1958 Dominator 99 engine breather

Back "in the day" when i used to thrash my 88 unmercifully i never had any issues with the standard breather.After "adjusting" the pipe on my99 to blow over the sprocket and speeding up the pump i found the rear tyre to get very oily!.I have now routed the breather into the oil tank which probably is not good for the oil acid levels. All in all i should probably left all standard,Does that help?.

Previously richard_rumsey wrote:

What is the best route for engine breather, where should it terminate?

Originally it just hung down to the rear of the engine which is ok but does tend to drip oil. Later models used a modified oil tank with the breather venting into the tank & then from the tank to the rear chain.

Regards, Tim

An engine in very good condition will suffer from approx. 1% blow-by due to piston ring to bore leakage. On a 600cc engine at 3,000rpm this equates to approx. 9 litres/minute of crankcase gases containing oil, fuel, water, acid and carbon amongst other stuff. I don't think it's a good idea to dump this into your oil tank where it will then contamintate the engines lubrication sysytem.

Better to install a breather/catch tank so the gases can vent to atmosphere and accumulated oil can be drawn back into the crankcase where it can then be filtered (if one is fitted) enroute to the oil tank. Rex Bunn advocates flushing the crankcases with fresh air via a check valve and exhausting the crankcase via a second check valve to atmosphere. I can't see any fault with this theory, although on a previous thread I was criticised by just about every other contributor.

On my Commando I have installed crankcase inlet and outlet, and recently an outlet on rocker box. Work in progress, I'll post the results after a few hundred miles testing.

Simon.

It normally points at the gearbox sprocket, lubricating the rear chain on long, hot summer rides. Not much use on short winter trips though. I have gone for the later oil tank with an oil separator tunnel and routed the engine breather to that, with the oil tank breather routing to the rear chainguard in the hope of lubricating the chain on long hot summer rides. If you overfill the oil tank, it does lubricate both the chain and therear tyre. Despite the dire warnings about contaminating the oil, the engine has hung together with this setup for over 25 years now. The advantage is that if you wet sump, the oil pumped out of the engine breather goes up to the oil tank rather than all over the garage floor. It might be worth mentioning that the 650/Altas range all had the engine breather routed to the oil tank and don't seem to have too many oil contamination issues. Problems with oil are all to do with short rides which don't allow the oil to get to working temperature and boil off the water and other nasties. Gordon.

Dear Mr Rumsey,

Buy a Duck's Bill breather from Hitchcock's Motorcycles, dare I say it, Royal Oilfield (sic) dealers. Part no is 140167. Tel no 01564 783192. Coat £3.50 incl p&p.

It takes 10minutes to attach to the breather pipe, You may have to shorten it a little with a Stanley knife and I would advocate that you put two cable tie-wraps around it to secure it to the outside diameter of the breather pipe because to two aren't exactly compatible. The breather induces a slight negative pressure in the crankcase so some oil leaks may tend to disappear. Downside is that it is so efficient that you may have to physically oil the chain.

I have three on my Inter and one on my ES2 - all working beautifully and minimising oil leaks.

Regards,

Peter Bolton

The Oilfield breathers do sound good. I wonder why my Enfield (now long gone) insisted in leaking oil if you exceeded 52 mph despite it. Not that it bothers me - I sold it to buy my Norton. The sensible option!

Previously Gordon Johnston wrote:

Problems with oil are all to do with short rides which don't allow the oil to get to working temperature and boil off the water and other nasties. Gordon.

Exactly my point, Gordon. One problem is water vapour condenses in the cold oil and collects at the bottom of the oil tank where it gradually corrodes the metal. Similar things happen to ferrous engine components, especially on engines used for short journeys and then stood for long periods, but it happens also to engines regularly run up to max. engine temp for long periods with the engine breather terminating in the oil tank. The oil by-passes the filter too,

Simon.

All

When I built my 650 SS engine last winter I fitted a catch tank (a WD40 can) between the battery box and oil tank, it fits perfectly in the round hole in the oil tank and battery box platform. In the top of the can are two copper pipes, one goes down approx a third of the way the engine breathes to this one, the second pipe goes just inside and no more this one vents the can to the rear chain. In the bottom of the can is a drain pipe with a small valve on the end, I occasionally drain the can of emulsified oil. At the same time I fitted a Paul Goff oil filter, the return flow from the oil filter goes to the oil tank tower connecting onto what was originally used as the engine breather connection. When I start up I can observe clean filtered oil dropping down into the tank, a most pleasing sight. And finally I left the original oil tank vent going to the rear chain, so it gets two lots of oil vapour air, one from the catch tank and one from the oil tank.

My catch tank is nicely out of sight and unless you knew it was there you would think the set up was standard.

Just in case you are wondering, the rocker gear is oil pressure fed so no need to rely on the scavange oil supply.

I have completed around 3000 miles since the begining of the summer and rode the bike from Aberdeen to Morpeth and back just the other weekend without incident.

Tony