2006 YZ478 kit problems

12 replies to this topic
  • davidvolk

Posted May 06, 2011 - 05:12 PM


I had a cracked cylinder so I decided to go with a new big bore kit from cylinder works.

I had a friend who is mechanically inclined come over and help me install the kit. We did everything according to the Yamaha book. Doing the timing chain was a pain.

I got it all buttoned up and went outside to kick it. It has WAY too much compression. Even when I pulled the spark plug out it is kicking over too hard. While we had the cylinder head off the piston traveled up and down the stroke very nicely, wiping the excess oil off as it was supposed to, so I know the piston is in correctly. The cam assembly isn't rocket science, and it was not my first 4 stroke top end rebuild, and about the 50th that Erik has done.

But something is wrong. Why is there so much compression? I did not kick it too many times, and I did NOT kick it hard, just moved the piston up and down a few times and realized there was a problem.

What should I check first? Could the cam chain adjuster be too tight? Where do I start?

Thanks for the help!

  • dalonar

Posted May 06, 2011 - 05:21 PM


even when I pulled the spark plug out it is kicking over too hard.

with the plug out, there is no possible way to get compression. something is binding

edit- ring gap?

  • davidvolk

Posted May 06, 2011 - 05:42 PM


with the plug out, there is no possible way to get compression. something is binding

edit- ring gap?

Ring gap was fine. What I meant by "Compression" is that there is far too much resistance when I tried to use the kick starter to move the engine.

When we had the cylinder head off the piston was traveling up and down smoothly.

I am going to remove the cam chain tensioner tomorrow and reinstall it. I am going to bet the issue is there. Like I said, it felt like it was just too hard to kick over, and my first thought was cam chain too tight. But because of bad luck in the past I didn't want to force the issue and try to start the bike. I wanted to ask opinions first!

Would the cam chain adjuster being out of wack cause the chain to be too tight, causing too much resistance?

  • grayracer513

Posted May 06, 2011 - 07:26 PM


Check your valve timing. Before you do that, be sure TDC indicated by the flywheel is actually TDC.

  • Octanee

Posted May 07, 2011 - 12:27 AM


i would say check your camshafts to make sure they are correct, the exhaust cam has its decompression feature on it, im thinking maybe if its not on right... but you say when the spark plug is out it is hard to kick over, and at that, i would think that there is something wrong with something else all together.....

  • davidvolk

Posted May 07, 2011 - 06:17 AM


It was the cam chain tensioner. When I pulled it out it was at the fully extended position. I could then use my hand to turn over the motor, as it should be. Stupid. Next stupid question, which direction do I turn the screw to get the plunger to retract? LOL. Downloading the manual (working on my bike at my parents shop rather than at home where my manual is!)

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  • davidvolk

Posted May 07, 2011 - 09:11 AM


Got it fixed. The cam tensioner was out all the way and just needed to be retracted. Put it in correctly and the bike fired on the 3rd kick.

  • jeffro667

Posted May 07, 2011 - 12:18 PM


Well, how does it run Dave?

  • erickdj

Posted May 09, 2011 - 09:45 AM


yeah, how does it run? did it knock you on your butt during the first test ride?

  • davidvolk

Posted May 13, 2011 - 08:27 AM


yeah, how does it run? did it knock you on your butt during the first test ride?

This thing is AWESOME!!!!!!!!!!!!!! Same jetting as the 450, but a TON more torque. It isn't violent, and I can pull all the gears at the track. I go into turns 2 gears higher than I used to, and I never have to touch the clutch. It sounds wicked too... much deeper sounding. I can just growl around the track at low RPM's, and short shifting is SOOOO easy. It doesn't stall easy either, which is nice.

On a rough track it can wear you out if you are riding in the proper gear, but I just ride a gear or two high all the time and just LOVE it!

I have about 3-4 hours on it so far. The best part was at a sand track on a long uphill when I just growled by a guy who was giving his 125 everything he had. It was like he was standing still when I blasted by him out of the turn. I was laughing.

For $400 I don't know how anyone could go wrong!

  • Hurricane

Posted May 13, 2011 - 10:12 AM


For $400 I don't know how anyone could go wrong!

What brand did you go with and where did you buy it from?

  • davidvolk

Posted May 14, 2011 - 03:20 AM


I bought a cylinder works kit on ebay. They are all around $400 new.

  • fouledout443

Posted October 19, 2014 - 01:49 PM


How did your bike turn out in the long run?


I know this is bringing up an old thread, but I have an 09 450f that im debating on buying a big bore cylinder for it while I have it torn down.


I have not checked the bore with a mic yet, but I have no idea how many hours were on the bike before I got it. 


Thanks for any help! 

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