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Comet Breathers

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
Hi - Quick intro' , picked up the Comet at the end of last summer and enjoyed a few hundred very comfortable miles but with an indicated alightly less than 70mph, a leak from the head and latterly a sosmokey exhaust I decided a head job was in order.

The presnt position is I think I'm suffering from Tech' overload, can anyone confirm that the 'elephant trunk' and a blocked off oil tank vent pipe go hand in hand?. My concern is that there will be a vacuum in the tnk and that this will lead to at the very least oil starvation.

Also, the crankcase breather behind the cylinder is totallt dry and clear, I've no reason to think there is a timed vent issue is this because the 'elephant trunk' set up negates the need for the standard breather?

Thanks in advance.
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
Most people who use an elephant trunk style breather just blank off the original timed breather. If your trunk breather just vents straight to atmosphere via a large bore pipe then leaving the timed breather operational will not be a problem. If your trunk breather is the type with a large ball valve in it to try and achieve a less than atmosphere in the crankcase then I believe the advise is to blank the timed breather off with a plug in the crankcase.

As for the oil tank breather, I have run for the last 30,000 miles with it blanked off without any problem. Just make sure the oil tank filler cap is the type with a breather hole in it and that the hole is not blocked with polish etc. That will be enough for the tank to breath.

Cheers
Simon.
 

aldeburgh

Well Known and Active Forum Website User
Non-VOC Member
Keep the standard breather on the machine but make sure it is opening and shutting at the right point during the piston cycle , as to any additional breathers make sure whatever you fit has a non-return valve fitted. If you can blow out as much wind as possible on the down strokes and keep as much wind as possible out of it during the upstrokes it should stay "oil tight"
As to oil tank breather , use a vented cap and or the vent pipe, on mine I use both but when I got the machine it had neither! the vent pipe was capped and the filler cap was not drilled ,its all a learning curve:cool:
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
OK , on a bit of a learning curve and the first thing that occurs to me is just something looks a similar fitting as on another marque don't make assumptions.

Thanks for the responses, yes what I had thought was a breather for the oil tank is actually the rear chain oiller and has been blanked off at some time so I'm quite happy to leave as is and oil chain as part of maintenance.

What I'm calling the 'elepant trunk' breather retro fit gubbins may not actually qualify for the name as it has a very crude right angled union tapped into the cover and certainly no check valve fitted. However it seems to be doing its job as the standard timed breather vent is as dry as and I'm actually quite happy as the thread has stripped and will need a helicoil fitting. It seems to be 1/2" 20 TPI can anyone confirm.

Many thanks for the responses and it is noted that there is a meet second Thursday , Weald Pub , Burgess Hill which is not too far from me. Holiday commitments apart I will get over there to introduce myself.
 

vibrac

Well Known and Active Forum Website User
VOC Member
there are two pages of threads with the word 'breather' in the search field and thats just the last year
I suggest a good read awaits you
 

Holger

Forum Website User
VOC Member
There is a better way of doing this which is a simple one way reed valve which is used on many motorcycles. You can get a cheap item to suit the Yamaha XS650 or pay more for a MG Le Mans 1000cc item. The reed valve would work perfectly with the elephant's trunk and hold the crankcases in vacuum while the engine is running.

I've had one fitted to my 850 Commando which works brilliantly and the machine is now 100% oil tight (No Kidding! Hand on Heart! I have witnesses! Honest!).

As part of the TPV 1200cc top end conversion on my Rapide which is nearly complete I have had the standard breather outlet bored straight through into the crankcase and fitted a full bore pipe fitting outlet. From there it will be a pipe and hose up an over the top with a reed valve located near the battery. This should work very effectively and hopefully be little more discrete than the elephant's trunk.

Holger
 

pa.jello

New Forum Website User
Non-VOC Member
Comet Breather

Hi - Like you I was a learner in 1982 and had to learn all the wrinkles about Comets. There is nothing wrong with the design and as already has been mentioned, the timing of the Breather Pinon on the shaft is essential. I opened out the slot on the pinion ( to allow a greater flow of pressure) and ensured the timing of the downward piston stroke was as per the degree timing when the slot/valve opened allowing the maximum gas to flow and to release the crankcase pressure. The use of a banjo with a plastic flexible tube to temporarily replace the evacuation pipe (A71/1AS) allows you to blow down the tube while rotating the crankshaft and checking the fitted timing degree disc to see that the air shuts off/on at the correct degree timing. If you get these correct, in my opinion there is no need for the Elephant trunk. The restrictor disc is also essential behind the rubber seal to prevent too much oil traveling to the overhead gear. With the valve lifter, I added an extra thick rubber washer behind E217 and added a spring to ensure the rubber remained seated on the case stopping oil from evacuating the case. This stopped the oil messing the gearbox and dripping onto the exhaust pipe.

Another trick is to use a light running in oil on a new engine and new rings as it allows the rings to bed in. If you use 50 grade oil, the oil will be pumped out into the exhaust and the rings will certainly not bed in at all.

My Comet has completed many thousands of miles since 1982 and does not use oil as you have experienced following the mods that I made.

I wish you well from NZ
 

stumpy lord

Well Known and Active Forum Website User
VOC Member
Their seems to be some confusion regarding how the upper frame member/oil tank breathes. The oil tank breaths through the small hole in the oil tank cap, and only through that hole. The device situated inside the filler neck is a chain oiler which takes a metered supply of oil tapped off the return oil flow to the oil tank, this is directed via a pipe at the rear of the oil tank on the rear chain. Unfortunately due to the fact that the oil flow is not constant [ due to engine revs and the viscosity of the oil, means it is just about impossible to get a satisfactory flow on to the rear chain being either to heavy or not . there is no breather pipe in the oil tank.
stumpy lord enough
Hi - Quick intro' , picked up the Comet at the end of last summer and enjoyed a few hundred very comfortable miles but with an indicated alightly less than 70mph, a leak from the head and latterly a sosmokey exhaust I decided a head job was in order.

The presnt position is I think I'm suffering from Tech' overload, can anyone confirm that the 'elephant trunk' and a blocked off oil tank vent pipe go hand in hand?. My concern is that there will be a vacuum in the tnk and that this will lead to at the very least oil starvation.

Also, the crankcase breather behind the cylinder is totallt dry and clear, I've no reason to think there is a timed vent issue is this because the 'elephant trunk' set up negates the need for the standard breather?

Thanks in advance.
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
G'day Stumpy,
On a B series there is. What is the pipe up in the filler neck next to the oil return pie?.
Their seems to be some confusion regarding how the upper frame member/oil tank breathes. The oil tank breaths through the small hole in the oil tank cap, and only through that hole. The device situated inside the filler neck is a chain oiler which takes a metered supply of oil tapped off the return oil flow to the oil tank, this is directed via a pipe at the rear of the oil tank on the rear chain. Unfortunately due to the fact that the oil flow is not constant [ due to engine revs and the viscosity of the oil, means it is just about impossible to get a satisfactory flow on to the rear chain being either to heavy or not . there is no breather pipe in the oil tank.
stumpy lord enough
 
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