28 replies to this topic

[SledNH]

What causes a motor to eat a clean hole through the middle of a new piston? I put 99 miles on and Now I have a hole in one of my pistons.

Someone said Its called leanburn, and I have an airleak in the crankcase, suggested that I install new crankseals.

I know this is not a motor forum, I just wanted to see what your input was, I have already rebuilt the engine once and am going to do it again.

[Paul D]

One word. "Detonation".

Too much air, not enough fuel, and compression cause the fuel charge to ignite prematurely cause that type of damage.

[PolarisCobra]

I would expect that it is due to a lean condition. This means that the air/fuel ratio is not right, too much air. A lean mixture will burn too hot, causing your piston hole.

The sixty-four-thousand question is why is it lean? I can give a short list. Remember, you have given no info about what you may have already done, so some of this may seem redundant to you.

Clogged/dirty carb
Carb jets too small
Carb boots cracked/leaking
Seals for crank on one side of the engine of the other

I am sure that there are other things, but you should start to get the idea. Probably want to get a factory service manual if you want to tackle this, or have an experienced shop have a look at it.

I agree that you don't want to continue to feed it pistions with out finding out what caused it.

Edited by PolarisCobra, 14 January 2009 - 05:01 PM.

[Tim]

As I remember you have an 88 89 yami ex570. I had crates of those things back in the day. There was a major airbox recall on those so make sure it was done. I find it hard to believe is was not but you never know.

[SledNH]

Wow, I didnt expect a good response THANKS!!

I removed the airbox and put in some UNI snowpods (mini oiled airfilters) on each carb in the preseason. I also rebuilt the engine with new machined cylinders, matched pistons (wiseco prolite), new top end gasket etc, I had the jugs machined professionally matched to my pistons. Good news is there is no noticeable cylinder wall damage since the hole was on top ($120 worth of machine work saved!!)

Funny thing is, when I bought the sled, the mag side piston was destroyed just like t his one. The sled has not been rejetted, although I have some bigger jets coming in the mail. The clutch side piston looks great, has a nice oily brown (coffee colored) pattern to it. This is desirable I am told.

I am going to have to open the crank case and replace the seal on the mag side. I ordered both seals just in case. I will also reseal the case with RTV sealant.

Any hints??

[plowking]

I'd check on what to use for sealant on case halves. I know when we seal the cases on our atv's,we use Yamabond,it is more durable to gas and oil and might not blow out as bad. Also, the cases surfaces have to be straight and smooth to seal properly. plowking

[Shawno]

More than likely you have a cracked boot, if you rebuilt the motor it could be assumed the carb would have been cleaned and jets checked.
Did you replace your crank seals when you rebuilt that could be a source for air coming in causing a lean condition. After rebuilding the engine did you ever stop on the side of the trail (this is not a wise ass comment just a real question) and take a look at your spark plugs to see their color? The plugs provide a lot of information for what is going on in your engine. Many people do not know that they can be an early warning to future problems. As I said this is not a dig at you it is for your information and others.

So next time you guy's are having you smokes or snacks or what ever on the trail take a minute to pull the plugs and take a peak, your results will be skewed if you let the sled idle after riding then shut it off. The sooner you shut it off the better representation of your engines running condition you have.

[SledNH]

Yeah the carbs were rebuilt, I have rebuilt carbs (holleys, carters, eddy's etc) for several years but these mikuni's were pretty straightforward and it was my first time to rebuild 2 stroke carb. The jets were fine, but I am going to step them up one size since I am using the airpods instead of the airbox. The carb boots were not cracked or checked.

I did not replace the crank seals when I did tyhe top end. I am going to pull the engine and open the crankcase halves as soon as this arctic air passes (no heated garage!!). I will replace the seals, and may pickup some of this yamabond when I get to rochester this week.

Is yamabond some sort of ATV sealant? Is it reasonably priced? I need some yamalube while I am at it.


PS: The plugs were good, when I checked them after 20 miles. The mag side did seem dryer but I didnt pay much attention to it. It all makes sense that the same side blew a piston again, so I am leaning toward a crank seal.

Edited by SledNH, 14 January 2009 - 06:06 PM.

[SledNH]

PS the sled ran great, I followed this break in procedure: (I do not have oil injection due to bad design?, I premix 30:1 with yamalube)

1. do not go past 3/4 throttle
2. do not let idle at a steady rpm, feather the throttle.
3. I retorqued head, cylinder bolts after few hours of riding.
4. let sled warm up before going out riding to prevent coldwash


I must admit I opened it up on a lake to see what it had for maybe 5 seconds. I almost shat myself when I reached 70 mph. This was a week before it blew though if that matters. This sled is no cadillac and it was all over the place. more experience has made it handle better though Again, the clutch side piston looks great!

Edited by SledNH, 14 January 2009 - 06:17 PM.

[Tehpibleu]

Heads up on Exciter 570

You couldn't have picked a worse sled "to deviate from specs"
with, IE removing the air box and putting on Air Filters NO NO NO...
That for sure aggrivated any other problem you might have....

All EX 570s should have been updated to 1991 Specs and
probably change airboxes.....The carbs are supposed to have
special Vent Tubes installed.....And If there is a Power Jet on
the bottom of the carb, it is supposed to be plugged off and a
bigger Main Jet installed.....

You need to go to a Good Authorized Yamaha Dealer and
get it done right to The New Update Specs that all dealers should
have.....It was a good trouble free sled when updated correctly...

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * **

SledNH, on Jan 14 2009, 04:50 PM, said:

What causes a motor to eat a clean hole through the middle of a new piston? I put 99 miles on and Now I have a hole in one of my pistons.

Someone said Its called leanburn, and I have an airleak in the crankcase, suggested that I install new crankseals.

I know this is not a motor forum, I just wanted to see what your input was, I have already rebuilt the engine once and am going to do it again.

[SledNH]

Yeah I have the updated carbs, and the airbox thing is an option, they (2 dealers) recommend a restrictive air filter pod. The UNI pods are supposed to have slightly more air resistance than the airbox so it does not cause a lean problem.

The power jets were blocked off per yamaha. The power jets caused a midrange lean condition, I would not think of using them.

The carbs are vented as required, and really you only need them if you use power jets. Yamaha blocked them off from the factory in 1989. I do not have the flatside 36mm mikunis you may be thinking of. Those were introduced in 1991 I believe.

I am sure that once this leanburn issue is dealt with the sled will be fine. You couldnt ask for a more straightforward sled. Engine, track, seat Its just that I am new to 2 stroke engines, and the various issues inherent. If this were a 4stroke I would have thought the spark plug hammered a hole in the piston!

Edited by SledNH, 14 January 2009 - 07:24 PM.

[smallengineguy]

You are using BPR9ES plugs correct? Those are the correct heat range. I'd say crank seal let go.

[SledNH]

smallengineguy, on Jan 14 2009, 07:56 PM, said:

You are using BPR9ES plugs correct? Those are the correct heat range. I'd say crank seal let go.

Yes I am using the BR9ES. I have some 8's but havent used them.

I am thinking the seal to which is what blew the piston for the previous owner

[smallengineguy]

Don't use 8's. They are a hotter plug.

[SledNH]

smallengineguy, on Jan 14 2009, 08:03 PM, said:

Don't use 8's. They are a hotter plug.

Ok, SEG. I will forget I have them....

One of the misgivings I have about opening the crank case in half is the stator, and all the electronic timing stuff there. I have read some other forums where people made huge mistakes putting this stuff back together. Can you give me any pointers?

I am thinking I should mark the plate(?) before removing it so I have my timing correct when I re assemble. But what actually sets the timing etc? I do have a chlymer manual but the B&W pics are hard to understand sometimes.

ALso I was told to watch out that I am not 180 degrees out of phase upon reassembly. ???

[Tehpibleu]

RE: VENT LINES......How long are they......?

SledNH, on Jan 14 2009, 07:23 PM, said:

Yeah I have the updated carbs, and the airbox thing is an option, they (2 dealers) recommend a restrictive air filter pod. The UNI pods are supposed to have slightly more air resistance than the airbox so it does not cause a lean problem.

The power jets were blocked off per yamaha. The power jets caused a midrange lean condition, I would not think of using them.

The carbs are vented as required, and really you only need them if you use power jets. Yamaha blocked them off from the factory in 1989. I do not have the flatside 36mm mikunis you may be thinking of. Those were introduced in 1991 I believe.

I am sure that once this leanburn issue is dealt with the sled will be fine. You couldnt ask for a more straightforward sled. Engine, track, seat Its just that I am new to 2 stroke engines, and the various issues inherent. If this were a 4stroke I would have thought the spark plug hammered a hole in the piston!

[SledNH]

Tehpibleu, on Jan 14 2009, 08:10 PM, said:

RE: VENT LINES......How long are they......?

2 per carb, they are (I would guess) about 10" long? They are banded together and wrap around and go under carbs.

You think something is wrong with them? I didnt think the length would affect the operation, but then again......Im the one with a blown sled

[Freedom Rider]

Loose or dry rotted air intake boot. Usually if crank seal is culprit it blows on the PTO side not the mag side.Mag side does not take the beating that the PTO side does with blown belts or hard engagements. The seal is also protected from elements by stator and ignition plate.Not nearly as much heat on mag side either. You are smart tearing it all of the way at this point. Get crank and cases cleaned up real good, install new genuine Yammi seals, and use yamma bond.

[SledNH]

Freedom Rider, on Jan 14 2009, 09:57 PM, said:

Loose or dry rotted air intake boot. Usually if crank seal is culprit it blows on the PTO side not the mag side.Mag side does not take the beating that the PTO side does with blown belts or hard engagements. The seal is also protected from elements by stator and ignition plate.Not nearly as much heat on mag side either. You are smart tearing it all of the way at this point. Get crank and cases cleaned up real good, install new genuine Yammi seals, and use yamma bond.

I am thinking someone may have replaced only the clutch side crank seal. The carb boots are soft and pliable, they may have been replaced, there are no cracks but I will check again very very closely.

I ordered some seals already from mfgsupply I think they are winderosa's. I didnt want to chance it at the delaer when I go next week. I definitely will use yamabond. My manual says Yamabond #4.

Should I loctite the crank case bolts? They are only torqued to like 16 ft/lbs if I recall (will make sure in manual though). That seems kind of loose. I have rebuilt big block fords mostly and I torque heads to 130 ft/lbs. I know t his isnt a ford, but it still seems like the torque values are weak, but I will resist torquing the crap out of them if I can (aluminum or not)

Edited by SledNH, 14 January 2009 - 11:09 PM.

[MATTtheCATZR800]

don t forget the jets they get a little green on the needle would hurt to jump up one size