2008 KX250F Cam (scuffing)

[IndyMX]

My brother noticed a new noise on his bike last Wednesday when we were riding..

I had a listen, it's definitely not a normal sound. Thinking that there may be something going on in the top end, I pulled the valve cover.

This is what caught my eye. This is the only cam lobe of the 4 that is doing this. The clearance on this valve is dead on spec.
The other intake is 1/1000th tighter. The exhaust valves are in spec.

There's no other indication of a problem. Oil level is fine, with no noticeable shavings or anything like that.

I'm thinking I might as well pull the head and cylinder to make sure everything is fine there.

Does this look like normal cam wear?? (the very tip of the lobe doesn't appear to be scuffed, just the front and back of the tip.)

 

[whenfoxforks-ruled]

Is it leaving abnormal wear marks on the rocker arm?

 

[IndyMX]

no rockers on this bike Bob.. the lobes run against the buckets...

I've not had a chance yet to pull them out, but I'm thinking it might be a good idea to tear it down, at least pull the cylinder.. that way I can check the whole top end very thoroughly..

 

[whenfoxforks-ruled]

Yeah yeah yeah, I was thinking push rod to begin with! Something is hanging up, bad guide? Causing the BUCKET to get too hot against the cam lobe?

 

[IndyMX]

Hmmm... I wonder..

Maybe something under the bucket??

I'll have it torn down this week. Need to get to the bottom of it.

 

[whenfoxforks-ruled]

Or the bucket hanging up?

 

[IndyMX]

Yeah.. I guess a tear down is in order..

Wonder if there's something under that bucket that shouldn't be there.

Wonder if a valve spring is starting to fail, and he was just lucky enough to notice and shut it off in time.

Hmm.. I almost can't wait to find out!

 

[Rich Rohrich]

Indy - How does he usually warm up the bike and what type of riding does he normally do?

 

[IndyMX]

We ride strictly track. No trails.

He usually lets it idle for until it'll run without the choke, then take a couple slow laps.

He's never on the rev limiter.. I'd say he tends to be very easy on the bike, comparing him to a lot of other guys I know.

 

[Rich Rohrich]

Is the wear greater on the front side or the backside of the lobe?

Do you know how did he break in on the bike?

 

[IndyMX]

It looks pretty even, as far as the width of the wear.. but the leading edge of the lobe seems to be wearing a bit more harshly.

He did a couple heat cycles. Been riding it for a year now. Never an issue until this past week.

 

[whenfoxforks-ruled]

Radiators are warm, its ready to ride.

 

[jsantapau]

I would test the valve spring to see if it weak and or possibley the bucket(cam follower) not setting in its bore correctly

 

[whenfoxforks-ruled]

How many seconds does a modern 4 stroke take for the oil to reach a full purged flow?

 

[IndyMX]

I don't know.. how long? If I was sure it wouldn't make a huge mess, I'd run it for a bit with the valve cover off, just to make sure it had good flow.

 

[Ol'89r]

Indy.

Hard to tell from the pics but, looks like he is losing the hardfacing on the cam. Possibly from dirt or a bucket binding. Could even be just a bad cam. Make sure the bucket is moving smoothly in it's bore and has no vertical scratch marks in the bucket bore. (Indication of dirt) Also, look at the wear pattern on top of the bucket. The bucket should rotate slightly in the bore giving an even wear pattern on top of the bucket. If the wear pattern looks different than the other buckets, it may be binding.

It is very, very, important to make sure you don't get any dirt or grit in the top end when you check or adjust your valves since it only takes one little piece of grit to cause this.

Just my $ .02

 

[IndyMX]

Thanks Terry, I'll check that..

 

[biglou]

I've posted this before, but, favorite bucket and shim tool = magnet on a stick. The magnet keeps the shim stuck in the bottom of the bucket in case the surface tension of the oil lets go. Very handy for reassembly as well. Also, if it's binding, you'll feel it. The magnet handle gives you more to grab onto than trying to grab that sharp-edged, slippery bucket with 1mm showing with your fingertips. :)

 

[IndyMX]

Yeah, I agree Loubre.. I used a magnet to pull the bucket.. But first, I tried to spin it in the bore with my finger. (that just sounded way wrong) It moved quite freely, no binding at all. There's no indication of a problem on the bucket at all. It looks brand new, top and sides.

 

[IndyMX]

one more

 

[Rich Rohrich]

Indy - That type of wear isn't going to cause noise.

 

[IndyMX]

I was sorta thinking that myself.. however, wasn't sure.

Hmmm.. Suppose I should pull the head and check the valves & piston??

 

[Patman]

How many hours on it?

 

[IndyMX]

I begged him to put an hour meter on it when he got it, but he refused..

I haven't a clue.

 

[Rich Rohrich]

Hmmm.. Suppose I should pull the head and check the valves & piston??

It's worth popping the top if only to get a reference to work from in terms of wear and tear.

My experience has been that cam scuffing like this in an engine that doesn't live at the redline is usually related to an excess of idling and slow speed running assuming the oil and filter is changed at the proper intervals, and the right oil filter is being used.

The area of greatest pressure on the cam (and the place where the oil has to work the hardest) is the nose of the cam . At idle and low speeds the cam has to work extra hard to overcome the seating pressure of the spring, at a time when oil pressure is fairly low.

This is the reason you have to break in a new cam by running it above 3000 rpm minimum, or run it in with super soft springs and then switch to full pressure race springs once the parts bed in. The extra rpm helps to sort of loft the bucket over the nose and keep the oil pressure up so there is a nice solid hydro-dynamic wedge between the parts so parts can bed in gently.

Plonk along type trail riders, guys who lug the engine, but especially guys who idle the bike for extended periods of time get this type of wear all the time. A high zinc oil might help, but there are no oils in existence that can cure the physics of springs designed to run safely to 13,500 rpm if you always run the engine at low speeds.

Looking at the specfics of his maintenance and his riding style should give you some useful clues as to why things are wearing this way.