Newbie Question (blown hole through piston)

[dunerider13]

Ok, I recently got a 83 250r, when i bought it i was told that it just had a new top end installed. It's ported and polished with I'm guessing a larger carb. I don't know too much about these. I'm a jetski guy. I can post a pic later of the carb to determine which one it is. Or can somebody let me know how to determine that. I know for a fact that it is a kehin carb.

The problem: I blew a hole through the piston WTF??? Now i've bored and honed the cylinder and got a new pistion and top end gasket kit. I'm on the last bore on this cylinder so i want this thing to run for a little while before i have to re-sleeve it.

Any ideas???

Sorry for being such newbie.

[voodoo1215]

What was your fuel/ oil mixture ratio. I bought an 82 R with the same issue... the owners child dumped straight gas in and rode off..

[deathman53]

I'm gonna say, too hot of a heat range for the plug or detonation. Do a compression test on the top end, use br8es plug. 32:1 gas/oil mixture. Let us know what the copression psi is.

[dunerider13]

I was running maxima k2 oil 32:1. I believe it was a br8es plug. When I get the new top end together I will check the compression. Do you know what it should be??

Also, I was doing some reading on this board and would carb jets have anything to do with this type of failure. I just blown away that there was a hole in the pistion.

Thanks

[Billy Golightly]

Holes in pistons are frequently caused by a lean air/fuel mixture condition.

[SYKO]

Holes in pistons are frequently caused by a lean air/fuel mixture condition.


I second that. what kind of shape was the air filter?? clogged up?? dirty? did it have the airbox cover on it?? other than the obvious hole in the piston can you tell if it had a recent top end done?? ( fresh gaskets, cross hatch marks on cyl) such as that??

[dunerider13]

Yeah I could def. tell it was a fresh rebuild. Everything looked new except the damn hole in the pistion. As you said the hatch marks were on the cylinder and the pistion looked new. The air filter is a k&n type and it was clean. Could it be caused by the wrong jetting or carb setting???

[Billy Golightly]

Your carb is more then likely to lean, if someone had the carb setup for an old dirty filter and recently put the fresh clean K&N on it you'll be flowing much more airflow then before, and you have to compensate for it.

[ceaserthethird]

Fatter Main Jett -

All way's start out Fat can't hurt ...

Too break in i would run 20:1 Ratio ! first full tank of gas should break in good ...

Check your main jett size and report back ?

Also if you have any after market reed's , exhaust ect ...

[Dirtcrasher]

Intake leaks are real common on these older ATC's. Start it up and spray some carb cleaner arount the carburetor to cylinder boot and reed block gasket. If the engine revs up, you have a leak.

[GPracer2500]

Where was the hole? In the middle of the piston or around some point on the edge?

[dunerider13]

the hole was basically in the center. if i can find it, i'll post a pic, but i was pissed when i saw that so i might have chucked it.

[GPracer2500]

the hole was basically in the center. if i can find it, i'll post a pic, but i was pissed when i saw that so i might have chucked it.

Holes in the very center of the piston are a tell-tale sign of pre-ignition. Pre-ignition is caused by some spot in the combustion chamber getting so hot it actually starts to glow and becomes an ignition source. The wrong spark plug is a common cause of this but not necessarily the only way it can happen.

With pre-ignition (not to be confused with auto-ignition, a.ka. detonation, spark knock, knocking, pinging, etc.) here's what happens to cause that failure: Some hot spot allows the mixture to ignite well before the spark plug fires. So now we've got a situation where the piston is traveling upwards and trying to compress an already burning (and expanding!) mixture. This is seriously bad news for the engine. As the engine tries to compress this burning, expanding mixture a tremedous amount of heat is produced--WAY more heat than the engine would normally see. The one place in the combustion chamber that is least able to disapate this heat to its surroundings is the middle of the piston. The metal is thin there and there's just no where for the heat to go. So it melts and the center of the piston breaks away.

Often this kind of failure is a cascading event. For example: your carb is jetted too lean, which causes detonation to begin (deto being the other abnormal combustion phenomenon), which in turn causes a hot spot to develop in the combustion chamber, which in turn allows a pre-ign condition to develop. Pre-ign and deto--while being seperate and distinct phenomenon--often go hand in hand. In other words, one can cause the other and vice versa.

I think Deathman53 is on the right track....

Correct plug?
What fuel are you using?
How much PSI was the engine pumping?
How sure are you the jetting is at least close to correct?

Stray thought--->a plug that is loose might still hold in most of the compression but not be seated well enough to dissapate its heat into the head thus allowing the plug to overheat. I once saw a barely loose plug run fine in a TRX250R until the tip of it melted away. Luckily it did not start to pre-ignite--but it easily could have.

Good luck.

[TimSr]

Im unclear on what what "blew a hole in the piston" means. A picture would help. A melted piston looks very differnet thana broken one. All the posts about running lean are relevant if the hole is MELTED through the piston. If something knocked a hole through the piston, it has nothing to do with carburetion.

[hadar]

GPracer is right on. I seen mented holes once before. It was on a snowmobile, guy had the wrong plugs in it. Run a cooler plug, check your ignition timing and main jet.

[dunerider13]

I don't have a pic sorry, though I do remember that it was melted. I mentioned that to a friend as we were pulling it apart saying that it looked like it got to hot and melted a hole through it.

Thank You gpracer for that explanation.

Now another noob question. What carb is this? I have a manual for the 83 atc and it looks nothing like the one I have. This is prob where the problem lied and it wasn't tuned properly.

After talking to the guy I bought it from, he only had the thing for a couple months and rode it once and only for a few minutes.

Thanks for the help here, I've got the cylinder, head, and exhaust back together now and I just need to figure out what carb this is so I can get a rebuild kit and start riding again.

[SYKO]

that is a large mikuni like 38 or 44 what is your main jet size? if its 360 its a 38 if its a 540 its a 44 and WAY to big for a non big bore motor.

[dunerider13]

printed on the main jet is 420RD. I'm assuming it's a 420 jet. Correct me if i'm wrong. And I measured the smaller bore on the carb and it measures 38mm. So i'm assuming its the 38mm one.

[SYKO]

ok then its a 38 but jetted all wrong, that carb is way to big for that air cooled motor unless you have a 300 kit on it, I have a 38 on my 85 and it feals great except a low end stumble I have in it

[dunerider13]

what do you suggest I go with then on the jet size? Do remember that this is a ported cylinder.

[SYKO]

well Im not sure what you need to run on a air fooler, hopefully some more mikuni fans will jump in and help