The Knowledge - Velocette Owners Club

ack to the bottom of the casing, & eventually back to the crankcase. Any 'timing' is no more than
striking a situation where pulsating air flow is reduced to minimum movement, - or even made static, at
which point oil has the chance to settle on the surrounding surfaces of the breather chamber, as well as
the inside surface of the large diameter vent tube & drain downwards. Further to this one would deduce
that as RPM rises, the oil would tend to settle out closer to the timing case, while if the engine was
running at low RPM, the fluctuations in the air direction would move towards the outlet vent tube,
whatever, theoretically if the point of comparatively static air can be kept within the confines of the
'valve chamber' 100 % of the oil held in suspension, should drop out & drain.!
ENGINE BREATHERS 'M' SERIES VELOCETTES - By Les Diener.
Over the years much has been written, & many designs of crankcase breathers used on various
machines, & without doubt the single cylinder engine presents the greatest challenge, due to the
amount of 'suck & blow' on each revolution. The greatest problem is to provide sufficient means for the
c/case to pass air only during the breathing cycle, & not a mixture of air & oil which, unless suppressed,
finishes up on the rear tire with possible disastrous consequences, & generally leaving a mess in the
process. The problem becomes rapidly worse with increasing engine speeds, & also when flywheel
assys. are more snugly fitted into c/cases, both situations demanding almost complete evacuation &
refill of the c/case voids during each revolution, whereas with slower revving engines, & those having
oodles of space around the flywheel assy. there is a form of 'cushion' on which to act. Reflecting back
over the years it would appear that in most cases c/case breathers were too small, but a notable
exception will be detected on most Italian machines. Moto Guzzi, Benelli, Morini, & others always had
huge c/case breathers which were mounted in a vertical position at some convenient spot on top of the
c/case, & with some form of 'trap-& -oil-drain' provided to separate oil & air. In later years one Japanese
example of this design was featured on the little CR 93 Honda, & whilst this was only a 125 cc racer, a
huge breather tube of some 20 mm I.D. hose went out along the upper frame member to the rear of the
machine, preceded by a large 'trap' housing, which no doubt would have contained some form of baffle
which would have allowed the oil to drain back into the sump. Timed breathers which have been used
on all KTT engines & others, have been reasonably successful but are not altogether foolproof as the
engine RPM goes beyond 6000, firstly because the orifices through which the air must pass is so
small,& also that very positive seals are required to allow the breather valve function to operate
efficiently. Disc or 'clacker' valves are much the same. So, in summarizing, the Italian concept would
appear to be the best compromise, in that BREATHERS must be 'BIG'. The design shown is what I
have on the 'ELDEE' classic racer, & whether by luck or coincidence, has turned out to be the most
efficient breather that I have ever used, for even with the engine running to 9000 RPM, not one drop of
oil leaves the trap housing, & this is evidenced in the 3/4 inch I.D. transparent plastic vent tube. The
'trap' can be fitted to all the earlier M series without problems, but on the later 'domed' timing chest
covers, a small 'platform' must be built up by tig welding. First it is desirable to drill 2 additional ½ inch
holes between c/case & the timing chest. Then drill a 7/32" hole, on an angle, underneath the timing
side main bearing, into the c/case, this is to always keep the timing chest free of oil build up. This is
only possible on later model c/cases, which have a large radius cast below the T/S main bearing. Next
drill a 5/8 " hole through the timing case cover in line with the magneto spindle, a 5/16" hole is then
drilled 11/16" below the centre of the 5/8" hole. (it may be necessary to skim the surface of the timing
cover at this point to provide a flat surface ) The housing is then mounted to the timing cover with the
5/16" hole directly in line with the annular groove in the housing & immediately below the 3/8" wide
drain slot. Drill & tap the 4 mounting holes, & fit the housing with jointing compound. ( At this point I
have checked photos of the ELDEE & noted that Les actually used 6 screws instead of the 4 shown in
his drawing ) Take the 3/4 inch clear PVC hose straight up as far as possible & route to the rear under
the seat to the rear of the machine, & if my experience is evidence, you have a breather that works !
Signed Les Diener.
#5417 It seems that there is room for comment here ? Re. Les Diener's experiments. As I remember it,
he started with the idea of a 'timed' device, & with this in mind, he machined a flat spot on the outside of
the timing case centred on the magneto spindle. The idea was to have a light circular body that was
ported internally, & vented back to the 'catch tin' via a hose in the usual manner. A light rotary valve
arrangement to be supported on the magneto spindle, but with ample running clearance to the body of
128