Lean burndown on 700 redhead WHY?

[SKMM00]

Hi, I bought a 01 mountain max 700 about two months ago. The odometer was around 9900km but the speedometer wasn’t working .. I never touched the carbs or anything because it seemed to be running perfectly and we are only a couple hundred few lower than where it came from.. I burnt about 2 1/2 tanks off fuel and the sled worked great....
...I came off the field onto a road and wrapped it out to get up to speed, about when I started letting off on the throttle it started to run only on two cylinders.. I slowed down and as I stopped it died.. when we tore it down in the shop the middle piston had started to burn down and the exhaust side skirt was broken off. Most of the pieces of piston were in the exhaust but I found a couple small pieces under the diaphragm in the fuel pump ....
the engine is stock as far as I know . It has a can and the airbox is drilled..the main jets are 143.8 all across.. we didn’t find any junk in the carbs or fuel pump or lines. When I drained the tank I found 1.5pine needles and a very small amount of dust or dirt-about what I would expect in a 17 year old sled......
Can anyone help me figure out what could have caused this. I’ll get some pics posted shortly...

 

[nst115]

Here are the pictures for you.

Sent from my Moto G (5) Plus using Tapatalk

 

[nst115]

Are all of the cylinders lean? Could the in tank filter or the fuel pump cause low fuel pressure and cause this?

Sent from my Moto G (5) Plus using Tapatalk

 

[shaggyzr2]

The air box being drilled will lean it out.

 

[Ding]

Looks lean on all three to me. Something limited the fuel or changed bowl pressure / intake pressure as suggested above. I would check the undersides of the pistons to see how hot they were getting. It looks like all three were quite hot.

 

[SKMM00]

Does it seem like my mains might all be jetted to lean?... but why didn’t my pto side burn down first ?..and why did it take so long to blow??

 

[nst115]

Looks lean on all three to me. Something limited the fuel or changed bowl pressure / intake pressure as suggested above. I would check the undersides of the pistons to see how hot they were getting. It looks like all three were quite hot.

What will the signs of a hot piston be on the bottom? discoloration?

 

[nst115]

Here are the pictures of pistons, can you see what you need to ding?

Sent from my Moto G (5) Plus using Tapatalk

 

[opsled]

Broken exhaust skirt..

That doesn't happen from lean burn down. Your center piston's top looks munched from pieces of that skirt getting pumped to the top. I don't see any lean issues looking at the other two.

Hard to say what broke that piston. Many possibilities especially if it's been apart before. Could have been cracked for a long time and finally gave it up.

opsled

 

[SKMM00]

I don’t believe it has ever been opened up..
But I did find a bunch of really fine silver specks in the coolant. One mechanic that looked at it thot it was probably some sort of additive because it doesn’t feel at all gritty when a guy rubs it between his fingers.. Does that shed any light on the subject? Could that have caused it to have trouble keeping cool???....

 

[nst115]

Pictures

Sent from my Moto G (5) Plus using Tapatalk

 

[backofthepackjack]

Broken exhaust skirt..

That doesn't happen from lean burn down. Your center piston's top looks munched from pieces of that skirt getting pumped to the top. I don't see any lean issues looking at the other two.

Hard to say what broke that piston. Many possibilities especially if it's been apart before. Could have been cracked for a long time and finally gave it up.

opsled

Yes and crankcases will have to be split and cleaned and crank bearings checked for damage.

 

[shaggyzr2]

Pictures

Sent from my Moto G (5) Plus using Tapatalk[/QUOTE

Sometimes you'll see those aluminum particles in your coolant, could be dust from machining from the factory, didn't get cleaned out good enough before engine was assembled. I had the same thing with my 700sx, changed coolant and never saw any aluminum particles again.

 

[NorthernSX]

A lot of times, a broken skirt can be caused by piston slap.
Happened to me on my 600 twin.
Cylinders had been honed just a bit too far, creating a larger bore.
So that causes the piston to be too small and slap around more in the bore, causing cracks, then broken skirts on the piston.

 

[SKMM00]

Thanks for the help..
My pistons were scored quite badly so I’m thinking it was just time for it to blow..
Anyway I bought a used motor from nst115 and he helped me put it in yesterday .. richened the mains a bit and cleaning the carbs and it’s running again ...
It’s got a slight hesitation when I punch it off of idle and sometimes midrange but I’d rather be rich than lean.....

When nst looked at my old motor he said he would think that it could have had from 16-20000 Km s on it .....

 

[SteIsa]

That is not lean, it is perfect jetted it is only worn out. That pistons should have been changed prior to that blow up. The clearance between the pistons skirt and cylinder got to big and it knocked the skirt of the piston.