Additional or alternative engine breather

[John Cone]

With reference to the Elephants trunk breather fitted to an enclosed "D" which uses the crashbars as a condenser. Which valve cap does the small vacuum pipe go on to. Mine has a breather made by Mac Read "hi Mac if your still out there" . I have a funny feeling that mine is on the wrong valve.

Also any ideas, normally when I start the bike the oil return is almost instant and with gusto, but at 4am this morning when returning home from work , I started her up and checked for return oil and there was only a smidgeon coming back. She has a twin start oil pump which is fairly new and plenty of oil in the tank.

 

[peterg]

Howdy Chaps,

Would not stray to far into comparisons with cars unless reference is to a dry sump application. If you'll recall the ratio of displaced volume to total engaged volume - in this case it is very generous in wet sump applications. To draw the discussion back to the m/c side...can't recall whether it's here or JTAN but some of us have Indian Fours. These machines can be run with the 2 inch diameter oil fill cap on the side of the top case over the sump removed with only a bit of wispy blue smoke wafting out. Fire up a high ratio displaced/engaged volume twin like an HD or Indian with the timing plug removed from the case and you will have a spectacular display of noise (turbulence and high velocity) and a cloud of oil vapors issuing forth. Besides demonstrating the complete contrast in operating environment, that is an extreme illustration of what I'd pointed out earlier, namely any gases drawn back into the case must be dealt with on the next crank movement to BDC - the signature of open breathers.

The point with a PCV - aptly name because it's primary function is knocking down positive crank case pressure to the level you desire - in this application is to draw down that pressure in successive crank revolutions after starting to a level achieved through the unit's internal ventsetting and/or your engaged volume's leagage (ie, breathing through the generator drive).

The example of gas velocity at the vent pipe diminishing means the pressure differential has decreased which therefore lowers the velocity at that exit point and the propensity for atomized oil to escape.

Some mention was made of scavenge pumps and cavitation if not assisted by positive crankcase pressure which may have merit. But, something to consider. Scavenge pumps on dry sump app's are generally much larger than their pressure counterparts because they are designed to evacuate aerated oil (more volume). Other applications where oil is subject to high aeration are turbochargers, but here, the sump tank allows the oil to stand momentarily before the scavenge pump draws it off for return to the main oil tank thus ensuring more oil over air content and therfore more efficiency. We don't have that here, windage and turbulence are conspiring to agitate the oil while the scavenge pump is attempting to work, therefore, if you can reduce those influences, what collects in the base of the case has more fluid content and the scavenge pump can therefore be more effective. I've not had my machines wet sump on a PCV but will readily admit to the longest riding session since fitment in January at 280 miles at 60 - 75mph, but, there's always that possibility, eh chaps?

 

[myself]

for a good breather system easy to fit in place of your exhaust valve cap contact conway motors. I fitted one to my rap. It uses a ducati valve and works perfectly. It increasd my mileage to a pint of oil by two thirds. i can't understand why they don't advertise it

 

[Tom Gaynor]

I'm not sure if I was the "Tom" mentioned but my breather take-off is the front valve cap. For interest, after much experiment, what I have ended up with is as follows: a short pipe from the cap to a PCV valve, and a slighly longer one to a catch bottle. The catch bottle (Millets, £3.99 0.75 litre water bottle clipped to my pannier frame) was necessary because at high speed (sustained 80 mph plus) the breather dumps oil from my (apparently) highly efficient two-start oil pump. This can easily be mistaken for a horrendous oil consumption of about 100 mpp. Now at 100 miles (fill-up time) I dump the oil back into the UFM. Idling along at 60-70 mph it doesn't dump oil, returns 400 mpp. Which points to one advantage of a catch bottle: one can monitor what is actually happening.

 

[Pharquarx]

Did we ever come up with a suitable solution for a Comet? I currently have only the case vent and pipe off of the left side of the engine. The vent line is routed over the dynamo and then drains onto the chain. I like the look and placement but it makes a real mess when parked. Is the Bunn Breather adaptable/suitable for a Comet?

 

[Tom Gaynor]

Singles (of any make) aren't a problem. The ideal is to have a breather input as close to the crank axis as possible, then a PCV, and, then, maybe, a pipe to a catch bottle. The catch bottle is there so you caN MONITOR HOW MUCH OIL IS BEING DUMPED. f*CK IT, caps lock accidentally hit again.
Unless you want to drill new holes, use the existing Comet breather input, a pipe, as large a bore and as short as feasible leading up, a PCV, and an exhaust leading to the bottle, which, if it remains empty, can be eliminated. If not, read on..
In my not normally noticeably humble opinion (a man of strong opinions, weakly held...) the perfect system is the Manx Norton / Matchless G50 system. The DS mainshaft is hollow. At the outer end is a flap (OK, PC) valve. No oil exits because oil is centrifuged away from the crank axis. There isn't so much as an oil STAIN on the fairing, after 15 seasons.
My Rudge, like a Vincent but more powerful, has such a system. The input is about 2" from the crank axis. There may be other Rudges that don't piss (sorry, I meant p*ss) oil from every joint, but I haven't seen them. Mine does not. Implicit in this is that your Comet breather INPUT becomes one of the rocker caps, or the front (exhaust) valve cap. (Plug the timed breather).
Got any pictures of the Rumi? Seen them in a (Maserati) museum near Modena. Heart-breakingly elegant.

Tom

 

[Pharquarx]

Thanks for the input Tom. I just posted two photos of the 1952 Moto Rumi Sport 125 that I have. Heart-breakingly sexy and elegant.
If your search for photos posted over the last day, it is there - Mi amore.

 

[bmetcalf]

That Rumi frame could use a vertical member up from the engine. It looks like a parallelogram that wants to get smashed flat.

 

[Pharquarx]

Added two more photos of the 1952 Moto Rumi Sport 125. Search under photos uploaded over the last day. All of the photos are titled "1952 Moto Rumi Sport 125". The first is a professional shot that I grabbed off of the internet when I was reseaching this model of bike, absolutely beautiful. The second is from a book on Moto Rumi, showing a machine from a private collection. No vertical member in either.