400EX engine noise @ high RPM

[chawness]

I am working on a 400EX for a friend and need some help. The quads timing chain had come off due to the bolts backing out of the cam gear. I replaced the timing chain, rear timing chain guide, oil pump, lower timing chain gear, cam, cam gear and the upper head. The quad starts and idles OK (not great) and runs pretty much normal while riding. However under full throttle high RPM there is a noise in the top end. I have not tore the engine apart again yet, I thought I'd get some opinions first from some people that have worked on these before.

[f4istunna]

Did you make sure the valves were adjusted in spec? Chain tensioner working like it should?

[chawness]

I adjusted the valves twice. .004 and .005

As far as the tensioner It appeared work correctly. How does a person check it?

[f4istunna]

As long as it isnt frozen up and the spring isnt shot it should be ok. I usually just make sure the chain gets nice and snug once i put the tensioner in and release it.


Did you put an aftermarket cam in? They make it alot noiser than stock.

Is the noise for sure coming from the motor? Ive had some high rev rattles that made me think my motor was gonna grenade alot of times and it ended up being loose bolts, skidplates etc? Just something to check out.

IS the noise under load (while riding) or just in nuetral revving? Or both?

[guinness77]

I have the very same problem with my 400ex. After a re-build and about 30 riding hours...WHEN THE ENGINE IS UNDER LOAD it sounds like one of the valves is hitting. Everyone said replace the timing chain. So... I replaced the timing chain, both timing chain sprockets, the chain tensioner, and made triple sure that valves are correctly adjusted. The cam is stock and in excellent shape.

What can it be?

[chawness]

I finally got around to working on this quad again. To see if it made the noise while not under load, I held the throttle wide open several times and the quad did not do it. It was against the rev limiter too. I tore the top end off afterward to make sure I had it all together correctly (bought a service manual). Once I got it back together I rode the quad for about 5 minutes and did hear the noise once. I think "f4istunna" is right in that the noise is not coming from the engine, I believe its from the chain and skid plate.

Just thought I'd report my findings.