THIRD TIME

[YZGreg]

Wow thats your problem regular gas will ping like crazy and melt pistons! Your bike is supposed to have a minimum of 91 octane. The guy I knew was running regular @ 32:1 but it was supposed to have premium.

[Woodsie]

Thing is, we ran probably twenty tanks of regular through it when we got it, without a problem. Thanks for the thought though, can guarantee it will have hi-test in it when it goes back together!

[YZGreg]

All I know is pre ignition will melt pistons and spark plugs, maybe the first time it happened the bike was a little worn but carbon raised the compression and then it started pinging. Hmm I'm stumped.

[Woodsie]

I hear ya! Anybody know if power valve could cause this????

[YZGreg]

No the PV wouldn't cause that it just changes the exhaust port timing to give you more bottom end, I would make sure it was free anyway just for the extra power.

[KTM250rm393]

Your problem is your plug.

Everyone that I know that has melted a 125 engine was because they run it full tilt with a BR8ES/B8ES plug. You should be running a BR9ES in a 125. The 8 is too hot of a plug for your bike.

I bet if you look at your owner's manual it will say BR9ES plug.

BTW, you should be running a resistor plug to save your CDI unit from getting backfeed by a non resistor plug.

My friend fried his 01 YZ125 last year because someone gave him a BR8ES plug when he fouled his BR9ES and he got like 2 mins of racing and melted a hole in his piston and before he never had a problem with his bike.

[02mncr250]

Sleeved cylinders are a pain,,,sometimes a pistin will stick, sometimes not,,,although burning aup is different that a piston sticking.

Where is the hole in the piston? Front (exhaust) or rear (intake)?

Jetting goes as follows

idle - pilot
1/2 to 3/4 - needle and needle jet
3/4 wot- main

Airfilter is oiled and not dry correct?

Just trying to get more info to help.

[Aus_Rider_22]

What is the difference between a BR9EG and BR9ES ??

THanks I run iridium version of BR9EG in my bike and it's great !

[AaronRider]

I dont think your problem is your fuel, unless it has been shaved for high compression it should be fine. I guess it could be the spark plug?

[Woodsie]

Book actually calls for a BR9EG. Talked to a lot of people that said B8ES was good. Like I said before we rode the bike for 3 months ...no problem. That's with a B8ES plug. There isn't a hole in the piston, just melted till the ring was stuck in the ring groove. Most damage is on the wrist-pin sides.

Thanks for all you guys helpin out.

[02mncr250]

bring the cylinder in and get a piston measureed for it. It is sticking for sure. IT IS NOT A JETTING ISSUE SINCE THE DAMAGE IS ON THE SIDES.

Find out what your sleeve is made of....steel or nikasil.

I reccomed doing one of two things.

1. best option. Buy a new cylinder and OEM piston and rings.
2. Second best. Get the cylinder honed and plated to accept a new sized piston. Your piston is seizing on the sides due to it expanding faster than what the cylinder is made of.

Jeff

[mxerncolo]

WEIRD.
Sometimes we overlook the obvious maybe:
1)You're 32:1 premix is 8 oz good quality 2-stroke oil like silkolene or Belray, Amsoil, etc mixed to 2 gal gas right?
2)The plug you were running was too hot, and even if you rode hard and fast the first three months, being new to the bike, it may not have been run as hard as you thought.
3) An intake leak that only occurs at way high RPM's ?
4) A bad cylinder altogether - stranger things have happened.
5) Go over your measurements - or take it to the shop with your measurements, and let them give it a try.
6) Good luck, and don't shoot it yet.... even if it is a Yamaha.
jim

[mxerncolo]

Re-read the post, and I'd seriously look into Jeff's first suggestion. That frying up the sides of a new piston - that's the cylinder. It's either damaged or has a perpetual "out-of-roundness" at high temps. I'd ditch it and start over w/ a new cylinder, and I betcha that'll solve your problem.
jim

[Woodsie]

You guys are all missing the point that the plug is melted.
I have to lean towards an airleak at high RPMs. But where?
Cylinder is sleeved. I'm going to get it bored, new piston etc.
Putting a bigger main jet in.
If it melts again, I'm gonna.........................!
Talking with a local bike mechanic that is gonna go over everything for me.
Thanks for all the help.

[KTM250rm393]

Aus_rider,

The difference between a BR9ES and BR9EG, the EG is fine wire electrode and runs about $1.50 more than the BR9ES.

Woodsie,

My friend with the 2001 YZ125 ran great too with a BR8ES plug, until he decided to to lay full throttle for 1 minute to catch up with me in some pasture riding and he melted a hole right through his piston. So I say you either have an air leak, or your bike was jetted too lean for WFO.

[02mncr250]

Woodsie,

You are saying the plug is melted,,,,,I think you are seeing aluminum from the piston on it , melted to it.....BUT the plug will not melt before the piston in a lean situation.

By the time you bore it,,,sleeve it,,,etc, you would be better off buying a new cylinder for 200.00 from Planet honda.

You are not having jetting issues since the issues are on the transfer ports, when a burn down occurs it usually happens on either the front of the cylinder (exhaust side) due to high temps and no cooling,,,,,gas cools the motor actually with octane.

How does the head look inside, are there nicks in it or is it smooth. Also, what color is the piston top? How far is your " wash" away from the edges of the piston?

You can do whatever you want,,,,,you asked for advice so here it is. But my reccomndation is your issue is with that sleeved cylinder, not your jetting. You may be putting in the wrong size piston since it is not stock.

Jeff

[Woodsie]

Man, the electode is GONE! Not just the gap closed with aluminum. Top of the new piston is black I suspect from melted steel dancing on the dome. Few small marks or gouges. Again I think its from the steel.
Call me crazy, but I think I'd rather have a cast iron cylinder than Nikosil. At least I can bore it. Buddy of mine went through CR jugs like candy. I don't have that kinda cash reserve.

[mtk]

Nikasil coatings are a lot harder than steel. In other words, they'll last a lot longer than a steel liner. If your buddy was destroying CR jugs, he had some other problem.

[02mncr250]

woodsie,

The biggest mistake made by people when putting jugs in place of nikasil is that the jugs contracts and expands at different rates than aluminum. Was that pro x piston aluminum? If so, i think that is where your issue was at.

A friend of mine had the same issue with his sled. Would ride WOT,,let off and it stick. Then he put forged pistons in and has not had a problem.

Nikisil lasts longer and over all performs better against wear. I reccomend using all OEM parts since Weisco pistons are usually junk.

If the top of the piston is black,,,you are jetted fine,,,,actually rich. Your wash should be good as well.

get a jug,,,,get an oem top end set and throw it back together. This is what i think happened

in order of course

1. Jug contracted,,,,,squeezed piston
2. ring chunk meleted out and went on top of the piston
3. Ring chunk took out spark plug electrode and both beat up the top of the piston


If the bike was lean,,,,it would not be black on the piston,,,it would be white to super light tan.

If you had an air leak,,,the bike would run like crap and run waaaaaay lean.

jeff

[KTM250rm393]

Jeff,

Come to think of it, my friends piston on his 01 YZ125 was white on top where the hole was burnt through the piston.