850 engine: oil sprays out from the tank cap

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pierodn

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Hi.
From the oil tank cap, i think for excessive oil pressure into the tank, comes out oil (cap and seal are not damaged).
Bike has the stock breather on the rear of the timing.
No external oil filter.
Oil is a monograd 50w; here the temperature is 15/25 degree.
What could be causing this?
Could be a violent oil return or the breather that emulsified the oil?.
What should I check?
Thank you.
Piero
 
Last edited:
2 questions. how much oil is coming back from the return line? if none or almost none you have a problem in the oil pump or pickup side and forcing it back through the breather. you could also have a big problem with ring seal.
 
bill said:
2 questions. how much oil is coming back from the return line? if none or almost none you have a problem in the oil pump or pickup side and forcing it back through the breather. you could also have a big problem with ring seal.
Click to expand...

Hi.
Oil return back.
Oil pump works fine.
No problems with ring seals.
Thanks.
Piero
 
Sounds like to much back pressure from your breather hose which will blow into your oil tank and force oil from your oil tank to blow out of the oil tank cap, if your oil tank is over filled would be worst or your breather hose is blocked that relives pressure from your oil tank.
I have been running my motor breather hose into a catch bottle and haven't run it to my oil tank for over 35 yeas now (but my 850 is set up in a Featherbed frame) I have a 1 ltr clear plastic catch bottle mounted between my rear engine mounts and the breather hose from my motor goes in the top of the bottle to the bottom of the bottle there is also enough space for th air to excape at the top of the bottle and if any oil that blows out of the breather sits on the bottom of the catch bottle, not much oil gets in there and only gets about a few mm of oil in a year.
I did this because of to much back pressure and was causing my oil tank cap to leak has never done it since removing the motor breather from the oil tank, I am also running a Yammy reed valve on my breather hose mounted where it comes out of the back of the timming cover, I also run my oil tank breather into the same catch bottle.

Ashley
 
Hi Pierro!

I sounds like your oil tank is not venting to the atmosphere. At the top of the tank is a short metal tube that should have a hose attached, where does this go to? It has to be clear of any obstruction to vent.
 
Not sure what is going on here. Can you describe what you see while idling engine and looking inside the tank neck? Does oil returning from engine get deflected by the deflector at the rear side of tank neck? Do you see oil enter tank from breather port at front side of neck? How full is your oil tank level? Check the vent line running down from tank neck to air filter Ham-Can rear side. It might be plugged up.
 
ashman said:
Sounds like to much back pressure from your breather hose which will blow into your oil tank and force oil from your oil tank to blow out of the oil tank cap, if your oil tank is over filled would be worst or your breather hose is blocked that relives pressure from your oil tank.
I have been running my motor breather hose into a catch bottle and haven't run it to my oil tank for over 35 yeas now (but my 850 is set up in a Featherbed frame) I have a 1 ltr clear plastic catch bottle mounted between my rear engine mounts and the breather hose from my motor goes in the top of the bottle to the bottom of the bottle there is also enough space for th air to excape at the top of the bottle and if any oil that blows out of the breather sits on the bottom of the catch bottle, not much oil gets in there and only gets about a few mm of oil in a year.
I did this because of to much back pressure and was causing my oil tank cap to leak has never done it since removing the motor breather from the oil tank, I am also running a Yammy reed valve on my breather hose mounted where it comes out of the back of the timming cover, I also run my oil tank breather into the same catch bottle.

Ashley
Click to expand...

Hi.
But why such high back pression from the breather?.
What i must check?.
Thanks.
Piero
 
If this is a Commando oiltank you have two breather hoses. The one coming from the engine enters the tube at the filler neck. The other hose further forward on the tank is the breather for the oiltank. There is some sort of de-mister inside that dome to clean the escaping air from oil. If this is blocked the air will leak out of the filler, depending how high you filled the tank it will take more or less oil with it. Never fill above max mark, stay a bit lower.
Be sure you have connected both hoses at the correct place, blow through the front hose back into the tank with open filler cap - there should be almost no resistance and you would hear the air entering the tank.
Had a customer with an almost molten piston with lots of blow-by on his 850 Commando, he still rode here for the repair and even with most of the RH compression going into the cases and from there into the oiltank there was no oil around the cap.
If this is a central oiltank made for the coffee racer brigade in shiny aluminium it usually has only one breather pipe (and often not even a drain screw). These lousy tanks can only breath through the (usually used) Monza cap - thereby making a mess of the bike as soon as you fill the tank more than half.....
 
All motors have back pressure from the crank case some more than others thats why we have breathers so the air excapes, without a reed valve the air can be sucked back in, on older Commandos the breather hose was vented out the back of the bike, thats one of the reason I vent mine into a catch bottle rather than the oil tank and also keeps the oil a lot cleaner, some may say it doesn't but I think it does, to much air presure in the oil tank and the oil will find a way to excape.

Ashley
 
WEAL Norton said:
If this is a Commando oiltank you have two breather hoses. The one coming from the engine enters the tube at the filler neck. The other hose further forward on the tank is the breather for the oiltank. There is some sort of de-mister inside that dome to clean the escaping air from oil. If this is blocked the air will leak out of the filler, depending how high you filled the tank it will take more or less oil with it. Never fill above max mark, stay a bit lower.
Be sure you have connected both hoses at the correct place, blow through the front hose back into the tank with open filler cap - there should be almost no resistance and you would hear the air entering the tank.
Had a customer with an almost molten piston with lots of blow-by on his 850 Commando, he still rode here for the repair and even with most of the RH compression going into the cases and from there into the oiltank there was no oil around the cap.
If this is a central oiltank made for the coffee racer brigade in shiny aluminium it usually has only one breather pipe (and often not even a drain screw). These lousy tanks can only breath through the (usually used) Monza cap - thereby making a mess of the bike as soon as you fill the tank more than half.....
Click to expand...

Hi.
The oil tank is an original commando.
Hoses well connected in places.
Could be blow-by?
Thank you.
Piero
 
ashman said:
All motors have back pressure from the crank case some more than others thats why we have breathers so the air excapes, without a reed valve the air can be sucked back in, on older Commandos the breather hose was vented out the back of the bike, thats one of the reason I vent mine into a catch bottle rather than the oil tank and also keeps the oil a lot cleaner, some may say it doesn't but I think it does, to much air presure in the oil tank and the oil will find a way to excape.

Ashley
Click to expand...

Hi.
I understand.
But as far as i know the 850 engine has the
Best oil line and breather system.
What do you think about blow by of air from pistons into the cases.
Thank you
Piero
 
With the breather hose in a bottle i can see that comes out a lot of aria, no oil.
Oil returns well into the tank.
No more oil emulsion with the breather in the bottle a part.
Ciao
Piero
 
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Piero. It does sound like blow by. I had a similar problem with my 850 before it died on me. Must have taken 3-4 years to die from the time I first bought the bike. When it died, I found the rings on one piston had flogged there way through all the oil lands and right through to the oil ring groove.
I suggest you make up an adapter to fit a compressed air line to the cylinders [ using a quick fit coupling like an Aro coupling ]. Test one at a time at TDC, lock the bike in top gear and jam the rear brake pedal. You may be best to remove one of the exhaust rocker covers. With all set up ready to goe and with the compressor up to pressure, connect the compressor hose to the plug adapter and listen for air leaking into the crank case. If you hear the air rushing into the c/case, your rings are stuffed. With good rings you may hear a very slight hissing. Most do this when the engine is hot but if its stuffed, it wont matter. A cold engine is likely to leak more past good rings than a hot engine. Ultimately, you may end up fitting new pistons kits. My engine is still std, the broken rings barely made a mark on the bore. I considered myself lucky.
My engine leaked oil like a sieve, and I had to make sure the oil cap was firmly tight or it would leak out like you have described.

Have fun.

Dereck
 
pierodn said:
With the breather hose in a bottle i can see that comes out a lot of aria, no oil.
Oil returns well into the tank.
No more oil emulsion with the breather in the bottle a part.
Ciao
Piero
Click to expand...

Thats right.
 
kerinorton said:
Piero. It does sound like blow by. I had a similar problem with my 850 before it died on me. Must have taken 3-4 years to die from the time I first bought the bike. When it died, I found the rings on one piston had flogged there way through all the oil lands and right through to the oil ring groove.
I suggest you make up an adapter to fit a compressed air line to the cylinders [ using a quick fit coupling like an Aro coupling ]. Test one at a time at TDC, lock the bike in top gear and jam the rear brake pedal. You may be best to remove one of the exhaust rocker covers. With all set up ready to goe and with the compressor up to pressure, connect the compressor hose to the plug adapter and listen for air leaking into the crank case. If you hear the air rushing into the c/case, your rings are stuffed. With good rings you may hear a very slight hissing. Most do this when the engine is hot but if its stuffed, it wont matter. A cold engine is likely to leak more past good rings than a hot engine. Ultimately, you may end up fitting new pistons kits. My engine is still std, the broken rings barely made a mark on the bore. I considered myself lucky.
My engine leaked oil like a sieve, and I had to make sure the oil cap was firmly tight or it would leak out like you have described.

Have fun.

Dereck
Click to expand...
Hi Derek,
i understand what you mean
Thank you you so much for your sugestions.
Piero
 
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