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Discussion Starter · #1 ·
I have a recurring crank seal question. I've had both crank seals replaced but still have a problem with gear oil being sucked into the crank case. If I reduce the gear oil volume to below 300ml then I don't get the gear oil smoke out the exhaust pipe, but anything more than that it becomes a mosquito bomb.

Any suggestions or possible solutions as to why new crank seals didn't solve this problem?

Thank you in advance.
 

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Wrong or dirty oil filter??? Maybe dirty air breather? Pure shots in the dark. Hope it isn't something more serious.(y)
 

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OK this might be a "dumb" question but.......this is a 2 cycle engine. Are you sure that you are supposed to be putting oil into the "gear box" ? Then are you sure that 300 ml is not enough ??
 

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I just checked and this is a two stoke engine. The bike relies on compression in the crankcase in order to force transfer of the fuel mix up into the combustion chamber via the transfer port and the seals need to be in perfect condition.

There would be a positive pressure in the cases until the falling piston forces the fuel mix up the transfer port into the combustion chamber. This, in theory, would be one time a negative pressure would exist in the crankcase

A short time later the piston rises causing the transferred fuel mix to be compressed prior to firing whilst the inlet port is exposed and a fresh charge is drawn in below the piston via the reed valve attached to the carb inlet.

At this point a strong vacuum would exist in the crankcase and this would be the obvious time where oil could be drawn in

If the reed valve is faulty it may cause oil to be sucked in but I cannot see where from as the valve is directly below the carb and not associated with the gearbox in any way I can think of.

If the engine has an oil pump as apposed to having pre mix it may be due to a faulty pump or incorrect throttle action/cable.

Good luck....Oh I would check those fluid levels as often those quoted are not as accurate as some would have you believe.
 

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Discussion Starter · #7 · (Edited by Moderator)
I just checked and this is a two stoke engine. The bike relies on compression in the crankcase in order to force transfer of the fuel mix up into the combustion chamber via the transfer port and the seals need to be in perfect condition.

There would be a positive pressure in the cases until the falling piston forces the fuel mix up the transfer port into the combustion chamber. This, in theory, would be one time a negative pressure would exist in the crankcase

A short time later the piston rises causing the transferred fuel mix to be compressed prior to firing whilst the inlet port is exposed and a fresh charge is drawn in below the piston via the reed valve attached to the carb inlet.

At this point a strong vacuum would exist in the crankcase and this would be the obvious time where oil could be drawn in

If the reed valve is faulty it may cause oil to be sucked in but I cannot see where from as the valve is directly below the carb and not associated with the gearbox in any way I can think of.

If the engine has an oil pump as apposed to having pre mix it may be due to a faulty pump or incorrect throttle action/cable.

Good luck....Oh I would check those fluid levels as often those quoted are not as accurate as some would have you believe.
Thanks so much Steve! I learned a lot reading your response. I think I will have a leak down test done and go from there.
 

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I had an additional though.
What if ALL of the old oil wasn't getting drained out.......so that it was over-filled ?
Probably not possible.
 
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