Primary oil sucked into engine.

Joined
Jul 30, 2009
Messages
191
Country flag
I rebuilt my 74 850 recently. After covering about 200 miles the primary started making an odd noise. I decided to check the primary chain tension. I found the chain was very dry and there was almost no oil in the primary. I had managed to seal the primary very well, and I hadn't noticed any leakage at all. At first I thought maybe I had forgotten to put any oil in. Further reflection and I remembered putting the oil in. I put the oil in quite a while before the top end was back on the engine and I remember taking out the level plug filling the primary with ATF and then replacing the plug, with a new Dowty type seal. I remember being very pleased that the O ring was holding the oil in perfectly. I attributed the perfect seal to the coating of Ezyturn grease that I had used. Also I think if I had forgotten to fill the oil then the primary would have made some bad sounds sooner. After I re filled the oil the primary sounded fine again and I have covered about another 40 miles now. I never had to adjust the primary chain. I think I must have caught the problem just in time.

I am pretty sure that the problem is my new reed valve breather that I installed on the side of the engine. I think I bought this thing about 5 years ago from Grandpaul. I think it is working really well and I am getting a strong vacuum in the crankcase. I have gap less total seal rings, and after the rebuild I checked the leak down and it is insignificant. I think the primary oil is getting sucked into the engine. I had this same problem on my Vincent, to solve it I put the seal in the other way around (with the spring on the outside) Has anyone else had this problem with too much vacuum in the crankcase causing the primary oil to go into the engine? Some of the breathers with reed valve like the ones designed by Jim Comstock must cause that problem as well?
 
The oil level should be too low for the oil to get sucked into the engine from the primary case as the oil level should just make contact with the bottom run of the triplex chain. Check the felt seal behind the clutch basket is in there and the big O-ring that surrounds the primary outer case against the inner case is not damaged. I would put some cardboard under the chain case with the bike on its centre-stand and run the engine in gear for a bit to see if you are drooling oil anywhere out. Check for cracks in the primary case or misalignment . Check that you have a paper gasket between the engine output to inner primary case. I would not put the engine output seal with the spring facing out. The little screw at the bottom most of the primary outer is a oil level screw and not a drain screw. The two large inspection caps should have o-rings in them as well as the smaller cap to check your timing mark.
Check the oil level on the center stand and not the side stand when filling the oil into the primary. Sorry if I am being too simplistic. I might have dumb ed it down too much.
:rolleyes:
 
Last edited:
I filled it till it came out the level plug. It's not much oil, just enough to cover the lowest part of the chain around the clutch. Oil isn't leaking out. The bike is very clean. I can park it and it doesn't leave spots. I think it is getting sucked past the seal behind the drive sprocket. This also happened on my Vincent, and I have heard of others who had this happen on Vincents. I am wondering mainly if anyone else has had this happen on a Commando.
 
I filled it till it came out the level plug. It's not much oil, just enough to cover the lowest part of the chain around the clutch. Oil isn't leaking out. The bike is very clean. I can park it and it doesn't leave spots. I think it is getting sucked past the seal behind the drive sprocket. This also happened on my Vincent, and I have heard of others who had this happen on Vincents. I am wondering mainly if anyone else has had this happen on a Commando.
Nope.
I've run XS650 as well as Comstock breather.
And, I way everfill my primary oil level.

Good luck solving the mystery.
 
Leaking thread on bottom primary chaincase fixing bolt? It’s a long shot, but what else is there ?

Gotta be honest though mate… my money is on you forgot that you drained the oil at some point…
 
The other leak point is the three bolts that hold the chaincase inner to the crankcase, I apply loctite both to seal and prevent them coming loose. But as others have noted oil leaking back into the crankcase is not a previously known issue with Commando engines especially with that frankly useless sealing arrangement behind the clutch.
 
I filled it till it came out the level plug. It's not much oil, just enough to cover the lowest part of the chain around the clutch. Oil isn't leaking out. The bike is very clean. I can park it and it doesn't leave spots. I think it is getting sucked past the seal behind the drive sprocket. This also happened on my Vincent, and I have heard of others who had this happen on Vincents. I am wondering mainly if anyone else has had this happen on a Commando.
If you think about it, after the chain picks up oil on the rearwards bottom run, it then flings it outward around the clutch and then, after that, away from the crank seal.
Unless you have a special port at the bottom of the primary/crank cases I can't see it being that. Unless a lower bolt or stud is missing or very loose and somehow the hole into the crankcase is open?
 
I filled it till it came out the level plug. It's not much oil, just enough to cover the lowest part of the chain around the clutch. Oil isn't leaking out. The bike is very clean. I can park it and it doesn't leave spots. I think it is getting sucked past the seal behind the drive sprocket. This also happened on my Vincent, and I have heard of others who had this happen on Vincents. I am wondering mainly if anyone else has had this happen on a Commando.
Nothing is getting sucked out of the primary to the engine. Not possible.
 
I rebuilt my 74 850 recently. After covering about 200 miles the primary started making an odd noise. I decided to check the primary chain tension. I found the chain was very dry and there was almost no oil in the primary. I had managed to seal the primary very well, and I hadn't noticed any leakage at all. At first I thought maybe I had forgotten to put any oil in. Further reflection and I remembered putting the oil in. I put the oil in quite a while before the top end was back on the engine and I remember taking out the level plug filling the primary with ATF and then replacing the plug, with a new Dowty type seal. I remember being very pleased that the O ring was holding the oil in perfectly. I attributed the perfect seal to the coating of Ezyturn grease that I had used. Also I think if I had forgotten to fill the oil then the primary would have made some bad sounds sooner. After I re filled the oil the primary sounded fine again and I have covered about another 40 miles now. I never had to adjust the primary chain. I think I must have caught the problem just in time.

I am pretty sure that the problem is my new reed valve breather that I installed on the side of the engine. I think I bought this thing about 5 years ago from Grandpaul. I think it is working really well and I am getting a strong vacuum in the crankcase. I have gap less total seal rings, and after the rebuild I checked the leak down and it is insignificant. I think the primary oil is getting sucked into the engine. I had this same problem on my Vincent, to solve it I put the seal in the other way around (with the spring on the outside) Has anyone else had this problem with too much vacuum in the crankcase causing the primary oil to go into the engine? Some of the breathers with reed valve like the ones designed by Jim Comstock must cause that problem as well?
A reed valve should not be creating a "strong vacuum" in the engine if it is working.

There are no passages from the primary to the engine that are low enough for oil to be sucked into the engine. On the other hand, the inner chain case to engine screws can leak when the engine is wet sumped and oil can enter the primary.

I've had two cases recently where a reed valve was not working (closed all the time), and lots of engine leaks were caused, but still I can't see that causing oil to be sucked out of the primary.

If you oiled things while putting the primary together and then forgot to actually add oil, it would likely be a while before you heard anything. The only thing of substance the oil does is lube the chain.

IMHO, applying anything to the rubber seal between the inner and outer chain case = leaks.
 
On my Vincent the oil was getting sucked from the primary into the engine, until I reversed the seal. It is OK now. The Vincent has the standard timed breather although it is set at non standard timings. On the Norton I will have to wait and see. If the oil doesn't go down then it means that I just forgot to put oil in there in the first place. If it goes down then it must be going into the engine because there isn't much on the ground.
 
On my Vincent the oil was getting sucked from the primary into the engine, until I reversed the seal. It is OK now. The Vincent has the standard timed breather although it is set at non standard timings. On the Norton I will have to wait and see. If the oil doesn't go down then it means that I just forgot to put oil in there in the first place. If it goes down then it must be going into the engine because there isn't much on the ground.
Keep us posted 👍👍👍
 
I also vote impossible. No way the ATF is being sucked out of the primary by vacuum pressure in the crankcase through a hole that is 4" above the level of the primary fluid. The more likely exit point is the lower edge of the primary joint. If it's ATF it might be harder to spot than motor oil...

Jim Schmidt (JSmotorsports) recommends using an O-ring on the crankshaft placed on the primary side of the crankshaft oil seal. It adds a little resistance to the seal by blocking the the lips of the oil seal. You can contact him for the details, of which I am not familiar...
 
Back
Top