DirtRider . Net MX, SX, Arena Cross, Off-Road Community
Dirt Rider . Net Text Version Home
Dirt Bike Dirt Bike Dirt Bike Dirt Bike

This is the text version of DirtRider.Net
Click Here for the Full Version


Pages: 1

KDX 200 Repeat topend failure

(Click here to view the original thread with full colors/images)

Posted by: specsys---------------------

1st failure, 4 point seisure, suspected to be due to excessively fast hard running causing piston to expand faster than bore.
Purchased new OEM cylinder, piston rings etc. At pick up of parts I queried the matching of piston to bore as the cylinder had no markings, whereas the piston was marked A. He checked with Kawasaki who advised that they no longer use A, B, C etc classifications for pistons and cylinders, they only produce one. At home I measured bore and piston with int and ext micrometers and had only .0015 inch clearance, this was less than recommended minimum of .0025 so I went to supplier. He measured with piston inserted in bore to hole below transfer port and used a feeler gauge and advised of a tight .002 clearance, and that this was ok. He fitted rings to piston.
Reassembled, ran 28:1 fuel with organic oil, clean filter new plug etc.
Started 1st kick, warmed up for ten minutes, let cool. Checked coolant and general temp, all ok. Ran up for 10 minutes gentle ride, up and down throttle and gears, stopped and checked, all ok. 1 Week later 30 minute gentle ride, all ok.
1 week later, approx 8 KM ride, never hitting wide open throttle all going great, and then burrr, loss of power, clutched it, pulled over, no compression as could kick over by hand.
Pull down: Piston has picked up at oil holes on both sides of piston and more significant pick up exactly at top ring gap location. "feels" oily. Top of piston is clean, with dark "straw" colour around perimeter, black in middle flowing through to front of piston. Clylinder is stuffed as plating damaged with vertical lines at front, and missing on inlet side.
I did not pull down the engine and check the bore for scuff marks after original running as my research suggested this was a waste of time and effort.
This cost me a fortune to rebuild, I did lots of research and was confident with original diagnosis. I was, and remain bemused by the suppliers measuring technique, and am convinced that my original measurements were correct.
I will now be engaged in an argument, and no doubt the same money.
More importantly, I want to fix it, and fix it properly.
Any comments appreciated


Posted by: blackduc98---------------------

I can't help you with diagnosis, but if you want a particular piston-to-bore clearance, then I would suggest sending your cylinder to some place like USChrome along with a piston of your choice (or you can buy a Wiseco piston from them). They will replate your cylinder (and repair it if necessary) and hone it to match the piston to whatever clearance you specify. It's cheaper than buying a new cylinder. Check out their web site: http://www.usnicom.com/


Posted by: 76GMC1500---------------------

We need pictures. I don't know what you mean by picked up or front of piston (it's round). A picture would really help us diagnose your problem.

What were your ring gaps? Did the piston rings butt?

You shouldn't have to take the engine apart to do routine cylinder inspections. Just look through the exhaust port.

I second what blackduc said about getting it replated and getting it machined to a specific piston to wall clearance. .0015" is tight for high performance engines, especially when running cast or forged alloys. Hypereutectic alloys can be run tighter.


Posted by: specsys---------------------

Just spoke to a reputable engine reconditioner who is adamant that the problem is not a clearance problem as their is no scuff or marking on the thrust faces of the piston. He is of the opinion that excessive heat has come down from the crown of the piston and become apparent on the thinner sections of the piston and exhaust areas.
But why!!!! The carb hasn't been rejetted, the radiators and water pump appear fine. The day before I had a mis firing plug (new) that was replaced the morning of failure. The reconditioner said that this may have initiated the failure in the approx two hours of running prior to failure through small damage that became worse. His words were that a misfiring plug didn't always mean running rich and could cause flashing and excess heat.


Posted by: specsys---------------------

I didn't check the ring gap as the supplier did this when I queried the clearance. he fitted the rings and checke the gap. Front of piston means the area against the exhaust port denoted by the arrow on the crown. Picked up means the aluminium of the piston has been "rubbed" up, or down, and left on the cylinder wall and or piston. Reconditioner advised that the thrust faces (exhaust and inlet areas) are heavier with the sides of the piston relieved "to hell" (less metal).


Posted by: KelvinKDX---------------------

Does the bike have an air leak anywhere that would be causing an excessively lean condition?


Posted by: wyatt---------------------

did the bike boil over or pressure up the cooling system at all. Some kdx's overheated due to bad head seal- or head warpage, causing detonation on piston crown and/or piston siezure. i have seen them run for miles after a rebuild only to overheat and sieze because the head wass warped and not resurfaced. Somewhere in the shop manual there should be a recommended piston to cyl clearance and or a ring gap tolerance if these arent followed nothing will work.


Posted by: ridejunky---------------------

You said you ran Organic oil. How fresh was it? They are far more unstable and break down much faster than petroleum based or synthetics could this be your problem?


Posted by: specsys---------------------

Coolant system showed no signs of overheating. The bottle level would rise and fall, and at pull down was normal. Organic oil was new, Castrol Two Stroke. head gasket showed no obvious signs of bypass, but to be honest, really hard to tell. Just checked head using a peice of glass and feeler gauges.there is between 2 and 4 thou clearance on both sides of head. Meaning, that a 2 thou will, and a 4 thou won't, pass under the two head bolts on either side. The single bolt at the rear, and the three bolts at the front sit firmly on the glass. Being unfamiliar with motorbikes, I am unsure if this clearance is within spec, nor if it can be taken up by the gasket.

Air leak and head warpage appear two possible contenders. With the spark plug being so new, less than an hour running, it is hard to tell if it was running lean. If the motor was so hot I don't understand why there wasn't an obvious "feel or smell" of hot. Fluid levels were normal.

Great support so far guys, keep it up.


Posted by: specsys---------------------

Small end replaced, big end checked prior to rebuild, and now, no obvious up or down movement. 21 Thou clearance on side play. But during inspection I found that the crankshaft has an obvious layer of oil on it, and a good look with a torch shows a small pool of oil in the bottom. This oil was not present at original rebuild.

So looks like the gear box case seal is leaking and head is warped.

Questions are: 1/ was the head warped causing the damage, or was it warped due to overheating.
2/ was the gear box seal damaged through overheating or overpressurisation or did it's leakage cause too lean a condition and subsequent overheating.
3/ if the gear box seal is gone does the oil cause too lean a condition, or can air also enter with subsequent leaning out.


Posted by: wyatt---------------------

residual oil in the crankcase is not unusual. A bad Gearbox side main seal will sometimes result in lots of smoke.

I would first recheck piston /cylinder clearance. Per manufactur specs. (oem and wiseco clearance are different) Check ring end gap. Cylinder bore for taper and out of roundness. The top of the cylinder and the head should both be checked for Warpage or flatness. At this point it is well worth the money to find a reputable shop to check this for you. The last step to be really sure is to pressure/ leak test your motor this will eliminate any possible air leak.


Posted by: blackduc98---------------------

Quote:
Originally Posted by specsys
Just spoke to a reputable engine reconditioner who is adamant that the problem is not a clearance problem as their is no scuff or marking on the thrust faces of the piston.
So where are the scuff marks? Pictures please! Words are extremely easy to misinterpret, particularly given the slang differences between dialects of English spoken around the world. For example, when you say something like "clearance is 21 thou" I have no idea what your unit of measure is. Please don't be offended - I don't mean to sound pedantic, I just want to make sure I have a clear understanding of what you're trying to tell us.

Quote:
He is of the opinion that excessive heat has come down from the crown of the piston and become apparent on the thinner sections of the piston and exhaust areas.
What does the underside of piston crown look like?
Quote:
The carb hasn't been rejetted
But how IS it jetted? Are you sure it isn't excessively lean? And what did the mis-firing plug look like? What does the latest plug look like? And what about the plug before your original rebuild effort? Can you see tiny specs of aluminum on the plug if you view it thru a magnifying lens? Photos would be helpful, but I said that already.


Posted by: 76GMC1500---------------------

Your reconditioner told you the exhaust side of the piston was the "heavy" side. This is not true. The sides of the piston with the wrist pin bosses are the heavy part. The piston is oval shaped beacuse of this. It is widest measing from the intake side to the exhaust side. The materials here are the thinnest and the piston expands the least. Around the wrist pin bosses, the piston is the thickest (heaviest) and expands more so the clearances have to be greater.

Excessive blow-by of the piston rings can overheat a piston and melt things down. If both piston rings collapsed, they could allow excessive combustion gasses to leak-by. Collapsed rings are usually caused by overexpanding them during installation.

Aluminum specs on the plug would be an indication of detonation which could overheat the piston crown.

We really want to see pictures.


Posted by: whenfoxforks-ruled---------------------

Does the bridging on an oem cylinder come relieved for expansion, did it seize in the area where cylinder bridging is in line with? Cylinder to piston clearance (.002) + bridge relief (model specific? .002?) total of .004 at bridge, less will cause seizure!


Posted by: hart125---------------------

I've had something similar recently that ended up being a bad carb. no matter how I jet the thing it always had lean areas. it wasn't a big problem when my cyl had a lot of clearance but with a new cyl and piston it got hot and you could tell most of the marks were above the ex port. That tells you it was a lean problem. I swapped out my carb with my dads 200 and it ran a lot better and a lot richer. it was jetted exactly the same as my orig carb!! I swapped out the carb. nobody's fault but I wanted to blame someone.


Posted by: specsys---------------------

Ok guyz, I can't figure how to post pictures, have sent a query through.
To recap, 91 KDX200, one owner from new until 6 months ago. No problems, motor never touched. Always run on organic oil. A french man, novice rider, from cold, 1st gear wide open throttle, about a mile on a flat beach before original failure. 4 point seizure, piston, rings and barrel ****ed.
New OEM barrel, piston and rings, clearance checked prior to assembly with internal and external micrometers with 1.5 thousands of an inch clearance. Barrel measured same over 8 nmeasurements, top to bottom, side to side. Piston tapered, with minimum clearance of 1.5 thousands of an inch measured just below the intake cut out on the piston. Motorbike shop mechanic checked clearances with piston in barrel and using a feeler gauge and said it was a tight 2 thou clearance, assembled rings to piston. I reassembled engine. Nothing else was done to engine. Kicked over by hand with no plug to check for any conflict, started on 1st kick when plug inserted.
Bike run for 2 ten minute periods with full cool down in between. one 30 minute ride through gears 1 to 3 with 1/2 to 2/3 full throttle, bike stalled and wouldn't restart. Diagnosed new plug misfiring with short between centre electrode insulation and internal face of screw thread. Mis firing plug was damp, and black. New plug installed and bike taken on 30 minute ride between 1/2 and maybe 80% ful throttle, all gears and loss of compression. New plug has very slight blackening, no visible signs of aluminium specks when viewed with magnifying glass. Underside of piston crown is clean, no visible signs of products of combustion. Crown of piston is clean with dark straw colour around perimeter and black in centre and out towards exhaust face of piston. Piston has scoured vertically exactly on oil holes at front of piston and at the pin of the top ring at the back of the piston. The thrust faces, dead centre front and back, have no significant marks or signs of contact. The cylinder head was not measured prior to assembly, but now has been checked to have a 3 thousands of an inch clearance on both sides. I can provice pictures to email accounts if you contact me at specsys@xtra.co.nz.
I spoke to another bike mechanic who said this is a typical clearance problem that was common on these and Susuki PE bikes. When he was working in the states he claims they used to take to the corners of new pistons with sand paper prior to assembly!
Cheers guyz.


Posted by: 76GMC1500---------------------

I do all of my image hosting at www.photobucket.com, it's really easy. Just copy and paste the link with the IMG code over to the forum.


Posted by: specsys---------------------

http://i118.photobucket.com/albums/...Piston001-1.jpg
http://i118.photobucket.com/albums/...DXPiston006.jpg
http://i118.photobucket.com/albums/...DXPiston005.jpg
http://i118.photobucket.com/albums/...DXPiston004.jpg
[IMG]


Posted by: specsys---------------------

http://i118.photobucket.com/albums/o106/specsys/KDXPiston003.jpg[/IMG]
http://i118.photobucket.com/albums/...DXPiston002.jpg
http://i118.photobucket.com/albums/...DXPiston001.jpg


Posted by: whenfoxforks-ruled---------------------

THIS IS CALLED:NO RELEIF AT THE BRIDGING! you need piston to cylinder clearance and bridge relief extra!


Posted by: BSWIFT---------------------

I may be way off base but this looks like a lean condition, overheat type seizure. Running the bike on the sand is a heavy load and a lean condition would show itself much sooner than moderate woods riding. The stated expansion of the piston appears to be evident in the pics suggesting that the clearances are too tight.
Also, being a 91, the air boot could have some cracking and allowing too much air, again, causing a lean condition.
What are the outside temps that you are riding in? The colder the outside temps, the leaner the engine is running.


Posted by: whenfoxforks-ruled---------------------

Quote:
Originally Posted by BSWIFT
I may be way off base but this looks like a lean condition, overheat type seizure. Running the bike on the sand is a heavy load and a lean condition would show itself much sooner than moderate woods riding. The stated expansion of the piston appears to be evident in the pics suggesting that the clearances are too tight.
Also, being a 91, the air boot could have some cracking and allowing too much air, again, causing a lean condition.
What are the outside temps that you are riding in? The colder the outside temps, the leaner the engine is running.

And the first run did sound like a cold seizure!


Posted by: specsys---------------------

It was about 19 degrees celcius, approx 69 farenheight. How could air boot cracks cause too lean a condition? Surely the carburettor venturi/jetting relationship determines the air fuel mix. I underside the impact of air leakage on the suction side of the carb as it adds extra air to the metered air fuel ratio leaving the carb resulting in an altered air fuel ratio. their was a small amount of filter oil sitting in the bottom of the air boot, and on the entry to the carb.


Posted by: whenfoxforks-ruled---------------------

Quote:
Originally Posted by specsys
It was about 19 degrees celcius, approx 69 farenheight. How could air boot cracks cause too lean a condition? Surely the carburettor venturi/jetting relationship determines the air fuel mix. I underside the impact of air leakage on the suction side of the carb as it adds extra air to the metered air fuel ratio leaving the carb resulting in an altered air fuel ratio. their was a small amount of filter oil sitting in the bottom of the air boot, and on the entry to the carb.

If it is cracked on intake side yes, it would make your engine run lean, but I don't think that is what happened to your engine.


Posted by: 76GMC1500---------------------

I would expect more damage on the crown from a lean condition, detonation is the usual mode of failure.

It's still possible that the piston was too tight in the bore. It is better to error on the loose side than the tight.


Posted by: hart125---------------------

thats not heat from lean.. that would be only on the exhaust side right in the center of the exhaust port and top of pistion. this is on the intake side and exhaust sides not center. could be too tight since the failure isn't near the top of piston, mostly at the middle or bottom which is tighter than the top. I think the sign to consider is that it happened right at the relief holes.. I haven't seen this before. I'd be sure that the premix is done right with really good oil.


Posted by: whenfoxforks-ruled---------------------

Bridge releif on the old iron cylinders was like .004 inch between the piston skirt and the bridge. You say that the piston to bore was a tight .002, good, but what about where them seizure marks are at? The oil holes are to provide lube to the bridge and insufficiant clearance at this point will cause seizure. Also on a cold seizure I would think its the same points. Either way the piston expanded or the bridge expanded, The manual usually covers this area.


Posted by: specsys---------------------

An assembly error caused this problem. The engine had overheated, it was not due to a clearance problem, nor premix ratio, type of oil, air fuel ratio, nor the way it was run in.

I made the mistake of having the air dam within the reed valve assembly 180 degrees out. This meant that the inlet air fuel mixture was shielded from the left hand side of the rear of the piston(inlet side) and subsequently a lack of lubrication caused excessive heat build up and final rooting of piston and cylinder lining.

The white plastic air dam is offset to receive the angled air from the carburettor rubber boot and distibute evenly to the inlet port. If rotated 180 degrees the off set is reversed and shields the left hand side of the engine.

An exspensive lesson learnt, take a bit more time in assembly (it was obvious in hindsight), or get a manual!!

The thing that caught me out ws teh lettering "DX" that I mounted right way up, whereas I should in fact have aligned the two tangs that are provided on the cylinder and mounting plate respectively.

It was through the forums help, discussion with a reconditioner for piston failure diagnosis and the motor bike mechanic for insisting that it was a lack of lubrication or reckless running in that I was able to build a picture of the nature of the failure and locate the now obvious assembly error.


Posted by: BSWIFT---------------------





Text Version Home





vBulletin Copyright ©2000 - 2009, Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2009 - Created by Stefan "Xenon" Kaeser