Is this cylinder wall okay?

[Daniel.]

Hi guys,

I think this is in the right section, please excuse if it's not.

I'm still learning the ropes and I wasn't sure if this amount of scoring is something to be worried about? I'm kinda getting the worst of it in these pics, everywhere else seems quite nice in comparison.

the crap sitting on the piston is from when I pulled the spark plug without blowing air down the hole first (still learning the ropes). I've got compression issues and i'm suspecting it's the valves (valve seats are blowing bubbles when i shoot compressed air up through exhaust and intake inlet) so i'm gonna be lapping those soon and i'll blow out the crap on top of the piston now while the heads off.
I don't suppose the scoring could indicate compression loss from the rings or anything like that could it?

Cheers,
Daniel.



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[my67xr]

what water cooled engine is that a 250cc 4V Zongshen ?
all looks pretty normal.

but do you have an oiled foam filter on the carby ?
the top of the piston looks a little dusted, dirt getting through the intake

 

[zwebx]

Up the top of the bore seems to be the issue, run your fingers over it if there are bumps then its unusable or wont last long (the rings wont if they seal at all)
looks like a yamaha yzf bore (or wr) 3valve inlet but i debut it

 

[motorman]

I would be honing that cylinder regardless

 

[zwebx]

I would be honing that cylinder regardless

unless its a nikacil..

WHAT BIKE IS IT?

 

[motorman]

Doesn't look like it to me?

 

[Hillz]

unless its a nikacil..

WHAT BIKE IS IT?

you can still hone nikasil

 

[Daniel.]

It is infact a yz250f and I always keep the foam filter well oiled and in place. I was under the impression these cylinders had that nikacil coating so I kinda wanted to avoid honing if possible. I don't remember feeling any bumps when I rubbed my finger along it last time but I'll have a more in depth look now and let ya know

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[Hillz]

unless its a nikacil..

WHAT BIKE IS IT?

you can still hone nikasil

looks like a thrashed out YZ250F to me

 

[zwebx]

you can still hone nikasil

looks like a thrashed out YZ250F to me

Needs to be done correctly. dont hone a nikacil with ball hone, normal hones get advice from a pro.. the normal stones get embedded in the nikacil bore coating..
maybe the best thing to do is go to a big bore kit as because re-coating is too much if the current coating is too far gone.

 

[Hillz]

WD40 & your normal Tri stone does the job... even motion

but it would say that barrel is cactus, piston looks horrible also

 

[zwebx]

Hi guys,

I think this is in the right section, please excuse if it's not.

I'm still learning the ropes and I wasn't sure if this amount of scoring is something to be worried about? I'm kinda getting the worst of it in these pics, everywhere else seems quite nice in comparison.

the crap sitting on the piston is from when I pulled the spark plug without blowing air down the hole first (still learning the ropes). I've got compression issues and i'm suspecting it's the valves (valve seats are blowing bubbles when i shoot compressed air up through exhaust and intake inlet) so i'm gonna be lapping those soon and i'll blow out the crap on top of the piston now while the heads off.
I don't suppose the scoring could indicate compression loss from the rings or anything like that could it?

Cheers,
Daniel.

View attachment 9615View attachment 9616View attachment 9617View attachment 9618

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"I've got compression issues and i'm suspecting it's the valves (valve seats are blowing bubbles when i shoot compressed air up through exhaust and intake inlet) so i'm gonna be lapping those soon and i'll blow out the crap on top of the piston now while the heads off."
Take the valves out, replace the seals while your at it.. inspect the valve stem for wear at max lift of the cam (approx with the spring out) and move it side to side it shouldnt move very far or at all..
turn the valve around and see if that changes anything (could be bent) put the valve in a drills chuck and spin it to see if its bent or anything like that that can also cause it to leak and then wear out the valve stem (if is a little bent) and then end up seizing..
Inspect the valve seat for dirt.. wear (pretty easy) and clean with brake cleaner will help get the carbon off
"I don't suppose the scoring could indicate compression loss from the rings or anything like that could it?"
This looks like normal wear on the bore tbh, to tell if the rings were warn take the piston off and look under the rings there will be carbon built up on it if compression was getting past (and if theres heaps then there was heaps of compression loss) also then you can take the rings off and measure the ring gap if it intrests you..
Also could have just overheated one time and came back like this..

If it were my bike i would replace the piston and bore with a BB kit or another cylinder
i would replace the valves.. and i would also inspect the cam chain for wear because it looks like its done some work.

Also when the piston is off check the big end bearing for up and down movement and the wrist pin for movement (up and down and diagonally)

Snaptrax recently rebuilt one of these.. Maybe just maybe he may have a bore laying around (he went bb kit)

 

[Daniel.]

Can you hone a cylinder without having to change piston rings?

Since the bike has been in my possession I've never thrashed it out. I got a receipt from the fella listing $1500 in parts and labour for a top end, honing was on the list. It was in the shop from May till September last year from memory because they were waiting for parts? Maybe he thrashed it before handing it over or something? Or maybe they just did a quick hone to remove some of the scoring that he had produced over the years instead of boring?

What are the reasons for a cylinder getting scores like this?

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[Daniel.]

"I've got compression issues and i'm suspecting it's the valves (valve seats are blowing bubbles when i shoot compressed air up through exhaust and intake inlet) so i'm gonna be lapping those soon and i'll blow out the crap on top of the piston now while the heads off."
Take the valves out, replace the seals while your at it.. inspect the valve stem for wear at max lift of the cam (approx with the spring out) and move it side to side it shouldnt move very far or at all..
turn the valve around and see if that changes anything (could be bent) put the valve in a drills chuck and spin it to see if its bent or anything like that that can also cause it to leak and then wear out the valve stem (if is a little bent) and then end up seizing..
Inspect the valve seat for dirt.. wear (pretty easy) and clean with brake cleaner will help get the carbon off
"I don't suppose the scoring could indicate compression loss from the rings or anything like that could it?"
This looks like normal wear on the bore tbh, to tell if the rings were warn take the piston off and look under the rings there will be carbon built up on it if compression was getting past (and if theres heaps then there was heaps of compression loss) also then you can take the rings off and measure the ring gap if it intrests you..
Also could have just overheated one time and came back like this..

If it were my bike i would replace the piston and bore with a BB kit or another cylinder
i would replace the valves.. and i would also inspect the cam chain for wear because it looks like its done some work.

Also when the piston is off check the big end bearing for up and down movement and the wrist pin for movement (up and down and diagonally)

Snaptrax recently rebuilt one of these.. Maybe just maybe he may have a bore laying around (he went bb kit)

Thanks mate, I'll go over all that tonight if I have time otherwise tomorrow. I'll have to check the receipt but there was definitely valve work done (whether they were replaced or just lapped I'll have to double check)

But I'm starting to suspect some sort of foul play. Still figuring it out though.

I'll have to look up some tuts about piston rings and that, I've never been passed the head before *gulp*

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[zwebx]

[video=youtube;vaFp8aYkHNA]https://www.youtube.com/watch?v=vaFp8aYkHNA[/video] That will get my point across..
you can only get nikacil a tiny tiny bit larger.. then you would end up on a c sized piston thats marginally larger and probably wont take out any of the wear on the top of the barrel.
my advice is dont take it to a guy.
REplace the bore with another and new piston and rings.
Do the head as posted in the other post and inspecting cam chain, guides

probably set you back around 800 bucks done at home... with a big bore.. put it all back together and re-jet if its bigbore..
Then ride for another 100 hrs and inspect later on knowing its not going the seize in that time.

 

[SwapperMX]

Never ever lap titanium valves for one. Some guys will use an old valve to lap the seat before installing a new valve, but there are better ways to do it. Also, square bore x stroke on all modern motocross 4 strokes puts a lot of wear on cylinders. Measure it and guaranteed it will be well out of spec. They should be replaced and are relatively inexpensive.

 

[Daniel.]

s.hit I wasn't aware you couldn't lap titanium valves.. How do you go about reseating valves in these bikes without forking out for a new set?

I've been wanting to sell this bike for a while now to get a registered wr or something so I'm not looking to spend massive bucks here

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[my67xr]

you normally just clean them up and get the seats recut in the head, or re-angled to suit your valves
then the springs will probably need reshimming to suit too.

 

[Cactus Jack]

If it's not blowing smoke as it revs higher or when backing off then the bore is OK ... Obviously a perfect bore with no scratches is the best way to go ... Loss of compression without any smoke issues indicates either loss of valve clearance or poor valve seal (IE valve face non concentric to the seat due to a bent valve or just the seating faces worn 'til the bucket touches the base circle of the cam) ... Since you've already checked the head and found leaks then the head is what you'd fix first . With late model motocross heads everything is critical . Old school valve seat cutting and lapping methods just aren't good enough . The guides need to be in spec , then the head is set up in a machine that machines the seats dead square and centred to the guide bores , they have to be perfect ... Here's an ebay listing from a head joint and in their description they explain everything you need to know ... YZ450F Head Service Machining Upgrade CRN PVD Titanium Valve Installed | eBay ... Bore scratches are caused by grit either in the sump oil OR getting in past the air filter ... Nikasil is an extremely hard coating but it can be damaged by abrasives such as sand and dirt grit ... A new big bore kit would obviously solve any possible bore problems ... I'm rebuilding a CRF150R motor and just bought an Athena 69 mm big bore kit . The cylinder has wider than stock water jackets and the cylinder sleeve is made of IRON ... They state that the piston should be checked and replaced after 15 running hours in order to keep the cylinder "tube" round ... And obviously the crank should be checked using the rap test since it won't last too long under the extra load of a BBK ... I also bought a stage 2 Hotcams cam to go along with the extra bore capacity ...

 

[Daniel.]

So I've pulled the valves out and had a look. There's a little bit of side to side movement in the valve guide.
Do you think the amount of carbon built up around the valve head and seat could be causing the compression loss? what do you guys use to break down and clean it? Brake cleaner/contact cleaner/degreaser/all of the above?

Anything critical here a beginners eye might be missing?

Really appreciate all your help guys!