Posts
5
Joined
10/18/2009
Location
US
Edited Date/Time
1/27/2012 2:23pm
In the beginning of June we bought a brand new 2009 yz250. In july, it detonated internally and wrecked the cylinder head, piston was destroyed, and cylinder was scored badly. We had it completely fixed, new cylinder, head, everything. Last week, the same exact thing happened again. we are fanatics about keeping the airbox clean and making sure nothing gets in through the box, filters are changed every ride, etc. The weather hasn't really changed drastically yet so i dont think the temperature change is an issue...we are considering getting a yz125 but if this is going to happen as often as it has than we might as well switch to suzuki, rm250. has anybody else had this problem??? responses would be really appreciated...
Thanks...
Thanks...
Any engine, intake, or exhaust mods (at all)? What fuel? Which oil at what ratio? Since you don't have a "location" in your profile, what altitude and air temps are you operating at?
SOMETHING is affecting your bike, but we need to know everything. This machine is not known for meltdowns under normal circumstances.
Photos of damaged parts would also help. Your description of "internal detonation" means very little. Detonation is something you can feel and hear, especially in a two stroke. If "knock" was your assassin, why did you ignore it??
The Shop
I agree with raceguy, I believe Yamaha's are well built and it very likely that it is not the bikes fault in your explanation of the engine failure.
Detonation shouldn't occur at normal temps.
Gys that ride on the torque peak (which is good) will detonate the YZ 125, and 250 on pump gas.
Anything 70-80 degrees and on pump I had to run stock pilot, stock main and stock needle with stock pipe...and one larger main with PC.
At 60 degrees on pump another bump in main...and sometimes a one richer needle clip.
Below 60 the engines output picks up so much that another main jet change wont cure the issue. Race gas necessary!
If you run a non oxygenated fuel at 100 percent (no pump) prepare to lean jetting considerably relative to pump fuel. On c12 I run a 170 main for temps 70-85ish.
At 60 degrees I found a 172 about perfect.
I've never seen a little bit of pre-detonation killing a motor, specially since these things are marketed for the average rider who's gonna run pump gas. I'm a Kawi rider myself, my bud's got a 2000 yz250, hasn't had this particular problem (although others problems have been encountered).
Good advice though. What can a guy who runs pump gas do to keep from blowing his engine? Roll the ignition timing back as far as you can go to keep it running?
Not all the 03+ YZ250's are setup close to needing race gas. My neighbor has an 05, and has never so much as pulled the carb bowl off. Desert riding, sand dunes with a paddle tire, 0-5K ft. elevation, 30 to 110 degree ambient temps, all on 91 pump piss that can be as much as 8 weeks old. No detonation. Admittedly he does not push the bike hard at all (as evidenced by the original piston still being in there), but it fires up with only a kick or two, runs great and has never even fouled a plug. He's quite happy with Yamaha for obvious reasons.
Pit Row
Lots of MX & drag racers get Cam 2 on their way to the track.
Someone also mentioned getting the squish band modified, and this is an excellent mod if you don't want to mess around with having to use higher octane fuel.
www.rb-designs.com head mods work really well on the yz head, and I believe the cost was only about $75, but well worth it.
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Jim J
http://www.motocrossactionmag.com/...42F98D09B16C18698E12
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