valve float

[killab443]

what happens when a valve" floats" what could break ect, can it cause timing to be messed up? To the naked eye would timing appear wrong?

 

[5810]

Valve Float A high-rpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain.

 

[Defiant]

Well, other than a cut-and-paste, think of valve float in this way: instead of thinking of the cam, think of the follower. Now, the follower is a roller-coaster car, and you're riding in it. Now the cam lobe is the track that the car is rolling on. You can wear a valve on your head if this helps the image. Oh, and there's no retaining wheels on the bottom of the car, you're counting on gravity (which is the valve springs) holding you on the track. And, the roller-coaster is in a tunnel, the ceiling of which represents the piston crown. It follows the track at a fairly constant height, with the same humps in it.
From the car, you see a bump in the tracks ahead. That's valve lift. Now, ideally, to open valves completely and right away, you'd want a simple step up to raise that valve as far as you can. But that'll make for kind of a bumpy ride for the coaster car, won't it? So to get to full-lift, there's a ramp leading up to the top of the bump. That's called an acceleration ramp, and it represents the leading edge of a cam lobe profile. It's just the right shape so the coaster-car will rise up quickly, but not so steep at the top that the coaster-car jumps at the crest of the hill. Oh, and as you rise on this track, the roof of the tunnel also rises so you don't bump your head on it.
Then, to get back down, you could again have just a cut-off hump and you'd drop straight down, but it would give you a rough ride as gravity (the valve springs) slams you back down. So, there's a decelleration ramp which lets you drop just fast enough that you don't get air.
As you can imagine, your coaster-car rides along the track just fine, up-down, up-down, up-etc.... up to a certain speed. But, go too fast, and you will pull air at the peak of the hump. That's valve float. If you float high enough, you bang your silly head on the roof (piston) and will bend the valve on the top of your head.
If you put wheels on the underside of the track so that you can't float, that's what's called a desmodromic valvetrain, which pulls you down after it pushes you up. Ducati motorcycles use this, and Mercedes did on the Gullwing.
Oh, and if someone grabs the end of the coaster track and slides it forward or backward, you'll start banging your head on the ceiling all the time- that's what goes on when a cam belt slips and your timing changes. A broken timing belt would be like being stuck on the top of the hump while someone moves the track and you keep hitting the ceiling.

In what I've come across so far, the DSM valvetrain isn't particularly susceptible to valve float until you get into insane rpms.