I understand the breather tube vents off excess crankcase gases but why all of a sudden does itstart spitting oil out?

 

has the bike sat for a long time?

 

As already mentioned this will occur if the bike has sat for a period of time. It results from oil leaking past the check valve in the oil pump. In extreme cases the valve must be re-seated or replaced. But it is a common occurence, normally to be ignored. Or put a can under there before you start it.

Also, what year is your bike? There are three different breather mechanisms: pre 77, 77 & 78, and 79 to 85.

 

it is a '71, I ride it a couple times a week. i did the top end in April and have put a few hundred miles on it. yesterday i rode it about 20 miles and when i stopped there was fresh oil under the bike and on the side of my rear wheel. i restarted it to see if i could see where it was coming from and it was spraying out of the vent tube. i rode it last night and this morning checking it when i stop and it seems to be intermittent, little spits once and a while. is this common or is there something else.

 

If the problem was oil passing the check valve while the bike was sitting, it would all come out during the few minutes after startup. Since it is continuing to come out during an extended ride then i would think there is another problem.

I am not familiar with the pre-77 breather system. Just that it is timed, and that when cams are installed there is a procedure to follow to ensure that the breather is again correctly timed.

Did this problem occur before the top end was done? As part of the top end rebuild, were the cams removed and re-installed?

 

it never happened before or after the rebuild until saturday. oil pressure is good, oil level is below the return tube, starts right up, idles fine/normal, I went on another ride yesterday afternoon and noticed a few oil spots on the tire but nothing major. Maybe it was a freak thing??

I know there is another vent above the oil pump and it is vented back into my oil tank, it was like this when i bought the bike. do you think it could be excess pressure against the scavange pump causing backup of oil? should the oil tank be vented into open air???

thanks

 

The scavenged oil is pushed into the breather oil trap by the flywheels.
The breather is timed to open when the pistons come down. The pressure created by the pistons coming down pushes the scavenged oil out of the trap and into the timing cover. This oil lubes the cam gears, idler gear, generator gear, rear chain lubricator, etc.

When the pistons go back up the breather valve closes and this creates a vacuum in the crankcase. The pressure in the timing cover has to go somewhere, and that is expelled through the breather tube.
If there is any excess oil contained in the air, it is thrown off by the oil slinger located on the generator drive gear.

When the bike has sat without being run for a while and there is excess oil in the crankcase, the oil slinger cannot handle all of it, so it gets puked out on the floor through the tube.

Once this happens there should be no more oil leaking from the tube, other than perhaps 1 or 2 drops when you stop. And that is mostly oil still clinging to the walls of the tube from the most recent puke. Certainly not enough oil is present to leave oil on the tire sidewall. This is in excess.

Are you sure you do not have a chain oiler on the bike?
If you do, are you certain that it is turned off? This is the only thing that would leave oil on the sidewall.

And the vent inside the oil tank is nothing more than an open ended stand pipe which sits above the oil level in the tank. No issues with that............. pg
PS: No, the oil tank vent does not have or need a vent to air outside the oil tank.

 

I am getting to the limit of my knowledge on this. Someone else will chime in.

The oil tank should not be vented to the open air. There should be a vent line running from the rear end of the gearcase [cam] cover to the top of the oil tank.

The input to the pump is at the rear end, bottom; output at the front end. You could check all of the lines to ensure that they are run correctly, and that they are clear, and that they are not kinked anywhere. Here is an oil line routing diagram for older Sportsters ...



Also note that in my limited experience, if the oil tank is overfilled it will come out somewhere. On my 1980 i have never been able to determine where, but it leaks somewhere and makes a mess when overfilled.

I do not understand this statement: "oil level is below the return tube".

 

I do not understand this statement: "oil level is below the return tube

What I mean by that Mick can best be explained by my hand drawn sketch below. The open ended stand pipe for the vent (as well as the identical oil return stand pipe) have open ends that are always above the oil level inside the tank...... pg

 

thanks guys; the chain oiler is off. when i first saw the oil, it was on the frame below the timing cover so i wiped it down and started the bike, it immediately started puking oil from the tube and slowly tapered off. would excessive oil flow through the pump cause this by pumping more than the scavange can handle (cavitation) BUURP?? I got on it and took it to 75mph 2 minutes before this happened?? the thing still runs and sounds great!