tweaked valve on yz 250 F

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#1
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I found the problem....wasn't the timing(past thread). I have a bent valve....i hope :think. I can't thimk of what else is wrong. with the cams out, i slowly push on the kick starter and i hear a hiss coming from one of the valves. There should be enough compression so i couldn't push through the compression stroke so air must be leaking from somewhere. Brand new gasket so it ain't that. Oh well time to get the valves done....poor me.:(
 

biglou

#2
An Unconventional Idea Here:

I read this thread earlier and a thought just occured to me. You may be able to pinpoint your leak by spraying a light soap & water solution around the gasket seam and then around the valves. If you can hear a hissing sound you should see bubbles immediately. I know this is totally unrelated, but this is a method of finding leaks in natural gas pipelines and home services (short of using the electronic sniffer). If you have a very slow leak, the bubbles will be very fine, almost fuzzy-looking. Keep in mind you will have to cycle the kick start lever to build pressure within the cylinder. If your leak is bad enough to actually hear it as you have described, I have a feeling you are going to see the problem right away.

I don't know if this is a BAD idea, having never done it on a dirtbike, but I would think that if you used the soapy water solution sparingly you shouldn't harm anything. Once you pinpoint your leak, you may want to remove the solution somehow. And, hopefully, if you have to take it in for repair, you will be more knowledgeable about your particular problem when you turn the bike over to the shop.

If anyone is certain that this is a very bad idea, please post! I haven't tried this but it seems to me that it should work.
Hope this helps. :)
 
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#3
The idea about the soap is not bad, and has been used for years -- but it's easier to use WD40 because the engine likes it much better than soap and water.

About the "bent" valve - leakage may not be caused by a bent valve, it could just be some crud stuck to the valve holding it open. It could also be that the valve seat wasn't ground correctly from the factory and it's allowing air to flow past.

Basically, here's the only real way to check this -- put the cams back in, button up the engine and do a leak-down compression test. A local shop can do this and will probably charge you about $25 - $40 if the engine/bike is clean.

If they do find leakage - then you'll take the head off and use either soapy water or WD40 to check for valve leaks. Turn the head upside-down, so the dome is facing you. Spray around the valves with your stuff, shoot compressed air towards the back side of the valves (next to the springs). Look for bubbles. If you see 'em, then you know which valve is leaky/bad. Check all the valves, it may be more than one.

Remove the suspect valve (only one at a time). Visually inspect it for unusual wear patterns, check the seat also. put the valve stem on a known flat surface, or hold a straight edge against it - check for a bent valve - it won't be much, so look carefully.

If none of these checks show problems, put a tiny amount of Prussian Blue (just enough to color the entire rim) on the valve where it contacts the seat, drop the valve into the guide and rotate it gently - once or twice. Now remove the valve and look for ink on the seat - it should be coated in a complete circle - if it isn't, then the seat is the problem. It may have dropped (almost fallen out), or it could have a dent in it.

If you're not comfortable performing these checks, you can take it to a reputable Yamaha dealer and have them do it for you. The thing is, some shops don't do very good head work, so make sure a good mechanic will be doing the work.

Hope this helps you out.
 
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#4
The piston only hit one valve, the middle back one. The mark it made isn't bad...barely noticeable. But i don't know if it was enough to tweak it. I've run out of idea's. I'm taking the head in tomorrow to see if its the valves. I can't see any gaps or anything by eye and i thought there would be if i could actually hear the hiss. What other things could it be to make it hiss???????? Plugs tight, gaskets good, Has to be the valves.......
 
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#5
If you can actually see a "dent" where the valve hit (even if it's small), then it smacked pretty hard. Normally they'll just leave a ghost pattern on the piston where there's less carbon buildup, and on the valve with a grey area.

I think you've diagnosed it well. They'll have to replace the valve and grind the seat to match the new (unbent) valve. The guide should also be checked. The other valves (and stem seals) should be replaced to, just on gp. They can do a 3 angle valve grind on the seats at the same time, this'll give the head a little better flow, so you'll get a bit more hp for all this work :)

Have you determined the reason it occurred?

This would be the first thing I'd be figuring out.
 
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#6
The reason it happened is cause....well...I'M A COMPLETE IDIOT and didn't tighten the oil plug in the frame and it pull out after 1 lap in my race and i siezed the motor. A ring stuck and the piston turned sideways in the cylinder than hit the head.:o I pulled the valve and it was straight. Good thing cause JUST ONE of those valves is 150$ CDN....titainium is pricy. They dont need to be bent after purchase either. After tearing my bike down about 90 times, i realized somehing sooooo obvious.....the rings need time to ceet.....duh. After i put it back together and kicked till mine and my buddy's leg was numb, it fired right up. runs great. Work of advise.......dont screw up your valves.....pricey $%^&!!!!!!!! :eek: Thanks for helpin out though!!!!!

PS Righty tighty,.......have to remember that one.;)
 
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#7
didn't tighten the oil plug in the frame and it pull out after 1 lap in my race and i siezed the motor.
D'oh! Hate it when that happens!

Lucky it was a relatively cheap fix.

You're not breaking it in on synthetic oil, right?
 
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#8
No, i'm not breaking it in using synthetic oil, but why? What the deal with that? If you call a new cylinder and piston+rings cheap.....you have tooooo much money:confused: . Actually its reasonable. It turned out to be a big bill for me cause i just ordered a pipe a week before i blew Betsy up, so my pay check what gone:silly: I was glad that those valves didn't get busted....ffeeeeewwwwww! With my new piston and PC pipe....my bike is a 125 eater, I almost feel bad....almost!
Well off to the races and now Betsey has me all happy.
Thanks for the help dude....much ablidged!!:)
 
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#9
If you call a new cylinder and piston+rings cheap.....you have tooooo much money
It was a cheap fix -- considering what could have happened with a low oil/no oil situation. You got away cheap!

No, i'm not breaking it in using synthetic oil, but why? What the deal with that?
I don't want to get into an oil debate, but basically, synthetics are too slick and don't allow rings to seat correctly.

Have fun!
 

jimt_yz400

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#10
But once broken in you should be OK to run a synthetic. It's only on break-in that it's a concern.