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answers on a postcard please...................

Comet

Well Known and Active Forum Website User
VOC Member
I need some help and advice please.

The Comet outfit was suffering on it's way back from the Dutch rally. A large amount of oil was being lost out of the breather. We thought it must have been a piston ring causing crankcase pressure. Today Dave and I stripped it down and the rings all seem okay. The bore seems to be okay as well. Valves seemingly are good. The bike was doing well for power but lost about 1 pint of oil in approx. 60 miles. We had by-passed the timed breather and instead just ran a breather from the top pushrod cap to a catch tank.
So ladies and gents. Any ideas as to what may be causing the problem. It seems to be some sort of crankcase pressure. The bike is not really burning much oil at all.

thank you in advance.

John
 

b'knighted

Well Known and Active Forum Website User
VOC Member
In my experience the Comet timed breather keeps the oil in. Adding an atmospheric breather will just prevent the timed breather from working as intended. If you do run a pipe from a valve cap it needs to be long enough for the slug of oily air to reciprocate rather than draining into a catch tank then sucking back air. I suppose you could try half filling the catch tank so that it sucks oil back up the breather but will blow bubbles if it needs to, but you'll need to blank off the timed breather for this to work.
 
Last edited:

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello John, What oil are useing, A Comet must be working hard with a chair, My comet got a bit smokey after i put some 10/40 in it, I normally use coarse 40, Much better after. Cheers Bill.
 

Len Matthews

Well Known and Active Forum Website User
VOC Member
Is this something new or has the Comet always been heavy on oil? Just wondering if the the oil feed hole in the liner might be too high therefore injecting oil above the scraper ring.
 

aldeburgh

Well Known and Active Forum Website User
Non-VOC Member
Prior to our comet dropping a loose liner I had actually got the breathing side of things sorted ,by a combination of setting the timed breather to open and shut at the right time and by adding a D cap to the inlet pushrod position with an 8mm petrol tank vent valve ,it was going really well...............
 

deejay499

Well Known and Active Forum Website User
VOC Member
John only had one breather, the timed one, which we blanked off and tried one on the valve cap. Never both at once. Will look at the height of the drilling in the liner. Pretty sure we drilled it after much measurement.
 

Comet

Well Known and Active Forum Website User
VOC Member
Have checked the drilling. It is definitely below the bottom ring of the piston- so all good there.
 

ray vinmad

VOC Drawings Manager
VOC Member
If the oil hole was too high you would be burning it, not blowing it out.

Perhaps the return side of the pump is not functioning fully.

Ray
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
I finished a complete rebuild of my Comet motor that included muff n liner, new piston n rings, all bearings, new cam, new oil pump etc.– about the only bits not renewed were the cases. In my Comet the timed breather internals have been removed. I have a breather connected to the timed breather outlet with the hose running vertical (well as best as I can get it) to a PCV valve under the tank; from there is a hose that eventually exits at the rear, attached to the rear stand. The importance of getting the hose vertical before the PCV cannot be overstressed; it ensures that the “slug” of oil in the hose can drain back into the cases when the motor is off.

When you think about it, the timed breather is open when the piston is descending, thus trying to pressurise the crankcase and then it’s closed when the piston is on the way up, creating a vacuum in the cases. The pressure/vacuum pulses cause a PCV to do exactly the same thing.

I next, in addition to my timed breather PCV, I fitted a “D”breather above the inlet pushrod tube, again with a PCV valve – the amount of oil coming from the original timed breather PCV fell away to almost nothing, but it sure poured out of the top breather. There is a very rich oil supply around the rockers – because of the convoluted path to the timed breather outlet there is a very poor oil supply to that.

Got to thinking that what I wanted to do was give all the gaskets etc. an easy life to reduce the possibility of leaks developing, so next I turned around the PCV valve connected to the “D” cap. Result is when the piston is on its down stroke, trying to pressurize the cases the “timed breather” PCV opens to relieve the pressure – at the same time the (much reduced) back pressure has the PCV on the“d” fitting firmly closed. On the piston UP stroke when it’s trying to create a vacuum in the cases the reverse happens. The “d” PCV opens allowing air into the cases, reducing the vacuum and the mild vacuum holds the timed breather PCV closed. I have a small air filter on the inlet of the "d" breather PCV.

Now there is NO oil exiting via the “d” PCV and just a small amount via the timed breather PCV. And so far – having covered 800 miles since the rebuild - the motor itself is leak free.

Martyn
 

aldeburgh

Well Known and Active Forum Website User
Non-VOC Member
Its all about wind , the standard Comet breather even if it is timed correctly , runs through quite restriction when it reaches the banjo and drilled screw ( The screw drillings can be opened up and the banjo swapped for one with a larger outlet spigot ) this all helps. To assist in getting rid of excessive wind it helps to have more outlet points BUT these must have clack valves fitted and ideally not be positioned so as to lose oil . When the piston descends I want as much unrestricted oil free wind to be blown out as possible , when it rises I want the wind to stay outside.
 
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