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Message started by serenity3743 on 10/03/11 at 10:16:12

Title: A lesson in valve mechanics
Post by serenity3743 on 10/03/11 at 10:16:12

I would like for all you experienced wrenchers out there to help the rest of us understand valves. Specifically, what happens when a valve "burns"? What symptoms would the engine exhibit if it has a burnt valve? What would a burnt valve look like internally? What causes valves to burn?

Let the discussion commence!

Title: Re: A lesson in valve mechanics
Post by verslagen1 on 10/03/11 at 11:32:24

http://youtu.be/tsQpTVkOUD0

http://youtu.be/n7YXgJPmV5M

http://youtu.be/tKDMqhqQ-WM

http://youtu.be/Umw4kVDuD1Q

Title: Re: A lesson in valve mechanics
Post by serenity3743 on 10/03/11 at 13:49:32

Thanks, Versy! So, a rough idle, a sucking exhaust, and a loss of compression are symptoms of the burnt valve. In the second video, I'm guessing it was cylinders #3 and #4 that had burnt valves. The stuff oozing in those cylinders was evidently water or other liquid demonstrating that the valves were not sealing.

As to what causes valves to burn, I know that an exhaust leak can cause it. Why is that the case, and are there any other causes?

Title: Re: A lesson in valve mechanics
Post by verslagen1 on 10/03/11 at 15:10:28

More or less, a burnt valve referrs to the valve loosing the sealing surface due to overheating.

This can be caused by lean mixture, incorrect valve adjustment, I'm sure there's more reasons just don't know anymore.

we all know that lean mixtures will drive the exhaust temps up. some one posted an aircraft egt graph awhile back.

also, the valve depends on contact with the head for cooling. being adjusted too tight will decrease contact time.

Title: Re: A lesson in valve mechanics
Post by SurfJunkie on 10/03/11 at 15:23:36

Versy has it covered.

Lean mixture can also cause the valves to "square".
Due to the higher temps, it overheats and stresses the valve, causing small cracks/weakness around the thinnest portion of the valve (typically right around the perimeter of the seat/edges of the valve. ) Often times these will eventually break off, thus causing a loss in compression. :)

Title: Re: A lesson in valve mechanics
Post by AlphaBarney on 10/03/11 at 15:40:06

Please explain what you mean by "rough idle". My bike has a very uneven idle when cold. It seems like it misses occasionally until it warms up, or I pull the choke out 1 click. Once it's warm it's even and strong.
Is this indicative of valve damage? I'm kinda worried now, would the rag trick in the you tube clip work with the stock exhaust? If so I'll give it a shot tonight.

Title: Re: A lesson in valve mechanics
Post by SurfJunkie on 10/03/11 at 15:53:22

I wouldn't necessarily think there was anything wrong on a cold machine needing the choke pulled out 1 or 2 clicks to idle correctly till warm.

Sounds like a good ol carbed cold motor to me :)

Title: Re: A lesson in valve mechanics
Post by verslagen1 on 10/03/11 at 16:40:53

5578647C755675667A716D140 wrote:

Worried? do a compression test and you'll know.

Title: Re: A lesson in valve mechanics
Post by Routy on 10/03/11 at 17:16:27

Thats another beauty of a single cylinder, not hard to tell which cylinder has the problem.....if there is one ;D

Title: Re: A lesson in valve mechanics
Post by Gyrobob on 10/03/11 at 18:16:36

23302726393432303B64550 wrote:

Just a small correction,..

Lean mixtures lower combustion temps. That graph showed the max temps happened at stoichiometric mixtures -- the so-called perfect mixture,.. 14.7:1

Valves might burn possibly from detonation, which will increase temps unevenly,... but if you gradually get leaner than 14.7:1, the temps will decrease.

Title: Re: A lesson in valve mechanics
Post by verslagen1 on 10/03/11 at 18:28:06

Gyr, did lycoming publish what the a/f's where, cause it's not on the chart?

6F515A474A474A280 wrote:

[quote author=0221292B282121283F4D0 link=1314085911/15#23 date=1314715625]
Mixture too lean

Combined with a high power setting this will result in high cylinder temperatures, possible detonation, very hot or burnt exhaust valves and maybe damage to the pistons resulting in a power failure and a possible off airport landing.

Just read it for what it says -- mixture too lean at wide open throttle will result in higher cylinder temperatures. Detonation is easy to explain when the valves get anywhere near hot enough to burn (cherry red) and aluminum pistons can get soft and blow out in those conditions as well…….

We’re splitting hairs here, methinks, but,.. to be technically accurate, a lean mixture doesn't raise CHT, it lowers CHT. Since you bring up aircooled aircraft examples, check out Lycoming’s chart:

The danger from lean temps is what happens from crappy flame front travel. When the flame front is not smooth and consistent, detonation (an explosion rather than a smooth burn) can cause mechanical damage.

One other comment, if the valves get hot enough to ignite the mixture prematurely, the result is preignition, not detonation. Other causes of preignition: timing advanced too far, and glowing hot carbon residue. The difference between the results of detonation vs preignition is moot,.. they both result in pinging (an explosion rather than a smooth burn) and probable damage if the pinging happens a lot.

[/quote]
http://i258.photobucket.com/albums/hh253/Gyrobob_theOriginal/Motorcycle/Miscellaneous/mixturevscylheadtempo1.jpg

Title: Re: A lesson in valve mechanics
Post by Gyrobob on 10/03/11 at 18:33:04

Vers asks, "Gyr, did lycoming publish what the a/f's where, cause it's not on the chart?"

I'm sure there's a chart somewhere like this that has actual A/F ratios on it. I'll check some old textbooks.

On the chart shown, 14.7 would be at peak CHT. The EGT continues increasing for a bit because near 14.7 there is still some burning going on in the first part of the header.