TRX250R.ORG
Workshop => Engine and Bottom End => Topic started by: GO OVRIT on April 20, 2014, 10:07:29 PM
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My 330 was running almost perfect. It felt a little off at wot in 4th-6th which I only get to use on one stretch of our track. I thought it felt rich, but now I don't know. It started easy, idled, and pulled good ever since I put it together this time last year. The other day, it ran great for a few 15 min hard runs. It never felt like it siezed and never shut off. It sat probably an hour, and when I went to load it, I couldn't get it started. I tried again at home with a new plug, and still nothing. It felt low on compression. I was in one of those moods where I just started pulling it down before I even put a guage on it. The piston has a little bit of burn on the front, but not close to the ring. The crown is uneven with a low spot the size of my thumbprint just back from the exhaust side. It's like it was right on the verge of blowing a hole through, but it cooled before it did. He cylinder is visually ok. No signs of seizure. I'm running 110/klotz super tech, a 38 a/s with 185, 58, dgh 4th clip that has a flat spot ground at the bottom, and a #7 slide. Ignition is a 96 cr250. Every time I've been too lean before its seized and caused more visual damage. Any ideas what I have going on.
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The people on here that are smarter then me are probably going to ask for a picture kind sir. Without one sounds like detonation. Either its getting hot and the fuel is igniting to early or you have to high of a compression ratio for the fuel octane you are running and its igniting early. (thats my random speculation but a picture will probably help people get you on the right path)
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Show us the piston crown, the skirts on the intake and exhaust side of the piston.
Show us the exhaust system and silencer.
Is the silencer a spark arrestor?
How old is the fuel and what type of container has it been stored in since purchased.
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It looks a little different now that it's pulled apart. The piston did burn down to the ring on the exhaust side. I never felt any seizure, but the front corners of the piston are both scared and the top ring is stuck. You can see slight marks in the cylinder, but can't feel anything. The pictures are not very good. The pipe is a standard trx5 from esr. Its probably been 8-10 hours since I packed the silencer. I got the fuel 3 weeks ago, and it stays in a sealed can inside my garage. It's cam2 r+m/2 is 110.
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For some reason. I can only get one pic per post to load.
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That's detonation and your jetting seems to be in the ballpark. I'd look to your stator side seal or a air leak somewhere. The reason I pointed out the stator seal it's often overlooked. Do a leak down before you run it again after a new top end, to ensure you don't have a leak. Also re-torque the head and cylinder base nuts after initial heat cycle.
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That's detonation and your jetting seems to be in the ballpark. I'd look to your stator side seal or a air leak somewhere. The reason I pointed out the stator seal it's often overlooked. Do a leak down before you run it again after a new top end, to ensure you don't have a leak. Also re-torque the head and cylinder base nuts after initial heat cycle.
What do you see that tells you the jetting is in the ballpark?
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Check out this link. I think it will help you in your search for the issue. http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html (http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html)
Look specifically under the section that says " scuffing on exhaust side only". This should give you some incite.
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I think you had a cold seize early on, and then a lean condition later on when it totally failed
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Check out this link. I think it will help you in your search for the issue. http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html
Look specifically under the section that says " scuffing on exhaust side only". This should give you some incite.
I went to the site whose link is above and looked at the article about two stroke seizure. There are a lot of great pictures of piston failures and a few good explanations of what caused the piston failure of the featured piston.
Some of the explanations of of what caused the piston failure were close to being correct for the type of piston failure, but he had the wrong picture.
Many of the explanations of of what caused certain types of piston failure were not even close to being right and also had the wrong picture.
I wish I had the time to spend all day reading all of the stuff that is on the web and adding comments based upon my engineering education and hands on experience. If I was grading the article on Piston Seizures I would give it a C- or a D+.
I would be happy to give you my opinion of what cause the piston failure in this thread but I still need close up pictures that are in focus and views of all sides of the piston as well as views of the underside of the piston.
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I'd love to hear your thoughts on it, but I don't have a good camera. As far as a cold seizure, I guess its possible, but I never ride/rev it before the cylinder is hot to the touch. As far as an air leak, I replaced everything and had zero gauge movement . I'll see what I can do about getting a camera.
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I hope you get some good pics ,because im interested in hearing what jerry has to say. is there scuffing on four corners of the piston, I can see one corner that is .can you see transfer wash on the top of the piston.If your running fat enough you can see clean spots on the piston were the transfers are and is the bottom side of the crown burnt black color
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Ok, I hope these are good enough.
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Ok, I hope these are good enough.
Ok, I hope these are good enough.
Visibly, the cylinder looks ok but shows signs of the rings being worn out for some time. Note the patches of blow-by above the exhaust port. Note the many other areas where the rings are not making even contact indicated by dull and shiny spots on the cylinder wall.
Lack of scuffs or seize marks on the intake side of piston shows that lubrication was still present between the piston and cylinder wall even though the piston clearance had disappeared and the piston was operating with a press-fit for clearance. The temperature on the exhaust side of the piston skirt and cylinder wall was enough hotter that the lubrication film could not hold up operating with a press-fit for clearance. When the lubrication film breaks down (reaches the critical temperature where the oil does not provide any lubrication) it only takes less than a second for the friction to generate enough heat to cause the surface of the piston to melt making the seize marks on the piston.
Scuffs and seizure marks on the skirt on the exhaust side of the piston, indicates the upper portion of the piston was larger than the lower portion of the piston. The seize marks being on the upper portion of the piston indicates the piston operating temperature was higher than what the piston was designed for. A properly designed and well-developed piston operating under full load and operating in the temperature range for which it was designed, will be round and have approximately the same diameter from top to bottom. At room temperature most pistons diameters are oval and tapered from top to bottom. Piston diameters are typically .005” to .015” smaller at the top than at the lower skirt. The oval/tapered shape we see on pistons at room temperature is the shape they need to have in order to move toward a shape that make full contact with the cylinder wall when the piston is up to normal operating temperature.
A piston will NEVER transfer aluminum to the cylinder wall or produce seizure marks on the piston if there is a working lubrication film. I have had numerous times in the controlled environment of the dyno room, using OEM pistons and plated cylinders, where the piston is over-heated and expands enough to have a press-fit for clearance. With the press-fit for piston clearance there is enough friction to stop the engine under full load and RPM. After such an occurrence, a tear down will show a polished bore without any aluminum on the cylinder wall or any seize marks on the piston. This is another reason that I believe in running a lot of good oil in the fuel.
Most lubricants cannot hold up to the tremendous heat generated in the areas where the piston to cylinder wall clearance has become a press fit, especially in iron sleeve cylinders. The surface temperature on the cylinder wall of an iron sleeve is considerably hotter than the surface of a plated aluminum cylinder wall with the same water jacket design and coolant flow rates. It may only take a second or less to generate enough heat to kill the oil film in these tight areas when the engine is under full load and high RPM and the piston clearance has become a press-fit.
The melted edge on the exhaust side of the piston indicates high exhaust temperatures. Ignition timing that is too retarded or an air/fuel mixture that is too lean in the RPM range where the engine was operating when the failure occurred will cause high exhaust temperatures. In this situation I would guess that the high exhaust temp. was due to a lean condition.
The under side of the piston shows that the piston crown has been operating near it’s thermal limit for some time. Oil and fuel vapors in the crankcase are in constant contact with the underside of the piston. Coking is a process that occurs when the temperature of a hydrocarbon liquid is raised to the point that a carbon like substance is formed. Anytime the underside of the piston reaches the coking temperature of the fuel/oil vapors, some of the vapors coke and start forming a carbon like substance on the underside of the piston crown. When the underside temperatures decrease the coking stops but previous carbon or discoloration remain.
The discoloration occurs where the temperatures are the highest. The discoloration usually starts in the center of the piston and moves toward the outer edge of the piston as the piston crown temperature increases. The bottom side of the piston will not instantly turn black if the piston crown temperature is raised high enough for a few seconds to cause the piston crown to “sag”. The discoloration takes a while to form as very thin layers form on top of each other until the color is black.
Engines that are only used for 300 ft. drag racing will not usually have the underside of the piston colored black. The underside of the piston is not usually hot enough long enough for the much discoloration to occur. Drag racers that only race the sand hills and spend 10 to 15 seconds under full throttle will often see the center of the piston black but the black will seldom reach the outer edge of the piston.
Engines (Dune bikes, Desert racers, Short track engines, and Moto-Cross) that are well tuned and run under prolonged ½ to full throttle with short periods of off throttle will usually have more discoloration on the under side of the piston than high horse power engines that are only at full throttle for 5 seconds or less then shut down.
In summary for the pistons in the Photos:
The piston crown temperature has been knocking on deaths door for some time. The piston temperature the last 5 to 10 seconds of life finally reached point that the piston clearance turned into a press-fit. The heat generated in the areas where the press-fit occurred created enough friction to melt the surface of the piston making the seize marks on the piston and stopping engine rotation. This engine/piston has approached death many times but the operating conditions changed in the nick of time to let it continue to live. I would estimate that the piston crown temperature has been on deaths door for an accumulated time of at least 15 minutes or more.
Things to do to reduce piston crown temperatures:
1. Reduce the amount of time the engine is held at wide-open throttle at any one time.
2. Eliminate ANY detonation the engine may be experiencing.
3. Increase the main jet size and or richen the needle.
4. Decrease the pipe restriction by improving the exhaust flow through the stinger and muffler.
5. Give the engine a few more seconds rest between periods of full throttle.
6. Increase the efficiency of the radiator to lower the average coolant temperatures.
7. Operate the engine in a manner for which the engine is designed. This means not to operate the engine in the RPM range after the power peak for more than a second or two. It is Ok to rev the engine into the RPM after the power peak when going through the gears but avoid operating the engine in this RPM range when the gearing is wrong and you are over-revved on a top speed pass or running it up a long steep sand hill and needing to shift to the next higher gear. Prolonged operation of the engine in the RPM range past the power peak can cause piston crown overheating due to insufficient exhaust port flow and insufficient scavenging inside the cylinder.
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Thanks, that is some good reading. Very informative. I spent good money on this engine and still considered it to be my "new" engine. I hadn't really thought about it, but since we built our new track I've spent a lot of time in over rev trying to figure it out. I've been recently thinking about a new pipe and silencer. I am concerned about your comments about the cylinder not being perfectly round and true.
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this is not my post ,but I want to say thanks to jerry ,I love seeing this kind of info posted
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hey jerry whats the most likely cause of the piston underside going black ? carb to lean, stinger diam to small, ign timing to much retard. what about plug to hot or exh timing to high
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hey jerry whats the most likely cause of the piston underside going black ? carb to lean, stinger diam to small, ign timing to much retard. what about plug to hot or exh timing to high
The underside of the piston “going black” is ONLY an indication of the surface temperature of the underside of the piston and the time the surface has been at a temperature high enough for the fuel and oil vapors that comes in contact with this surface to coke. The underside surface of the piston crown does not care where the heat comes from or what caused the temperature to rise to a level for coking to occur.
The underside of the piston going black does not tell us anything about the air fuel mixture. The presence or absence or the length of what many tuners call the mixture ring at the base of the porcelain insulator in a spark plug, does not tell us anything about the air fuel ratio. These two areas where soot or carbon form are just indicators of the temperature of these two specific areas and cannot measure air fuel ratio. At low temperatures the porcelain insulator will remain white until it’s temperature us raised to the temperature where fuel and oil coke to form deposits. As the temperature of the insulator increases the deposits start to burn away at the tip of the insulator and the dark ring of soot or deposits move downward. If the temperature of the insulator is raised sufficiently high the dark ring completely disappears. The insulator can withstand high enough temperatures to burn the insulator to a clean white appearance. The aluminum above the underside of the piston cannot withstand high enough temperatures for the underside to burn off the dark layer of soot and carbon. The piston crown (surface of the piston that is exposed to direct combustion chamber heat and pressure) will often burn off the majority of the carbon when combustion temperatures are excessively high or the piston crown is not designed for the long periods of wide open throttle the engine may be experiencing
If you have a brand new piston and lean the jetting enough to drive the engine into run-away detonation or……. reduce the stinger diameter enough to drive the engine into run-away detonation or……. advance the ignition timing enough to drive the engine into run-away detonation or ………..run fuel with low enough octane to drive the engine into run-away detonation or……… install a heat range that will cause instant pre-ignition that will drive the engine into run-away detonation, there will not be enough time for enough fuel and oil vapors to contact the underside of the piston to make it black before a hole is melted in the top of the piston.
If we mistune the carb, ignition, or some other component enough to drive the piston crown temperature high enough for these crankcase vapors to coke on the underside of the piston, but not high enough to drive the engine into occasional detonation counts, the discoloration process begins. The degree of blackness depends upon the actual temperature of the underside of the piston and the amount of time this surface is exposed to the mixture in the crankcase at a high enough temperature for the crankcase mixture to coke on the underside of the piston. Hard crusty carbon deposits will form on top of the black areas where the temperature is raised even higher. I do not know what temperature is required for the crusty carbon deposits to form on the underside, but when I see the dry carbon the piston crown temperature is very close to the temperature where the crown is about to melt.
I believe the majority of the current Wiseco and Wossner piston crowns I have looked at are too thin and not designed to dissipate the heat from the center of the piston into the rings and skirts at the rate at which many of the engines are capable of driving heat into the top of the piston. I had a few phone conversations with one of the engineers at Wiseco about 10 years ago just after they started marketing the Pro-lite line of pistons. I was having piston crown overheating on a lot of the new pistons on engines that were run at sustained high RPM and at lot of wide open throttle. These engines were used for road racing, shifter karts that were run on big tracks, micro sprints, flat trackers, and some pro moto-crossers. I did not have this problem on the older pistons on the same engines prior to the new Pro-lite line of pistons.
According to this engineer the new owners of Wiseco (Dover Diversified) needed to drastically raise the price of their pistons to pay for all of the new overhead (Stock holder dividends, and all of the CEOs, CFOs and unneeded management teams that drain the money from companies that are publicly owned. Prior to this buyout Wiseco was a family owned company. Marketing had to come up with an excuse to justify the higher prices so they came out with the Pro-lite line of pistons. Engineer were told to make the pistons a lot lighter because the average racer and shade tree engine builders believe light pistons produce more power than heavier pistons. A light piston helps reduce the stress on con rods and bearings but often does so at the expense of reducing the thermal loads the piston can take before it fails. As we increase the power of the engine the thermal load of the piston needs to increase not decrease.
The majority of the weight was taken away from the outer edges of the piston crown because that was an easy place and about the only place left where weight could be removed. The Wiseco engineers did not want to remove the material in the outer edges because they were decreasing the size of the pathway where heat flowed from the center areas of the crown into the rings and skirts. As a result of the new Pro-lite line of pistons I feel Wiseco instantly became a company that primarily produces pistons for drag racing and or cannot be tuned for max power if used for applications that require longer periods of full throttle. Testing and observation of customer pistons has shown me that the new thin parallel piston crown thickness will not tolerate sustained full load without the temperatures in the center building until crown sags or blows through. A piston designed for sustained and or ultra high power levels will have a piston crown that is thinner in the center and the thickness exponentially increasing toward the outer edges of the piston. Cut any of the OEM pistons in half through the piston crown and tell me what you see. You will generally find a piston crown designed to dissipate heat and not a piston that was designed so that marketing can brag about how much lighter there pistons are than an OEM piston.
I feel all of the pistons being made for the big bore after market cylinders are also designed for drag racing and to have lesser effect in upsetting the balance of a crankshaft and a connecting rod that was designed for the weight of a 250 piston. I believe the aftermarket cylinders and the piston manufactures the cylinder manufactures are using were attempting to minimize the stress on the OEM connecting rod and balance of an OEM crank, led to a line of pistons that will not take the thermal load that the new cylinders are capable of producing especially when the ports and pipes are taken to the next level of power or higher.
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Very interesting read. I just went out and looked through some old pistons (stock and bb) and see what you're talking about. All of my big bore pistons have eventually failed, and the underside of the crown is crusty black. None of my stock pistons have failed and all are clean inside. Are there any optons other than Wiesco and Wossner? Has anyone added thickness to the crown or come up with any other solution to make the engine more tolerant of the heat. I guess welding would create other weak points in the piston. Obviously the best thing is to reduce the heat by the proper tuning, pipe, and riding style, just wondering if there are any other safeguards.
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these were the last 3 ive used. left one seized at the 2'oclock and 7 oclock position but i think it had more to do with the carb tune being wrong for the gear it was operating in. other 2 didnt fail but you could tell they was gettin hot at the front side near the top ring. im guessin the large castor ratio kept it running. im thinking the stinger diam is whats causing it.
carb tune seemed good
water temp was pretty normal
no deto
110 gas
comp ratio around 13:1
#8 plug
timing wasnt advanced or retarded from stock position
stinger diam is 1.148" the best i can measure. thats why im thinking this might be the problem. unless the wiseco crowns are just to thin
(http://i1081.photobucket.com/albums/j357/doddledo/IMG_20140514_203339_zps91b35721.jpg)
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Were you trying to tune it for max power or max piston life? When you have the right piston for your application you should be able to tune it for max power and live. .......Then it would not matter what gear you are using.
Can you imagine how reliable our daily rides and tow vehicles would be if we could only hammer it on a hill pulling a trailer for 5 to 10 seconds and then have to let the engine cool for a few minutes between blasts to prevent the tops of the pistons from caving in.
I had a guy bring in a new 2014 CTS Cadillac to dyno a couple of weeks ago. It had about 3000 miles on it and is planning on putting a turbo or twin turbos on it. He wanted to get some baseline power numbers as well as punish it for a while to see if it would hold up. I made 11 wide open throttle 5th gear back-to-back pulls. Each wide open throttle pull went from about 1500 RPM to 6500 RPM lasting about 25 to 30 seconds each, with just enough time between pulls (about 10 seconds) to slow down the rollers to start another pull. After the second run the power stabilized and the remaining runs laid on top of each within a couple of HP throughout the whole RPM range.
6500 RPM in 5 th gear was 138 MPH. The guy told me it has 8 speeds.