TTR-90 new bike / bent valve

[Jon K.]

My TT-R 90 has a bent valve. The bike is still new, I have not changed the oil yet. It is out of warranty, we got it quite some time ago.

Anyone have similar experience?

My baby girl was just riding along, shut it off, when we tried to crank it back up it has no compression, and a bunch of clearance under the valve.

I haven't pulled it down yet, maybe something else has happened, but I don't know what it might be.

 

[Senior KX Rider]

I bought Erics XR-70 new in 01 and he chose not to ride it for a while and stayed on his JR 50. In 02 he started riding the XR and with about 5 hours on it it did the same thing you describe. It was a bent valve and I have no idea why. Those motors are said to be bulletproof. :eek: Have not had any trouble with it since

 

[Rich Rohrich]

Patman's kid bent an exhaust valve on his XR80 when it was fairly new. I figured he did it on purpose so Pat would spring for a trick engine rebuild :) I never got the details on exactly what happened but it looked like the valve lofted off the nose of the cam at excessive rpm and poked the piston.

If a spring rusts while it's sitting it can cause some pretty weird things to happen, so doing a visual check along with checking the springs for open pressure and free length is usually a good place to start.

 

[Jon K.]

I got out to the shop and have cleared off a place on the bench, but havn't pulled the engine down yet.

We had been running the thing all day, and certainly no excess RPMs.

As soon as I get it pulled down, I will report back.

Thanks

 

[Patman]

I really don't know what happened with the bent XR80 valve but considering he came off of a 2 stroke and was getting comfortable with the power and weight of the new ride I suspect he reached the end of the available power supply and over rev'ed. That is not an issue now for sure though! :aj:

 

[Dewster]

I figured he did it on purpose so Pat would spring for a trick engine rebuild

How do you bend a valve on purpose? I don't think over rev will do that. Jumped chain maybe... but over rev :think: .

If so let me know, I've been needing a reason to put Thumper's 340 kit in my bike. :aj:

 

[Patman]

I suspect he floated the valve with too many revs and it kissed the top of the piston.

 

[XRpredator]

Yikes! I'll have to keep an eye on Pred2's TTR90. So far, it's on its second year with no real problems. Heck, I finally even cleaned the air filter!

 

[Jon K.]

I got it pulled down last night. Sure enough the valve had kissed the piston, though certainly not from over-reving.

Best I can tell, over the winter, the bike was stored with the fuel left on. :bang: This spring, I did indeed have to go through the carb to get the goo out of it.

Now the funny part. And this is really just a theory. At some point the carb must have percolated fuel down through the intake and it looked to hyave settled on the (closed) valve, evaporated from there, and formed a fair amount of goo on the valve itself.

When we went riding last Sunday, the valve must have heated to the point of cooking the goo into . . . . . . well . . . . . some sort of harder goo. The valve is coated with the stuff! I am hypothesizing that the valve got mired in this stuff and hung open.

Anyone ever hear of such a thing?

Oh, and getting that valve out was interesting. To get the intake valve out of the one-piece head, of course the intake rocker must be removed. To get the rocker out of the head, the cam must be removed. To get the cam out of the head, the friggin' exhaust rocker must be removed! :eek: Then comes the part where you try to compress the retainer and remove the itty-bitty split keepers.

Anyway, I need a valve, and I expect the thing to run forever!

Jonny

 

[Patman]

I wouldn't sweat it Pred, my son has now been renamed "BrandonWho" after his buddy BillyWho. It seems they both can break a bowling ball with a rubber mallet.

 

[Rich Rohrich]

When we went riding last Sunday, the valve must have heated to the point of cooking the goo into . . . . . . well . . . . . some sort of harder goo. The valve is coated with the stuff! I am hypothesizing that the valve got mired in this stuff and hung open.

Anyone ever hear of such a thing?

Sure. I've seen valves get hung up in the guide due to crusty oil deposits. Once that happens having the valve hit the the piston is pretty easy.
:ohmy:

How do you bend a valve on purpose? I don't think over rev will do that. Jumped chain maybe... but over rev :think: .

It's pretty easy to understand if you think about where the valves are in relationship to the piston, and why it's most likely to happen to the exhaust valves.

The piston sort of chases the exhaust valve near Top Dead Center (TDC) on it's closing cycle.
The intake is just starting to open and the exhaust is closing when the piston is near TDC. The piston is moving up as it approaches TDC while the exhaust valve is trying to get back on it's seat . On cams with reasonable amounts of overlap the exhaust valve is still pretty far open as the piston is hitting TDC. If you over-rev the engine the valve and rocker (in the case of the XR) or cam bucket can lose contact with the camshaft. When this happens it is no longer following the cam profile so the exhaust valve can easily be open too much when the piston is near TDC. If it's open more than the available valve relief clearance in the piston, then the two make contact and parts get bent. This is usually what people are referring to when they describe valve float or loft. This normally happens at high rpm, but as valve springs wear and lose some of their open pressure the rpm at which this "float" can occur much earlier. Low performance engines like the little XR don't have a lot of extra valve spring pressure to work with in a big over-rev situation so there tends to be less of a margin for error. Rev limiters will usually protect you from this situation but down shifting under load at high rpm will mechanically drive the rpm well past the normal rev limit and can easily get those valves floating around.

Valve clearances that are too loose can render the cam's closing ramp ineffective and cause seat bounce which can also damage the valves.

After I pulled the engine apart on Pat's son's XR80 it looked like valve float from over-revving was the root cause of his bent exhaust valve. Exhaust valves generally require as much as twice the piston to valve clearance that intake valves do. With intakes the valves are opening the fastest as the piston is moving away so the intake valve tends to chase the piston and dont require quite as much clearance.


Hopefully this makes some sense.

 

[Dewster]

Wow. I never heard of "valve floating" but it makes sense.

Learn something new every day. :cool:

 

[Vic]

Hopefully this makes some sense.

Perfect sense. Really good explanation.

:cool:

 

[Tony Tice]

I'm telling you all

Whomever this Rich guys is, he knows his chit man! :aj: :worship: :worship: :worship: :worship:

That was an excellent description. Text book stuff.

Tony

PS. Honest to God, Rich, I couldn't help myself. That brought a tear to my eye it was so perfect.

 

[Jon K.]

I felt that the "valve chasing the piston" vs the "piston chasing the valve" conveyed just the right amount of drama and intrigue.

 

[Tony Tice]

I agree.

Two Thumbs up.

What next Ebert? Lets head over to SMJ and see what they are viewing.

Tony :ride: :laugh: :|

 

[Rich Rohrich]

You guys are killing me. :rotfl:

Thanks for the kind words. :cool: