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Discussion Starter · #1 ·
hey I have an ex 440 and it blows pistons about every 100 km. I have done almost everything it could be...except the cdi box. Could someone tell me if it is that or if it could be something else that i might not have checked....and if someone could maybe find a cdi box from that year and tell me about it. Thanx alot for your help

Mikey
 

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What do you mean "blows pistons"? CDI's just provide spark. Blown pistons are usually caused 2 ways: running way too lean, or not enough oil. Are the cylinders OK? If it's just the piston, it's too lean. Re-jet it richer.
 

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Discussion Starter · #3 ·
the piston is not being blown, it just puts a hole in the center of the piston. Oil injection is known to be good and so is carburation. And it is the stock plugs so it is not those. But i am not quite sure about the timing.

Mikey
 

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You should hear it pinging quite loudly when under acceleration if it's timing. The pinging is pre-ignition, which will eventually burn a hole. What do the plugs look like when you take them out? They should be a dark brown in color. If they are light tan/white it's too lean.
 

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Discussion Starter · #6 ·
So basically it could be the timing or it is too lean?....i don't really know about the plugs. Its at a friends house right now

thanx
Mikey
 

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Discussion Starter · #9 ·
hey....sorry i haven't replied for a long time...my computer crashed. The guy i bought it from said that the piston goes into a "four corner seizure" The piston seizes up...on the fan side i think. He said that he had a yamaha dealer ship do the seals on the crank....is it possible that air could be seeping in through the crank somehow?

Thanx
 

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Sounds like an air leak somewhere. Do a leakdown test and that will tell you where the air is leaking from.
 

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Discussion Starter · #12 ·
Somebody said that they did this engine with silicone.......is it possible that thats where there is an air leakage
 

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is it one piston blowing each time? is the carb good and clean, and the carb boots not torn or leaking? and crankseals are a big cause, doubt the timing is off unless someone messed with it......
 

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Discussion Starter · #14 ·
ok....i got it alll together.....i hope that it doesn't pop another piston.....how long would it take for it to blow a piston if it is sucking in air somwhere? Would it have already blown a piston if i had it wide open for 4-5 seconds?

Mikey
 

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Maybe not, but check that plug to see how lean it's running. It should be a dark brown, and not a gray or white. If it is, it's still WAY too lean. That way you can fix it before it messes up again.
 

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I have the answer..

True story here....
I burned a hole in about 4 pistons years ago on the same sled. I tried everything until I handed the engine to a dealer and said "fix it". Well he put a new crank in it and did this and that and it ran the best it ever did, until the rest of the sled fell apart.
Long story in full version.
I finally asked why I kept burning out pistons. he asked me if I ever sunk it, I did so I told him about it and he said there was a small amount uf fungus growing on the inside of the mail jet. same side as the burnouts.
Just replace the main jet with a new one. get the same one.
ran strong and great, untill the track fell off.....
 
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