Oil Pumping through breather Part 2

Yorkie · 2026-08-20T06:13:32-0600

A few weeks back I posted that my recently rebuilt 72 Commando motor was pumping oil out of the breather into the oil tank. Nothing was coming out of the oil return pipe inside the tank. It was suggested that I had the oil pump gasket incorrectly orientated and that proved to be correct. However, it did not solve the oil pumping tthrough the breather. The oil is gathering in the crankcase and is not being scavenged by the pump. I stripped the timing side and replaced the oil pump with another one I had in stock. I checked the oil line junction block for gasket orientation and blocked pipes: all good. I checked the spin-on filter and replaced the filter. I checked the return tube to the tank and blew out the tube to verify the hole inside the tank was not blocked I replaced all the oil lines. I think I have the oil lines correctly hooked up - oil tank feed line to pipe on outside of junction block. Return side to filter mount marked IN, Out pipe going to oil tank return pipe. The bike still pumps oil I am not sure what to look at anymore. The bike has a AMR anti-wetsump mod (ironic) and an RGM breather on the back of the crankcase. Thoughts?

gatsby · 2026-08-20T06:22:09-0600

Can you post some pics of all of your connection so we can see the routing?

Carl H · 2026-08-20T06:43:23-0600

Yorkie said:
A few weeks back I posted that my recently rebuilt 72 Commando motor was pumping oil out of the breather into the oil tank. Nothing was coming out of the oil return pipe inside the tank. It was suggested that I had the oil pump gasket incorrectly orientated and that proved to be correct. However, it did not solve the oil pumping tthrough the breather. The oil is gathering in the crankcase and is not being scavenged by the pump. I stripped the timing side and replaced the oil pump with another one I had in stock. I checked the oil line junction block for gasket orientation and blocked pipes: all good. I checked the spin-on filter and replaced the filter. I checked the return tube to the tank and blew out the tube to verify the hole inside the tank was not blocked I replaced all the oil lines. I think I have the oil lines correctly hooked up - oil tank feed line to pipe on outside of junction block. Return side to filter mount marked IN, Out pipe going to oil tank return pipe. The bike still pumps oil I am not sure what to look at anymore. The bike has a AMR anti-wetsump mod (ironic) and an RGM breather on the back of the crankcase. Thoughts?

IF there is a lot of sumped oil in the base they can or will move it thru the oil vent. So be sure the bike sump has been drained dry. . Also the hole and or drilling in the cases can be blocked or has something that obstructing oil, which is happening IF you have no oil return to the tank via the way it should come. When you had the pump off did you blow air through the hole on the pump gasket surface and hear and feel it coming out of the drain plug in the cases. You could also spray wd 40 the case holes to see what dribbles out

You could disconnect the inner return hose at the engine and put a temporary hose on it to check if oil is flowing into a "catch"container.

Call AMR did you install it properly and also remove the RGM thing and replace it with a piece of tubing Or take the "guts" out of it. So you have no things that Norton didn't use stuff in it. Be sure to "prime the pump "properly so you ncan make oil pressure. Does it look like the cases have ever been split and sealed poorly with RTV or any the other gasket surfaces with excess RTV .

velo54 · 2026-08-20T07:21:33-0600

If it is a 72 standard crankcase it has the oil pick up point at the front of the crankcase. The spinning crank pushes all the oil in the crankcase to the rear where the RGM valve is which pumps the oil back to the tank via the breather hose. Modifying the crankcase to put the pickup point at the rear of the crankcase is a big job. Strip the engine. I did the modification, blocked the standard breather and installed a breather at back of the timing chest, like later bikes, put a reed valve in the line and drilled a few extra holes to connect crankcase to timing chest as much as possible. None of your return oil is going through the filter, like a lot of bikes with reed valves bolted to the crankcase. Also installed Holland anti sumping valve, so don't need the crankcase reed valve to evacuate oil in the sump on start up. Graham

Yorkie · 2026-08-20T07:29:21-0600

gatsby said:
Can you post some pics of all of your connection so we can see the routing?

velo54 said:
If it is a 72 standard crankcase it has the oil pick up point at the front of the crankcase. The spinning crank pushes all the oil in the crankcase to the rear where the RGM valve is which pumps the oil back to the tank via the breather hose. Modifying the crankcase to put the pickup point at the rear of the crankcase is a big job. Strip the engine. I did the modification, blocked the standard breather and installed a breather at back of the timing chest, like later bikes, put a reed valve in the line and drilled a few extra holes to connect crankcase to timing chest as much as possible. None of your return oil is going through the filter, like a lot of bikes with reed valves bolted to the crankcase. Also installed Holland anti sumping valve, so don't need the crankcase reed valve to evacuate oil in the sump on start up. Graham

Thank you for your reply. When I first built the engine I blanked off the rear breather and added a reed valve to the magneto blanking plate. It did the same thing where it blew the oil through the reed valve. There is something fundamental that I’m missing. These bikes never pumped oil through the breather when new.

velo54 · 2026-08-20T07:35:11-0600

Maybe , as Carl suggested, the line from crankcase to the oil pump may be blocked. Take oil pump off and blow compressed air in the hole to check.

Carl H · 2026-08-20T07:37:43-0600

velo54 said:
If it is a 72 standard crankcase it has the oil pick up point at the front of the crankcase. The spinning crank pushes all the oil in the crankcase to the rear where the RGM valve is which pumps the oil back to the tank via the breather hose. Modifying the crankcase to put the pickup point at the rear of the crankcase is a big job. Strip the engine. I did the modification, blocked the standard breather and installed a breather at back of the timing chest, like later bikes, put a reed valve in the line and drilled a few extra holes to connect crankcase to timing chest as much as possible. None of your return oil is going through the filter, like a lot of bikes with reed valves bolted to the crankcase. Also installed Holland anti sumping valve, so don't need the crankcase reed valve to evacuate oil in the sump on start up. Graham

Not really, I never had problems with my Combats. But I did learn that you must prevent them from wet sumping. My Current Combat never returns oil via the Combat vent, BUT I do have a manual shut off valve and a Oil Pressure gauge as the safety to be sure I have the valve on. I do not want to return unfiltered oil to the tank for many reasons, the main one is contamination of the whole oil system which caused many Combats major issues.

velo54 · 2026-08-20T07:58:32-0600

Carl, maybe the RGM breather valve that Yorkie is using is a reed valve. As you say, the standard combat breather won't pump any oil.

batrider · 2026-08-20T08:28:47-0600

My Combat oil pump once sucked up what looked like a tab from the cam thrust washer. Some of it was stuck in the short passage from the sump and it also chewed up the pump return gears. I replaced the oil pump and cleared the passageways. I've read about this same thing happening to others.

Carl H · 2026-08-20T09:23:54-0600

velo54 said:
Carl, maybe the RGM breather valve that Yorkie is using is a reed valve. As you say, the standard combat breather won't pump any oil.

My current Combat doesn't return oil maybe mist , because I have a shut off valve and it is never wet sumped.

When wet sumped a stock Combat system does return oil, But not as well as one as a one way valve (reed). I used to desump my first street Combat and race bike by kicking it over and stopping at BDC and it was a real oil gusher In about 10 times the base pressure could MT most of the oil in the base. But I did see finally that it wasn't a good idea for the reason I stated on my current Combat I finished about 5 years ago. I know 2 people that destroyed newly reworked engines because they had check valves in them. It happens and it is expensive.

Yorkie said:
Thank you for your reply. When I first built the engine I blanked off the rear breather and added a reed valve to the magneto blanking plate. It did the same thing where it blew the oil through the reed valve. There is something fundamental that I’m missing. These bikes never pumped oil through the breather when new.

So you have a valve in a similar place as an 850 vent. If you did there is only oil flung around by the timing gears in that area.? So how much oil is actually being returned in 10 seconds or so via the vent. And do you actually have oil pressure so you don't grenade things while testing. IS the bike really full of sumped oil in the base? How much does the oil tank level change after it sits and after you actually run it. As a desperation move Remove the blanking cover and put rags all over the place and run the bike briefly and actually see how much oil is being flung around there. 850s don't have issues with oil in the vent. When you do all I said let me know, My Gal pal rather have me do work around the property than write on the computer. Luckily she is not here now.

possm · 2026-08-20T16:33:59-0600

About 1980 I raced a Speedway Sidecar with a Supercharged Combat engine in it, the breather pumped all the oil on to the track, worked better than the scavenge!

Torontonian · 2026-08-20T16:53:24-0600

Unbolt ( 1 Bolt ) the oil junction to the motor . See if the small gasket was fitted backwards . Pull the timing cover and the oil pump . See if the oil pump gasket was not reversed on assembly .

TRWham · 2026-08-20T20:28:04-0600

It’s a closed system, so the oil must return via some path or all the oil ends up in the sump and the tank runs dry. If the scavenge side is not returning oil, then how do you expect the oil to return to the tank other than via the breather? The breather moves the entire displacement of the engine, so there is airflow to carry oil, especially if the sump, timing side and everything else below is stuffed with oil. It seems your scavenge path must be blocked. Why?

Yorkie · 2026-08-20T21:57:59-0600

Torontonian said:
Unbolt ( 1 Bolt ) the oil junction to the motor . See if the small gasket was fitted backwards . Pull the timing cover and the oil pump . See if the oil pump gasket was not reversed on assembly .

Done those things. All gaskets are correctly orientated.

Oil Pumping through breather Part 2

Similar threads