1980 440 exciter keeps burning holes in piston

T

terryvmax

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michigan
Hi Ive got a exciter that burnt a hole in the piston for the second time in 100 miles . Both times the sled was under a load. the first time was towing another sled. The second time riding double with a heavy passenger pulling a ice shanty. Looked at piston images, said it was detonation caused by bad crank seal mag side, timing advanced, too lean air mix, low octane fuel. I checked timing and was right on. If it was too lean wouldn't it affect other piston? The other piston looks fine. The plug looked the right color. The octane was 87 , same thing wouldn't it affect both pistons? I don't want to put It back together again until sure why it keeps happening. Is there any way to tell if the crank seal is bad without tearing it down and looking at it? Thanks In Advance.DSCN0327.jpgDSCN0327.jpg
 
Many years ago I had an 80 440 exciter, same thing happened to me. It ran great for a couple years, burnt a hole in the piston, fixed it rode it for a few hours and the same thing happened again. The first time it happend I was running it wide open on a very cold day so I'm thinking I was running it too lean, not sure why it did it again though.
 
Had this exact thing happen to my 77 tigre....turned out to be a cracked carb boot. Sucking air through the crack caused a serious lean condition in just that cylinder. Id almost guarentee yours is sucking air somewhere.
Id really look every possible thing over before running it again. If you cant find anything, try the carb cleaner trick while its idling. Spray it around the seals, carb boots,base gasket....etc and see if there is any rpm change
Good luck, nothing worse then toasting a motor and not knowwing what caused it :(
 
Had a ZR700 do the same thing to me. Tried all the tricks to find a leak. 3 pistons later, I decided to take it all the way down. Ended up finding a sheared flywheel key. I know you checked timing but something to think about.
 
I replaced the boot last time it went down. Looked a little cracked but didn't jump out at me. Tried to eliminate every thing. Wouldn't be able to try the carb cleaner trick till I rebuild it. Hoping to find the problem before that. The plug heat setting is correct. I don't know much about crank seals, could it suck air there? If I cant find something concrete might sell it as a parts sled. Putting too much money in a 34 yr. old sled.
 
34 years old.

Crank seals!!

Those sleds are bullet proof but no seal is sure to last that long.

Yes, a bad seal will cause your problem.

opsled
 
I will 100% agree with opsled on this one ........I rebuild these motors all the time and every time they are burned down it is a bad crank seal .....they are cheap for winderosa ones or you can spend about 4 times as much for OEM ones heck most motors I rebuild from the early 90's get crank seals right away .......they do not last forever .....even my races sleds the motors come apart every year and everything is checked and re ringed I check the feel of the crank seals and look at the lips if anything questionable they get replaced ......in my many years of racing I have only blown a few motors and it was due to jetting it on the ragged edge not because of a air leak

also leak down tests do not always show bad crank seals.......learned that long ago
 
there is 2 ways to do it ....one rip the whole motor down or 2 just remove the bottom have of the case and slightly lift the cranks to get the crank seals off ....special tools required would be a clutch puller and flywheel puller ...make sure to mark the stator before removal
 
I would look for a crank seal leak, crank case leak between the halves. Double check timing. 1.5 mm btdc, jet 145 main, and double check ohm on spark plug caps and coil. VR98 for spark plugs
 
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