Breather

Dear Nortoners,

In order to install my 1972 seven fifty Commando engine in my Slimline Featherbed frame, I had to remove the crankcase breather which is located at the rear / bottom of the crankcase. I replaced it by a breather located at the inner / upper / rear part of the timing case (as on a 850 as far as I know). Two 8 mm holes drilled in the right half of the crankcase allow the air (and oil if any) to go from the crankcase to the timing case and to exit via the breather. The breatyher is simply a pipe screwed in the case and continued by a 10 millimeters (ID) rubber hose.
Still with me guys ?

That hose connects the breather to the oil tank (standard Dominator one), and I think that is the reason why I can't prevent the oil from leaking via the oil tank cap. I assume that the breather sort of "pressurizes" the oil thank (despite the genuine breather whose ID is only 4 millimeters). A test showed me that the engine sucks and blows air very strongly through the breather when running.

So I envisage to disconnect the breather hose from the oil tank to see if my assumption is right, but I don't like the idea of letting the hose breathing in open air, with the risk of inhaling dust or sand or other non-mechanical friendly material.

I noticed on some pictures that some guys fit a sort of nice-looking filter at the end of their breather hose.

Any idea of where I could find one?

Any comment on my assumption?

Many thanks / merci beaucoup !

Hi Laurent, so at the end you over pressurize your oil tank , my guess : why not fit a reed valve between engine and oil tank , after few minutes , your engine should not pulse out any more air ........and/or , same way but on your oil tank (I suppose it has a breather for letting air out, if not fit one, if yes fit a reed valve on it..........will see if other people will have some more ideas!

I had no leak issues nor even any oil misting [Q-tip tested] inside breather hose on a Combat plumbed like yours though with a Krank PCV installed up at frame where vent hose turns 90's to aim at oil tank. Tank retained factory vent hose aimed straight down on RH side and never dripped. I also didn't fill tank but to very tip of dip stix. So if rings half way sealing the factory hose ID's are adequate. The oil cap seal may need renewing too. Shouldn't need a oil tank hose filter as about on operation conditons should cause lower tank pressure than extertior and even if it did likely would only be pulses of not enough flow to draw dust all the way up vent tube into tank. But its Cdo's so may have to escalate further.

I don't know why you had to change the breather from where it was to fit the Featherbed frame as the engine mounts are the same withe as the Commando mounts, but the breather does suck air back in with out a reed valve, as for running the breather hose back to the oil tank it will pressurise the oil tank and blow out the oil cap and leak all over your oil tank even with a breather in the oil tank, I went through all this when I first put my 850 into a Featherbed frame , but my simple fix was to remove it from the oil tank and run it into a catch bottle that sits beween the engine mounts at the back of the gear box I have been running it like that for over 33 years now, I use a 1 ltr catch bottle with a air hole in the top to let the air out, it takes a long time to even get a little bit of oil in the catch bottle, I empty it about once a year and only get small bit of oil from the bottle, its also wise to have a clear bottle so you can see inside it.

Hope this helps

Ashley

You might try installing a froth tower into the oil tank, with an oil mist scrubber. See


atlas-dommie-oil-mist-scrubber-t19002.html#p240772

The tower is simply a 1 inch ID steel tube that could easily be welded in the roof of the oil tank.

Many thanks to all of you,

Ashley, the reason why I had to relocate the breather is that I kept the footrests as they are on a Dominator, ie with their splindle too close to the crankcase to leave room for the breather.

Considering your answers and other discussions (eg: the post on 750 breather on this site), I'll go for a reed valve (a Yam XS 650 one) as you suggest Pierre-François.

Cheers,

Laurent

Posted this once before. It's a small K&N filter. I just let it dangle behind the transmission. Very little oil if any comes out of the oil tank breather hose. (This is at the end of the hose that used to go to the air cleaner ham can on my '72.)

http://www.knfilters.com/vent.htm

laurentdom said:

Many thanks to all of you,


Considering your answers and other discussions (eg: the post on 750 breather on this site), I'll go for a reed valve (a Yam XS 650 one) as you suggest Pierre-François.

Cheers,

Laurent

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I think your problem with oil weeping from the oil filler cap is due to a lack of adequate oil tank venting. Tha XS reed valve will be of benefit to de-pressurize the crankcase, and will stop oil weeps at the engine seams, but will not relieve the oil tank pressure, in fact may even increase it (if I am understanding your application correctly).

Go with the XS valve...it is a good mod, but you need to address the oil tank vent as well.

Another thought has occurred to me....perhaps your oil weeps at the oil filler cap are actually coming from under the angle bracket tab that is the top tank mount. This is a weak Atlas/Dommie point and known to be a problem. vibration broke one or more of the spot welds holding the tab onto the tank on my Atlas. Oil seeped out near the filler cap. The fix for this is to cut off the tab, grind the remains flush with the tank roof, then weld in a 14 ga. tab. I found the metal so fatigued under the tab and full of spidery cracks, that it was necessary to weld a patch over the entire tank roof. Give your tab a critical look. If it is not leaking now, weld 2 gussets behind the tab to firm it up and head off a future problem.

Slick

Laurent,
I'm thinking your motor will wet sump badly if your cases haven't been modified by moving the oil pick up to the rear of the case where all the oil pools. All the 750 motors with the rear case breather returned just as much oil if not more back to the oil tank as the oil pump did. I'm not sure how much room is needed to miss what's in the way but Jim Comstocks reed breather that screws on in the original breather location is way thinner than the original. I think mounted it's about 3/4" - 1" thick so maybe that would give enough room to work.
Just my thoughts.

Many thanks for your clever comments gents.

Nortarly, we modified the crankcase as per Mick Hemmings method and drilled two 8 millimeters holes in the timing-side crankcase (as per Old Britts method). It does not seem to wet-sump up to now, but difficult to verify since I use a tap on the IN oil line to ensure there is no wet sump.

Anyway, I like your idea of checking whether Jim's breather could be installed.

And I will check soon that she doesn't suffer of the Atlas-Dommie oil tank syndrom ...

All the prior normal to comnoz level effective one way valves generally vent into oil tank so extra good crankcase draw down advice would add more pressure pulses to oil and so still avoids solution for oil tank leakage. There could be extra ring blow by thats aggravating oil tank pulses so if engine itself not leaking don't expect oil tank to improve by a PCV. Cracks to poor seals or something plugging last vent hose to consider.

Drill a 1 or 2mm hole in the oil cap to keep pressure from building up. I get a just a small spot of oil on the outside of the cap after highway speeds.

You need a check valve and all the problems will be gone. I use an XS650 breather valve (on advice from this forum), http://www.mikesxs.net/product/15-0677.html

Please explain to dumb ole hobot how a check valve between engine and oil tank can fix oil tank leaks? No mention of engine weepage in this instant case so maybe getting confused on pressure physics but I'm open to be set straighter on my advice to this fellas messer. PCV are cat's meow on Commando engines of course.

hobot said:

Please explain to dumb ole hobot how a check valve between engine and oil tank can fix oil tank leaks? No mention of engine weepage in this instant case so maybe getting confused on pressure physics but I'm open to be set straighter on my advice to this fellas messer. PCV are cat's meow on Commando engines of course.

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The huge volume of air blowing in and out of the oil tank causes the oil vapor to blow out the vent. Using a Comnoz, XS650, or whatever check valve you choose reduces this to blowby gases only, greatly reduced volume into the oil tank, thus, the end of the splooge out the tank. I tried it and saw first hand. Thanks again to those who came before me and recommend the breather valves.

That makes some sense to me, if the initial extra efficient one way valve out huffs don't push oil out the tank some before the system settles down to less pressure to puff out much. What confuses me is my 2 Combats with a number of events towards over case pressure, like worn out ring gap blow by and red zone and beyond operation, > caused all engine seals to fail-leak but no oil came out their tanks or final vent hose, though pre-Peel's tank weeped some bouncing on my paths till I re-did oil cap seal. Will be educational to see how its solved one way or 3 at once.

Roadrash said:

Drill a 1 or 2mm hole in the oil cap to keep pressure from building up. I get a just a small spot of oil on the outside of the cap after highway speeds.

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if the oil tank is properly vented, a small hole in the cap is superfluous . My Atlas has a 10 mm vent tube coming off the tank. See the link I first posted, showing the froth tower in the later Atlas/Dommie oil tank, which shows the vent tube ( right side of froth tower, near top) fourth and last photos.

Slick

hobot said:

That makes some sense to me, if the initial extra efficient one way valve out huffs don't push oil out the tank some before the system settles down to less pressure to puff out much. What confuses me is my 2 Combats with a number of events towards over case pressure, like worn out ring gap blow by and red zone and beyond operation, > caused all engine seals to fail-leak but no oil came out their tanks or final vent hose, though pre-Peel's tank weeped some bouncing on my paths till I re-did oil cap seal. Will be educational to see how its solved one way or 3 at once.

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For your super high speed operation, the Comnoz valve is accepted as the right choice. A brilliant design.

Ya seem to react as if I don't like good breathers when the fact is comnoz and all the rest are not good enough for Peel so active exhaust sucker planned with valve not to suck seals inside out. i found out about valve float on 4th day on 1st Combat and on 7th day was doing tire heat burns out before pulling clutch drop wheelie to leap ahead of wheelie barred drag bike then letting Combat rip into red zone so took most the 1/8 mile before Combat got out horse powered and have lived through Peel bouncing tach need to invisible, yet with the now 'taboo' looked down on factory system and old school venting via TS case out simple PCV still kept the oil inside. I don't know why this instant fella gets oil tank mess so hope to learn something when and if he does. My buddy Wes '71 still has the old cam time vent hose yet don't make a mess out engine or tank either so we must be doing something wrong a long time now but don't care. i was pretty scared doing my first X-mass tree's and did get hurt badly but will never forget the roar of the crowds each set up with announcer calling it a SNORTON Norton and pin drop silence when my knee let R foot touch my hip from the side on first step away and collapse plus the way the local bad boys accepted me into inner circles.

Nanny nanny boo boo, I have two.