Valve clearances WAYY off 2006 Yz450

I just bought a 06 YZ450F with a hard start issue. Got it for a screaming deal and couldn't pass it up even with the known issue.

Previous owner says when the bike gets hot it starts losing power and its hard to start when hot. Sounds like valves to me so I went ahead and took it apart to check the clearances. It almost seems like someone shimmed the intake valves at exhaust valve specs. Valves tighten up as they get worn and need adjustment, not losen.

My intake valves can fit a .279mm easily. The Exhaust valves fit a .305mm in easily.

I have not pulled the caps to see what shims are in there yet but that is the next step.

Anyone have input on this? Just a careless valve job or is there something else that could be wrong? It is at TDC



I've seen the shims themselves as well as the tips of the valves wear from the lifters spinning on them. When this happens the clearance will increase. Pull the shims out and see if there is a circle pattern worn in the center of them where they contact the valves.

They have circles worn into them on both sides. The intake shims are as follows from right to left 1.84 1.85 1.85 - exhaust shims 1.80 1.80

I will shim it to spec but that just seems like a huge jump for .30 to .20 to get the exhaust back in and from .28 to .10 on the intake side

Thanks for the quick response!

Actually, the heat related problem sounds a lot more like the carb than anything else to me, but I wouldn't rule out a coil or stator problem, either.

Even though KJ has brought up an accurate potential problem, the fact that all of the valves are loose reduces the likelihood of it being a worn shim or valve stem, as such a failure would be random and unusual, rather than uniform across the set. The possibility that it's carbon holding the valves off their seat slightly would usually also not be uniformly distributed over all 5 valves. Because of this, I would suspect the previous owner's mechanical skills first. You should inspect the cam caps for serious galling or excess clearance. This can allow the cams to tilt up away from the valve lifters as the chain pulls down on the sprocket and increase clearance. With that big a shim (185 ish), clearance should not be that large.

If you don't find a problem, try setting them correctly and try it out.

How is the compression?



You can see the circles worn on the shims themselves. There are two caps that have very noticably different wear marks then the others. Do they wear marks indicate anything?

The cam journals look to be smooth and in good shape. The motor is clean, no carbon buildup that I can see anywhere. There is no galling on the cam itself or the journals. Im thinking it was shimmed incorrcetly by the previous owner.

The outer ring of odd looking wear marks on those two lifters is from the auto decompression pin striking the lifter while the AD is active (less than 700 RPM), and is normal. The fact that there are two such lifters means he mixed them around instead of putting them back in their original positions as he should have.

Ok so basically there was a nut job doing the valve adjustments. I made a drastic shim change and am close, taking it back apart now to dial it in.

Thanks for the quick responses! I will post up how she runs when I get it back together!

As far as the timing goes...Should I worry more about the 12 vs 13 pins or the punched marks lining up properly with the head at TDC.

I read a thread on here early and never came to a conclusion. If I set it to 12pins the exhaust mark is higher then the cylinder head while the intake is perfect. At 13 pins both the exhaust and intake marks linke up perfect.

What happens when you set the timing and put the chain tensioner in? Make sure that when the slack is taken out of the chain then both dots will line up with the head. Be sure to always double check the timing after the tensioner is in.

Edited by KJ790

I would think about replacing the valves. They were obviously out of spec before when the previous owner or someone adjusted them. You can shim these titanium valves once when they go out of spec, after that they wear quickly once the wear coating is worn off. I've shimmed them twice for customers who don't want to buy valves yet and the adjustment doesn't last that long

I'd replace all 5 along with the valve seals and have the seats cut. You won't have to worry about valves again for a while. Sounds like u got a good deal especially for a 06

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