The Knowledge - Velocette Owners Club

has no pipe leading from it. It seems to be a breather with an internal pipe leading to the top of the tank.
Is it? If it is, why does it come out of the tank near the bottom? Should it have a pipe on it and if so
should it just exit somewhere convenient or should it be piped to somewhere in particular?
#765 Regarding the additional engine breather. Apparently at certain rpm it was found that crankcase
pressure prevented adequate draining of oil from the rockers / cylinder head. An additional breather
was added from the timing chest, underneath the magneto, to the top of the oil tank. This then vented
via a tube from the froth tower to the rear of the bike. This was introduced in the mid sixties, but it is an
often found modification - my machine included. I believe prior to this normal crankcase venting was
through the chaincase. A previous owner of my machine even vented the chaincase via a snazzy little
breather tube arranged from the chaincase inspection cover. I think the small 1/4 bore pipe at the
bottom of the tank was a breather which was also used to lubricate the chain (this being blanked off on
my machine). A cartridge conversion is available via the Velo owners club which replaces the felt filter
with a paper cartridge type. This fits in the original filter tube and is spring mounted which allows oil to
bypass the filter if it becomes blocked.
#766 Reference to previous postings will yield a lot of info on the breather subject and in general what
Brian says here is correct. However, I can see no advantage to the breather tube from the chaincase
since this is already very free to breathe due to the huge hole through which the shock absorber spring
protrudes, and secondly, the 1/4" bore pipe at the bottom of the tank really should not be led to the
chain in an attempt to lubricate it. This is the tank breather which should vent to atmosphere well away
from the chain since the acidic gasses that exude from this pipe will help to cause premature corrosion
of the drive train. Mine is led along the rear subframe and simply vents to the rear of the machine
where it can do no damage.
L #828 My apologies for taking so long to get back to you on this one. My oil tank is one which
began life as a standard Venom tank and has been modified to Thruxton spec including relocating the
mounting position rearwards to provide clearance for a long velocity stack or inlet manifold, and is now
also chamfered on the inner rear corner to clear the rear mudguard, so the actual position of the
breather pipe connections may not be quite where you would want them on your machine. However,
the 21.375" crankcase to oil tank breather hose is 10mm bore reinforced PVC, pushed on to a 1½" long
stub of fairly thin walled tube which I think is 3/8" ID. Off hand I can't remember the dimensions of the
crankcase connection but the hose is a good fit there too. The tank connection is welded about ½" from
the top of the tank, just behind the filter chamber in a position which will clear the frame down tube. The
tank breather is also relocated about 2" below and 1" to the rear of the engine breather connection in a
position to clear the rear mudguard. This is ¼" tube and extends up inside the tank to the top of the
froth tower, and has a length of ¼" ID reinforced PVC hose 27½" long venting to atmosphere at the rear
of the machine.
L #1223 Many modern engines employ a one-way check valve in the crankcase breather system.
Wouldn't a 500cc Venom engine benefit from a check valve in the breather system? Can anyone here
supply any info on past efforts to use such a valve?
#1224 I have fitted to my Thruxton a Moto Guzzi Mk11 Le Mans breather as advocated by Royce
Greasy in Classic Mechanics and it has being very successful. The box and pipe outlets needed
alteration plus a couple of outlet pipes blanking off, the box contains a one way valve and I found it
"cleaned up" my Thruxton brilliantly. It must be fitted so it is above the oil tank so excess oil drains back
into the tank but with a little thought this is easily achieved.
#1235 I have not seen the breather article either but I have been using a reed valve in my Thruxton
breather for some time. I might add this works most effectively if you have fitted a seal to the end of the
drive side mainshaft. Otherwise air is just going to be pulled right back in at that point. Interesting that
the KTT engine is fitted with a timed breath (same effect) but Velocette did not see fit to add it to later
models.
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