[stemmol]

I was riding my r when it suddenly stopped running. i noticed a loss of all compression. opend the head to find there was a hole burnt straight through the top of the piston. What could have caused this?

 

[rook]

too much timing advance, lean, cheap gas.

 

[Michael88R]

I forget which but either pre-ignition or detonation. One of them can cause a hole in the piston. Many things can cause this condition. Some which have already been mentioned. Gas, timing, lean, etc.

 

[chiefshocker]

If you've been running it before without problems..timing should be fine.. prob too lean, your mix is off, jetting is off or your octane is too low. Was this ride in a different elevation?

 

[NormalZ]

I forget which but either pre-ignition or detonation. One of them can cause a hole in the piston. Many things can cause this condition. Some which have already been mentioned. Gas, timing, lean, etc.

Yep, the hole in the middle of the piston is from pre-ignition. Detonation issues will usually cause pitting around the edges of the piston, and possibly rings/ring land damage, if bad enough.

The pre-ignition is caused by hotspots in the cylinder, which could be caused by running too lean, too much compression for the octane you're running - it's going to be something along those lines causing it.

 

[voelker]

did this about a year ago. im pretty sure it was the jetting, but i havent touched this motor since i took this picture.

 

[stemmol]

Yes that is what happened. I just rebuilt the engine it had tons of compression and i was running 87 maybey thats what did it in.

 

[BLACKeR]

whats your compression now?

 

[2muchquad]

87?wow you should have premium even for a stock motor.I knew a guy that did in his 94 rm125 mx bike running 87oct.Your lucky you didnt "vent" your cases

 

[TwOsTrOke]

Leaky seals in the bottom end could have caused this also. Certainly a very lean condition.

 

[stemmol]

i have about 195lbs of compression. No air leaks did the test. Im going to run a richer 32:1 110 mix

 

[mxduner]

a hole in the center is jetting/lean condition, and advanced timing as mentioned before will burn the exhaust side of the piston and cause damage to the ring lands

 

[osoloe1]

No one mentioned too hot of a plug? You should have an NGK BR8ES...if it's a BR7ES then even a slightly lean condition will kill the piston quickly.

 

[darylb]

Did you change the mix ratio for the gas/oil before that ride that blew up your motor? If your motor is jetted correctly for a given gas/oil ratio, increasing the oil in the gas will lean out the air/fuel mixture. If you decide to change your fuel/oil mix you have to check your jetting (air/fuel) as well. The volume of liquid flowing out of the jets is constant so when you add more oil to the fuel/oil mix there is less fuel in the volume of liquid.

 

[C-Leigh Racing]

a hole in the center is jetting/lean condition, and advanced timing as mentioned before will burn the exhaust side of the piston and cause damage to the ring lands

You sure thats right.
Neil

 

[ky250r]

I just pulled the head on my 250r this weekend to check out a leaky head gasket problem and I find this, guess Im lucky I pulled it before it was too late, I been running pump gas in it though,

 

[C-Leigh Racing]

Yep,,, ol pump gas strikes again.
When you pull the cylinder off, take the piston off & inspect the underside of the piston see if any metals gone where it almost melted through. If it has, then a complete rebuild is in order because all the sprayingof aluminum down into the bottom end.
Neil

 

[ky250r]

well I did pull the cylinder sunday night and I was pretty lucky in this case. theres a small bulge on the underside where it started to melt through, and it might be hard to spot, but theres about a quarter inch long crack in that spot also. but it sucks cause this was still a fairly fresh topend, rebuilt since mid september this year. but either way, this gives me a chance to send out the jug for porting and this time gonna run a wiseco racer's choice coated piston. I ran one hard for almost over a year and it broke the sleeve on the cylinder before the piston broke. also getting the cylinder top decked cause theres a few gouges that I think is where my gasket leak is coming from. and also gonna run straight race fuel like I used too, 110 octane. but heres the pics of the piston,

 

[C-Leigh Racing]

With all the baked on oil up in the piston top, you can hardly tell how much is missing but can see it is just a little.
Those little pieces had to go somewhere & down in the bottom end only place they could go.

You said something about getting the cylinder decked because the gasket leaked, that could cause the piston to be burned like that.
Sucking coolant & then times sucking in raw air & causing a way lean condition on top of the piston. I could see it happening, but I would still check the ignition to be sure its set right & go to the race gas for sure. Race gas removes a lot of guessing, at least up to a certian compression amount. Compression up around 230,,,250 lbs, you near bout need to check race fuel if it came from a service station.
Neil

 

[ky250r]

You said something about getting the cylinder decked because the gasket leaked, that could cause the piston to be burned like that.
Sucking coolant & then times sucking in raw air & causing a way lean condition on top of the piston. I could see it happening, but I would still check the ignition to be sure its set right & go to the race gas for sure. Race gas removes a lot of guessing, at least up to a certian compression amount. Compression up around 230,,,250 lbs, you near bout need to check race fuel if it came from a service station.
Neil

I dont plan on having that much compression, maybe around 195 to 200psi at the very most. I got a cr ignition and as far as I know its set at the stock position. I did test out the spark and it makes a pretty big blue spark, even with a old plug. do you think maybe a b9es plug would work a little better? I dont understand completely how the "hot and cold" heat ranges work, I just know that the hight the number the colder the plug.