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my back ground is with race car engines circle track and drag not sleds, which i have had sleds for years, but I don't know a lot about porting and such. Here is my problem, a friend bought a 900 moutian cat, takes it out first ride, makes it like two miles at around 5000 feet elevation, burns a piston. He gets the guy he bought it from to do the labor and fix it. He goes out again makes it like 5 miles burns a piston again. I don't know all the particulars, but the guy he bought it from helps him fix it again and fixes it with some cash. It's now the next season he takes it out, and rides for maybe 20 miles burns both pistons. So, it sits this fall. He finally brings me the sled, which i think was hard for his pride to do. I tear the engine down and the engine crank case is full of antifreeze. I have no idea how it got in there and has sat for several months that way the cylnders are scored very bad. I had to have them welded and plated had the crank rebuilt and straighted was out of phase the cylinders were ported and one had a hole drilled through the exhaust valve port the other had a hole in the cyilnder betwween the transfer ports. I showed the cylinders to a couple guys that are cat guys that know them, and they both said i had one early cylinder and one newer cylinder, but should work and live. I have second thoughts about this engine becuase of all the trouble he has had all ready, so I tell him I want to go riding with him the frist time he rides it and he is good with that anyway. I assemble the engine and put it in the sled and know i have to cover all the bases so i clean the carbs really good and get the jet sizes 420 pto 430 mag on mains 40 piolts and needle in the middle clip.Ii was told the engine was a slp performance eddition with twin pipes v force 3 reeds i call slp and talk to jerry very very helpful. He wants to know compresion ratio and if the cylinders have numbers stamped becuase they stamp them no # s that would be theres and i measure the quench with soilder when jerry is on the phone .065 quench so jerry tells me he really wants to help and i need to find out who ported the cylinders and that there jetting for there kit is 420 main 430 main 50 piolt middle needle position at 6000 to 9000 if im un loading at 5000 and going up to 10000 i might want to go up to 430 and 440 with his kit well i pull the heads off and cc them its 37 ccs on the mag head and 38 ccs on the pto head so i think to be safe run 2 gallons of 100LL to a tank of gas. My brother and i take the sled to the hill i mix the gas 50to 1 with oil and run straight 100 octane gas. We unload at 6500 feet and the sled runs good but off the bottom to 5000 rpm seems like it might be fat and blub berry and after 5000 rpm runs hard. It's like 9 degrees out side and 7500 feet elevation then i jetted the sled to be safe i thought at 450 460 and 50 piolt. I run it wot for about 150 yards and pull plugs and they look possiblly alittle lean but safe with about an 1/8 of an inch wash. So i ride the sled about half the day and not pushing it reall hard but make proably 10 climbs wot and pull then do another 150 to 200 yard wot pull and pull the plugs every thing looks good but maybe leaa little bit but not severe think it might need more fuel but we go to the truck to trade sleds and its minus 11 degrees so i think the jetting is ok i call the kid up that night and tell him i want to go riding with him so let me know as soon as he can when i can go with him. Which he now works and partly lives 225 miles away so weeks go by and he calls and wants me to go up to there familys cabin for a couple days and go riding with him. I can't go so he comes last night and gets the sled. I tell him not to run wot for more than 100 hards first tank of gas becuase of the oil in fuel and it leans it down some and should be ok after that so off he goes. He calls me this morning and his uncles sled went for a swim and wants to know what to do, so i tell him. About two hours later he calls me and says that his sled has now burned a piston and siezed solid. Waiting for him to bring it back to me but im baffeld on the lean out becuase i fattend it up and checked wash and it was good and as soon as i give it back its blown up. can any one help me please?????? 
P.S. I don't know what the port timing is. I feel that I am experienced with jetting and getting sleds to run well and usually live, so I am baffled.
dan
P.S. I don't know what the port timing is. I feel that I am experienced with jetting and getting sleds to run well and usually live, so I am baffled.
dan
