one valve.  Also Rocky Mountain ATV sells a quality valve spring compressor very reasonable, or ebay.

I am going to just take the motor into my local dealer...and have them quickly just pop all the keepers so can see how the valve to guide fit feels.

Disassembled some valves (springs keepers out so I can move the valve in and out).  With the valve opened about .25", there is .007"-.009" radial play.  Not right.... 

Who did the head?

Who did the head?


I am not going to say till they have a chance to address the issue...

Edited by Blutarsky

Great.  So what is the point of this thread then?

Read my original post.  I went thought how I did the troubleshooting...and noted what I found.  I was asking for others input....not coming here to bash someone who has not had a chance to make it right yet...  Sheesh...

OK...it was Millenium...and they stepped up and made it right.  I told them what I found...they said that does not sound right....send it in.  As soon as it got in, the called me, and said the clearances did not look right.  It was a long time since they did the original work, so I knew it was going to be tought for them to justify fixing it for 100% free.  What they did was charge me for the parts ($50!) and ate the labor.  Working in mfg...I understand how it is hard to hide a PO...but labor in the shop you can slide through.  So major props to Millenium. They stepped up and totall redid the head.  Got it back, everthing was right, and it runs great.

very good of them to fix it the way it should have been.

Good to hear cuz I'm sending a head off today too them!

Also, the re-lining they did on the cylinder was THE best I have seen.  It was so straight.  No more than .0001" out at any point.  Just perfect. 

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