YZ 250 Breakdown Please Help
- Thread starter trial_07
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Never clearly seen this detonation issue? Was the sparkplug getting loose by itself? That piston could have went simply from too many hours and a worn skirt, but you obviously can not check that now!
I tend to agree with whenfoxforks in that it was likely a piston worn beyond limits and failed due to piston slap.
To solve my detonation problem I retarded the timing, milled the head to get a .050" squishband and jetting. Poor cooling can also contribute to detonation. Getting the squishband set properly is the secret on these bikes. If you are at a low altitude you will need to get the head space enlarged/adjusted as well to avoid too high of a compression ratio.
If the bike was actually higher compression it would have been achieved by machining the head or cylinder. This is mostly the cause of any detonation if there was in fact detonation occuring. So at this point it needs at least a crank assembly, cylinder, piston and maybe a head or some cases if it gouged them. If it were me I would be cracking the cases ASAP to get the big picture. You can pretty much rest assured the bike will run fine with all new parts.
Send the head to Eric and tell your tale. Best method to lower the compression, and least effect performance. Still curious, how it ever first came to the conclusion the engine was detonating?
- Apr 26, 2004
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My friend had many problems with the engine. We suspect detonation as a cause because he could hear it pinging. First time, a small chunk of the piston above the ring, exhaust side, broke off. Second time, the tip of the sparkplug broke and damaged the piston which only counted two hours. You know about the third time. All this in one season. Here are some pics from when the tip broke off:
The tip broke off when he landed and snapped the throttle.
I'll talk to him about sending the head to Eric, the season here is pretty much over anyway.
The tip broke off when he landed and snapped the throttle.
I'll talk to him about sending the head to Eric, the season here is pretty much over anyway.
Well there is 2 pistons with no signs of detonation. Pre ignition more than likely. The engine is not high compression. If you suspect detonation, stop immediately and check the plug. Detonation would burn off all them rich looking deposits. Or the service limit went over on the, piston deck for number 1, the ground strap fell off number 2, and number 3 went over and the skirt broke.
Fuel, carbon deposits, or plug too hot.
Were the cylinder and/or head modified in any way from thier stock form? I don't really see detonation either, that usually leaves a funky lighter color on top of the piston. Usually low octane fuel or too hot a spark plug are to blame. I've heard of lean jetting contributing also. What happens is that the combustion chamber temps get too high and can even melt the piston. Low octane fuel burns quicker so it ends up igniting too soon, stressing the piston. This increases cylinder pressure and temperature. If the plug is too hot in the haet range it will cause the combustioin chamber temps to rise too. In some cases where guys run a cheaper plug like the br*es that's too hot the ground strap can break off and you know what happens then. Lean jetting will cause the mix to be more volitile as well and burn hotter.trial_07 said:
If all else was stock, including the plug, I would lean towards the pipe having leaned the jetting or that concoction of low temp car fuel and your buddy didn't jet the bike after installing the pipe?
Now that is one I have seen on here, more than once. People in the name of "getting more for their dollar" by mixing already blended fuels together? 110 octane plus 91=? You have no way of KNOWING exactly what you just concocted, well the lab thing. This case, I know he took an unleaded, oxygenated pump fuel, and mixed it with a leaded, oxygenated race fuel. Both were concocted in their own specific purpose/blend. They have similar contents, and some different! Damn near every japanese 2 stroke stock race bike will run fine on decent premium fuel. Pre ignition and detonation have the knock diagnosis, but a quick look at the plug will tell you for real.
That's my point foxforks, everything is junk anyways so as long it gets rebuilt to stock specs it should run fine. I would recommend running the highest grade pump fuel you can find with nothing mixed in. High grade pump gas is 93 or 94 octane here, that's usually fine for anything under 13:1 or so compression ratio. I'm not 100% positive, but I think 2 strokes are usually less than 10:1.
Get it rebuilt and jetted and see if it still pings or knocks. Make sure to get a piston to cylinder clearance measurement before you assemble it.
Get it rebuilt and jetted and see if it still pings or knocks. Make sure to get a piston to cylinder clearance measurement before you assemble it.
Has he been buying pump fuel from the same place? How has he been storing the vp and concoction? No sealed can, it will draw moisture out of the air, and hold it in the can. The oxygenate in the vp u2(isn't this discontinued?), more alcohol? We already know 2 strokes do not benefit from the extra cooling properties of the lead. And the lead does bizarre things when more oxygenate is introduced! The first piston pic screams "water". Take a sample of the pump fuel and carefully, put a lid on a clear jar, let it sit a day or so, out of light or temperature fluctuations, any water will separate. A simple change of stations or gas handling could be it. And even with the pipe, I am pretty sure he does not need real high octane, 91~93 should be fine, as long as it is not contaminated. It will run in a car, not a dirtbike. Cars can actually run with water added to the combustion process. Steam does make power, not in a dirtbike.
