What could have caused that?!? (Broken valve)

[giligan]

Hi guys, my bike suddently had no power while riding on the highway. A compression test gave me 0 compression on cylinder # 2. Can someone tell me the possible reasons??? I have a 99 ZRX1100 with pods , dynojet stage 3 swap and also installed blockoff plates.

Image link:
http://www.facebook.com/album.php?ai...7&l=b5e6bcea0d

Thanks for helping!

Eric

[Gearhead119]

usually when a valve is burnt like that it is a sign that that vavle adjustment was too tight. ... the missing chunk is caused by the valve being held open ... slightly.. after ignition on the power stroke the hot combustion gases act kinda like a plasma cutter on the edge of the valve. Did you check the clearences before you dissasembled the motor?

[giligan]

No. I was too curious to know what was wrong... Thanks for the quick reply & explanation.

[Gearhead119]

Does the valve look burnt? not broke? from the looks of the picture that cylinder seems to be running leaner than the others.. or just got cleaned a bit more... once the valve seat/valve face seal is comprimised the whole gets big quick. The only other thing that I have seen break on a valve is .. alot of valves are 2 pieces... head and stem. I have seen the entire head fall off the valve. Do too the good condition of the surounding combustion chamber I would say the a foreign object is unlikely..

[giligan]

It's the second time that a exhaust valve breaks on my bike. Both times, I was accelerating hard with throttle fully opened... both broken valves have the same pattern. I took some other pictures with the first broken valve. Like you said, it looks like it was plasma-cutted, or melted. What are the signs of a burned valve? I will have to dissassemble the head in order to compare the valve seats.

More photos:
http://www.facebook.com/album.php?ai...7&l=b5e6bcea0d

Thanks again!

[Gearhead119]

There are a few things that can cause a burnt valve... either a foreign material such as carbon holds the valve open and allows it to get torched. another is the contact patch on the valve/valve seat is too narrow causing premature wear on the valve... this allows the valve to sink into the head (cupping) and eventually tigten all the way up. once the clearence is gone and more heat (high speed runs) is introduced, the metal expands allowing the valve to never completly close... thus torching the valve. Another is if the contact patch is too wide... a wide patch doesnt allow for proper carbon removal and leading back to scenerio #1. The contact patch should be roughly 1/3 the width of the valve face and towards the outer half of the valve edge.

Was the previous valve replacement the same one that burned up this time also? if so I would check the seat. use some lapping compound to make sure there are no low spots in the seat. i would suggest getting the seats (atleast the one that got torched) touched up. If it was touched up the previous time... you might recheck the installed valve stem hight. sometimes the valve tips need ground to get the vavle adjustment back in the middle of the shim range.

Are you using the stock valve springs? cams? sometimes aggressive cams and heavy valve springs can cause premature wear on valves too.

[giligan]

Original cams and springs are used in the motor. I can't remember if it's the same valve as the first time, but probably.. When installing the new valve, I used special stuff (kind of sand paste) applied on the valve and rotated against the seat in order to make that patch look okay. It seems that I was not doing it right.

Is it possible also that the lean running cylinder could cause the valve to melt?

Your comments are greatly appreciated! Thank you!

[DaddyDave]

sure looks exactly same size & shape of a shim ???

[compressor59]

I would question if your valve clearance might have been too tight. Also I have been told that you are not suppose to use lapping compound to lap the valves in because of some kind of coating or hard face applied to the valves. That is what Kwick told me when I was putting mine together.

[bkra13]

sure looks exactly same size & shape of a shim ???

Wonder if someone dropped a shim onto the head, forgot to clean it up and it's floating around and causing problems?

[Gearhead119]

Wonder if someone dropped a shim onto the head, forgot to clean it up and it's floating around and causing problems?

only three ways to get a shim into the combustion chamber... through the carbs, back through the exhaust, or through the piston (big problem!). If there is a shim floating around it would be either in the valve train area or in the bottom end somewhere. If there was a foreign material floating around in it and he was riding the bike it would have destroyed the head.

only foreign object could posibbly be is the screws that hold the butterfly plate to the throttle shaft. They are peened on the backside and RARLY come out... I have seen it though.... usually does lots of damage and you can see smashed screw marks in the head.

Some vavles shouldn't be lapped in but in this case.. I would lighty check it to see if the seat is going to seal properly.... if you don't want to use laping compound you can use prussian blue (spelling).. comes in a tube like lapping compound but is a dark blue paste with no grit... carful with it though.. you start putting it on your finger.. then it is on your arm.. then its on your face... then your back.... then the bottom of your foot!! That shit seems to get everywhere!!!!!!!!! Kinda like anti seize.

[giligan]

I'm sure it's not a lost shim that causes that problem. It's probably me and I assume it. That's experience I would say. I have to admit that starting the bike for the first time after doing a mecanic job is very exciting!

[1slomofo]

The factory service manual does in fact call for lapping the valves after cutting the seats.