Now What Hapened guys?

[MountainMax]

Another destructive trip for me this weekend, all was going well since my last rebuild ( about 5 tanks ago ) but this time I melted a piston on the mag side (last time i had a leaky head gasket and broke the top of piston #1). Anyhow I don't know why it burned down, there was no leaking base seal, and my jetting was all good, after a WOT run across a small lake she was doing about 170 on the speedo and she shut off, towed it home and this is what i found the piston looked like, I checked the carbs for dirt, nothing in the jets but a found two small chunks of hardened material in the bottom of the float bowl, and there wasn't alot of oil down in the base or on the crank, my rod bearing and crank bearing seem fine though, checked my bender direce oil injection and that all seems fine also. I had .5 litre of injection oil thrown in with each tank of fuel also since the last rebuild so I doubt it was lack of oil. by looking at the pistons what do you guys think? Im going with the peice of dirt sucked up and blocked my main jet leaning her out and overheating that cylinder, but could a piston melt that fast, like say 20 seconds flat out? I don't know what else to look for. Opinions please.............

 

[terret725]

thats def a leab burn down. could be a dirty carb but id go more with a leaky seal some ware. like around the intake boot, crank seals, and even exaust some times cause cause it. what kind of mods do u have? bost bottle, porting, head gasket mod, big bore?

 

[daman]

Yup, lean from somthing, you sure jetting was on?

 

[MountainMax]

im sure my jetting was on but like i said i found a couple peices of solid chunky salt like peices about 1 mm wide in the float bowl, there is fairly new crank seals in there and the intake and exhaust manifolds and boots were and are still ok. My mods are a bender 800 big bore and boost bottle/triple pipes running 165 main jets (bender calls for 160's) and she ran fine last year with those jets at -30, yesterday was 2 degrees above.....

 

[daman]

It was probly the crud then..the other two are fine right?

 

[Turk]

Get rid of the Bender direct oil injection unit. I had a bud install one & immediately burned it down twice. He went back to the stock oil injection system & no more probs.

 

[MountainMax]

Thanks Turk, I wondered if losing oil would do this, it's now history, i got to connect up the oil hose back into the fuel pump and try to block the ports in the carbs now....... Could this possibly cause a lean burn down like this though?

 

[daman]

That sure looks like a lean sqweek with the melted alum, out of oil
really don't do that, unless she got so hot..dunno....

 

[MountainMax]

She never showed hot on the coolant temp guage when it hapened it was down about 140 degrees, but i tell ya, there is no oil on the crank or down in the base pooled up like I normally see when pulling the cylinder off.......

 

[daman]

but i tell ya, there is no oil on the crank or down in the base pooled up like I normally see when pulling the cylinder off.......

Yea not good then...check the crank out good..

 

[crewchief47]

She never showed hot on the coolant temp guage when it hapened it was down about 140 degrees, but i tell ya, there is no oil on the crank or down in the base pooled up like I normally see when pulling the cylinder off.......

I'd be willing to say that it was the crud that caused the burn down. If the guage didn't show hot, it's probably because the fuel went from regular flow to zero when the crud moved into position. It's not lean in the sense there wasn't enough gas, it's lean in the sense that it's starved, and that will burn down real quick that way. I had a similar experience on a 94 600. Blasting across the lake and then wham, burn down with zero warning. Found a piece of crud in the bowl. Piston looked much the same although the cylinder was fine. Of course those 94's drank oil.

I may be wrong, and with the time you have invested in this motor, you should make sure someone else agrees with the above.

 

[MountainMax]

Thanks for the opinions, i think it was the crud in the carb also, but I am going to replace both crank seals and fully clean out the carbs and remove the bender oil injection and start fresh for next season, while im at it i will check the crank out throughly for bearings and out of round. Thanks again guys........

 

[daman]

Good luck MM....

 

[woolyviper]

if the crud was big enough to block the main jet, than I would blame that - that sure looks like a really lean condition - you need gas to cool the pistons, and when that isn't there, the piston just melts.

 

[terret725]

if u have a boost bottle i would do a leak test on the nipples on the intake when u get it back together. spay some carb cleaner or starter fluid around them when the sled is idling and if it idels up u have a leak. i put a bost bottle on at the beginning of the season and burned down the first ride cause i didn't do a check. needless to say i don't have the boost bottle on any more and have a rebuilt top end.

 

[kimoaj]

Sorry to read this, burndowns always suck.
I`ve ridden sleds with float bowls that was so full of shit you wouldnt believe it, non of them burned down.(well my friends V-Max 4 burned down, I did find some dirt in the bowl, but the main jet was open as the day it left the factory). I`m not saying it wasn`t the reason for your burndown, but I would have more faith in that theory if you pulled the bowl and found a clogged main jet. Not like now when you found something in the bowl.
The chance for the crud to stay under the main jet for such a long time that it seized, is minimal. With gas constantly entering the bowl, the sled/engine moving around/vibrating, I doubt the crud would block the jet.
Although I`m not a pro I`m just saying what I think.
What pistons were you running?. Did you check cylinder-->piston clearance when you rebuilt it?.