2007 crossfire 800 keeps blowing up

[almerv]

I just recently picked up a blown up 2007 crossfire 800 with 3900 miles. Here's the story from the previous owner. It blew up on him twice. the first time he just thought it needed a top so he rebuilt it new pistons rings gaskets and one new cylinder because it melted the piston and scored the cylinder. Shortly after the rebuild it blew up again within like the first 100 miles. So a buddy of his said that a crank seal was out which would cause the cylinder to run lean and get hot. He bought a new used lower unit and then lost interest and put it for sale. That's where I picked up I bought one new cylinder the other cylinder was still in great shape. I bought new pistons and rings for both sides and a whole gasket set. This is not the first snowmobile or engine I've ever rebuilt, I torqued everything to manufactures spec and then did a pressure test and everything checked out great nothing leaking and held pressure. After getting the engine in the sled I emptied the oil tank the best I could, there was still maybe 1/4 of a quart left in the bottom. I mixed the gas in the tank to 50:1 plus the oil injector mixing whatever was left in the oil tank. I figured it was getting plenty of oil. i drove it around the yard putting it through heat cycles breaking it in. after a few runs around the yard i decided to take it down some trails I still haven't gotten the sled past 1/2 throttle I've been riding it easy. while in the trail I was about 10min into the ride and it died on me I got off pulled on it and it fired back up everything seemed to be okay so i kept going. I went another 300yrds down the trail and it died again this time it pulled a little harder i never started it I opened the hood and looked at the coolant tank and it was almost overflowing. when I started riding the coolant level was between full and empty it was on the lower side towards empty but nothing to worry about. I pulled the clutch side plug just to look it over and it looked great didn't look to rich or lean so i put it back together and it started back up and it ran great and never died again, but I only put maybe another 10miles on it tops after that. I got home i pulled it into the garage and took the exhaust off to inspect the pistons and what do you know the mag side piston is scored bad so i removed the head and the cylinder was shot. now the motor might have 30miles on it since I've rebuilt it. i did notice one thing when I was tearing the head off, when i went to take the power valves off i must have forgotten to tighten the bolts to the power valves because they were only finger loose which I'm sure cant be great so maybe that is the reason it burnt up. I'm thinking that the crank seals were fine when the previous owner had it because this is the 3rd time its blown up within a short amount of time. I'm leaning towards a fueling issue where its not getting enough fuel leaning it out but im at a loss currently of where to even start because i have a feeling if I just put another top end in it its just going to burn up again. if anyone has any ideas of where to start or has seen any issue like this on these 800sid greatly appreciate the feed back. thanks.

 

[deanross]

I’d be interested in hearing some ideas also, as my sons M1000 is doing the same thing. But unfortunately can’t give you any help. You might try posting this in the Arctic Cat forum.

 

[Readymedic]

Which pistons are in the sled, and you replace with? Always go OEM. The best tolerance. They are aluminum. WISECO is garbage. They are forged. They heat up at different temps on different areas, they are not reliable. You have to let them warm up for along time. So they expand equally.
All you ever have to do for a 2 stroke is 3 heat cycles. Thats it, then you rip it. It could be your break in period isen't quite right. Heat up 3 times. Between each cycle, let it cool off completely. Third time, let it cool completely, let it idle and go. Very easy to get a 4 corner seizure, then its done.
One thing you never mentioned...did you have the power valves out?? Other than the bolts loose. If you have them upside down...by by piston.. They have a curve to them. That curve passes by the piston..
If you were lean, your piston would be melted, rings would be melted to the piston. Pieces everywhere, and pitting on the crown.

 

[almerv]

Which pistons are in the sled, and you replace with? Always go OEM. The best tolerance. They are aluminum. WISECO is garbage. They are forged. They heat up at different temps on different areas, they are not reliable. You have to let them warm up for along time. So they expand equally.
All you ever have to do for a 2 stroke is 3 heat cycles. Thats it, then you rip it. It could be your break in period isen't quite right. Heat up 3 times. Between each cycle, let it cool off completely. Third time, let it cool completely, let it idle and go. Very easy to get a 4 corner seizure, then its done.
One thing you never mentioned...did you have the power valves out?? Other than the bolts loose. If you have them upside down...by by piston.. They have a curve to them. That curve passes by the piston..
If you were lean, your piston would be melted, rings would be melted to the piston. Pieces everywhere, and pitting on the crown.

I bought spi pistons that were recommended to me by another mechanic I put them in my other sled that I rebuilt a few years ago and couldn’t complain about them so far. But I believe OEM pistons are the best and that is what I will go with this time. I had the power valves out and I put them in the correct way but forgot to tighten the 2 bolts down I did not adjust the power valves while I had them out ether. It was definitely a lean condition the rings were stuck and the piston was melted. I checked the fuel pump and benched tested the injectors everything seemed good. Fuel pump was at 43psi. I went through the stator and everything checked out hood there. so I’m thinking it was that power valve being loose and sucking air. The previous owner had issues with the clutch side and now mine was on the mag side.