Last March motor burned down with a hole in the mag side piston. Did a top end rebuild this summer with 2 new pistons. I thought the problem was because oil pump set slightly lean and running wot at -10. Took it out this morning for a 10 mi ride and sled shut off at stop sign. Took out plugs and I see a crater starting to form on the mag side piston. I was running extra oil in the fuel for break-in. Carbs were cleaned and set to factory specs during rebuilt. Anyone have any ideas on what I should do now?
Help with z570 motor
- Thread starter krash
- Start date
Hole/crater in the middle means lean condition. Can be air leak in base gasket, crank seal, intake tract. Can be carb settings off or dirty. Can also be bad rings that allow blow by and bad case pressure. LOW oil causes crank bearing failure and piston scoring (rings and along piston skirts. Don't take it apart. Take it to someone who can pressure test the motor. You can take to carbs apart and check them agains factory settings. two more things. Water can also cause this so carefully take the carbs off. Not letting any fuel spill from the bowl. Drain all of the fuel from the carbs into a glass jar and let it settle for 15 minutes. If you see water, that can be all, or part of your problem. make sure your are running the correct plugs. Cats do not like the incorrect plugs.
A hole in the top of the piston, directly below the plug, is all timing. the timing is off by a few degrees. If it only does it on one cyl. I am afraid you may have spun the crank out of index. I have seen this on a watercraft engine, exactly the same thing. Only way to fix the crank is to have it trued. You can check the crank indexing if you have a cyl. timing indicator dial. Or have a good mechanic check it, it will be obvious if it is spun. The TDC of the pistons should be 180 apart, anything else, and its spun.
A lean condition will burn off the exhaust side of the piston first, as it gets the hottest first.
A lean condition will burn off the exhaust side of the piston first, as it gets the hottest first.
trailshredder
Member
No doubt an air leak...spray carb cleaner around the engine and carbs while sled is running...it will start dying (flooding out) when you find your leak. Good luck.
trailshredder
Member
Could also be mag side crank seal which you can not get at with the carb cleaner trick.
Started to tear it down tonight. Noticed a missing bolt on mag side y pipe. Not sure if that has anything to do with the problem. Here is a picture of the piston. Crater starting to form on top and some starting to wear or top exhaust side of piston. Does this help anyone on the problem?
Attachments
too hot and too lean, either by fuel/air mix or timing. My bet would be the crank seal behind the stator plate. Some are replaceable w/out splitting the cases. If not, just flip it over and remove the bottom half only, replace the seal, check the bearings, seal the case halves and re-assemble. Make sure you find the problem before you fix it again.
The scuff marks were probably from the melted pieces of the first piston failure, as they fell to the bottom and re-solidified, them scraped through between the crank cheek and the case. The pieces probably went out the exhaust port at some point. If the case is not cracked where the scrapes are, dont sweat it. You can check the crank for runout on a couple of V-blocks. One on each end, and a dial indicator, checking at various points along the crank. While rotating the crank, it should be within a thousands or two. If it is way out, it needs to be trued. To check the index, it needs to be assembled. The pistons should be exactly 180 degrees apart. You have a TDC mark on the flywheel for the #1 piston, you need to have another mark at 180 deg. from that to check the other piston. A degree wheel is the best way. If it sound like more than you can do, have a local machine shop (engine re-builder) check it, or the dealer if you trust them.
Let us know what you find out, good luck. Jim
Let us know what you find out, good luck. Jim