2006 YZ450F athena 478 big bore kit failed leak test

Guys, i need some opinions on this i picked up a clean 2006 YZ450f off a guy for $2000 he said it had 3 hrs on the rebuild. He bought it off of someone and realized its too much power for him and his track is pretty small and the bike scared him so he was selling it for what he paid. The hr meter says 3 hrs.

The amount of aftermarket parts is sick i added up all the retail cost and its over $7000 in parts, I got a deal on it.  So i change the oil and go to start it and it fires up no problem 7th kick, im not used to start a 4 stroke pig as its been 3 yrs since i had a 08 yz450 and as most know every yz has its own starting procedure it likes. 

So i noticed as i kick its pretty easy to kick a lot easier than my eric gorr yz300 2 smoke so i come on here and i read about doing a leak down test. i borrow a leak tester from my buddy and i do the test and it says 80-90% leakage i attached a image so i then take off valve cover and check timing and find out the exhaust is one tooth off and rotate exhaust cam one tooth towards intake cam side and repeat the test same thing but i noticed that its alot harder to kick over and there is alot more compression as when i kick its alot harder and with the spark plug out i can hear a very loud woosh sound much louder than before when piston comes TDC and forces the air out I can hear air coming out of airbox and then i check clearances and they seem tight so i take off cams and repeat test same thing so now im wondering if the guy lied and i have a old piston and there is blowby but i dont hear any air coming from crank case.


next thing i do is remove the head and i find out that the piston is in fact new, cylinder is new and when i pull the cylinder i see that the crank is a new hotrods crank so im happy about that.


so i bring the head to work today and start to disassemble and remove the 1st intake valve and i see that it looks like a kibblewhite stainless with the dual spring setup. i take some mold cleaner and some scotch brite and in less than a minute the thing is al shiny and polished so the guy didnt lie to the guy i bought the bike off of and did rebuild the whole motor. I did notice on a couple of valve seat areas there was carbon build up, it looks like the guy lugged the 450 around his yard and he did comment that his yard and track were small so i could see him lugging it and loading it up. so my thought is to get some carbon cleaner and clean up the head and valve seats and reassemble. ive attached pics what do you guys think and do you think that small bits of carbon in the seat area could cause for the low readings during the leak test?

 Also with the kibblewhite SS valves can i buy some lapping compound and a lapper shaft and lap these?










What you need to know is whether the KW's are nitride coated or just hardened.  Some SS valves are nitride coated much the same as Ti valves because it's a less expensive procedure.  Those that are coated can't be lapped for the same reason you never lap Ti valves.  Personally, unless they leak after you reassemble the head, I wouldn't bother with it.

What you need to know is whether the KW's are nitride coated or just hardened.  Some SS valves are nitride coated much the same as Ti valves because it's a less expensive procedure.  Those that are coated can't be lapped for the same reason you never lap Ti valves.  Personally, unless they leak after you reassemble the head, I wouldn't bother with it.

So gray do you think those small pcs of carbon on the valve seat could have cause the leakage to that level? I shoulda just left it alone but i wanted to make sure the bike was rebuilt. I did get a steal on it. check out the parts list. all for $2000


CF frame guards l-r $100 CF skid plate $80 CF fork guards $100 CF chain guard $40 CF rear brake guard $40 Carbon tank guard $95 CF frame guards footpeg area $100 protaper bars $100 yosh pro series ti-carbon slipon $600 fmf ti powerbomb header $200 Fork start $50 billet plugs $50 Billet oil fill $25 Billet gas cap $50 Applied triples $400 270 mm front brake rotor & bracket $270 rad frt wheel $460 rad rear wheel $460 moose lock grips $25 boyseen clutch cover $100 boyseen water pump $200 fuel screw $25 r&d power float bowl adj leak jet $250 asv lever frt brake lever $60 asv lever clutch lever & perch $150 ride eng frt brake line mount $30 ride eng frt brake top $25 hammer head rear brake pedal $180 hammer head shifter $70 holeshot device $110 hammer head rear brake clevis $45 hammer head rear axle blocks $50 hammer case guard $35 oil filter cover $40 asv front brake lever $50 braking rear brke rotor $105 athena 480 kit $850 step seat $100 fast way foot pegs $135, Xwing carb wing $100, Preload rings $50,

JD Jet kit                           


silicone hose kit                           


hot start                         


The carbon could have caused it, yes.  Also, the carbon on the stems may have held the valves a bit light on the seat. 

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