The Knowledge - Velocette Owners Club

amount which drops out onto the walls of the pipe actually remains there when the engine stops.
L #4066 At long last my '55 MAC is getting a long overdue rebuild (to my usual standards!) I have
flattened all joint surfaces in a bid to slow the leaks, and have drilled and fitted a breather under the
magneto. The question is what do I do with the pipe? Can it be just run to the back of the bike or must it
go via the oil tank? The MAC tank would need some pipework welded in to do it properly, but it has no
froth tower, just a raised filler cap. Any suggestions would be most welcome.
#4069 The crankcase breather must breathe into the oil tank since it will carry a lot of oil out with the
breathed vapour. This is normally separated out in the tank, and the froth tower system keeps the oil in
the tank and only dry (ish) vapour out of the rear breather pipe. If you don't have a froth tower you can
only hope that not too much oil finds it's way out of the breather hole in the tank cap, but this hole really
isn't big enough to allow the engine to breathe correctly and I would recommend that you invest in
having the tank modified to incorporate the full late breather system plumbing.
#4072 There are two critical lengths for the pipes to balance the natural frequency of the engine.
Without them it is said that the oil will be evacuated. Someone will tell you the correct lengths, or if not
we will find the information in the Veloplonk draw and respond.
#4086 The adaptor to oil tank pipe should be 3/8 bore 21.375". Oil tank to wheel 27.5"
#4088 Please advise where exactly do you take these measurements from? They seem very tight
tolerance!
#4092 You don't take the measurements from any point to any point, they are just the length of the
pipes.
#4070 Agreed, I have tried several times to vent the timing chest breather directly out to the rear of the
bike, even going as far as putting a baffle inside the timing chest in front of the outlet hole. All in vein,
there is just too much oil flying around in the timing chest. In desperation, during the summer rally, I
routed the hose into the side of a plastic bottle, the air free to flow out the top. This worked fine except I
had to empty the can back into the oil tank at every stop! You mentioned you drilled the breather tube
hole under the mag. Make sure you also have the required holes leading from the crank case to the
timing chest.
#4111 My first attempt to correct excessive oil breathing on my VM (aka "The Torrey Canyon") involved
fitting the standard BSP hose connection under the magneto and piping it to a small cast alloy box
mounted on a bracket under the dualseat. The other end of the box was piped to the general area of
the rear number plate. I ran a flexible quarter inch pipe from the bottom of this box through a hole
drilled in the top of a scruffy old oil tank cap and stuffed the box with pot scourers. There were BSP
stud couplings everywhere but the installation was fairly tidy and it just about worked. Until, after about
3000 miles I looked inside the alloy box to find a disgusting mixture of mayonaise and rust - no sign of
the pot scourers. That's what happens if you get the breather pipe lengths wrong. Since then I have
done the proper job as outlined in Rod Burris' book but I've only piped to the oil tank; no external pipe
to the rear. The timing chest / rocker box behaves as a plenum chamber and, to a limited extent, so
does the oil tank; hence the reduced oil loss to the rear wheel when the tank is half full.
#4105 I reproduce here exactly Les's article. It appears that I am mistaken in that over the years Les & I
discussed, (mostly by mail ) various ideas on breathers & other things, & while I am sure that at one
point he was playing with some sort of timed 'valving' ideas, this article & the only drawing that I have
which is listed numerically, has no corresponding 'valve' drawing as I mentioned in my last mail, & I
have to draw the conclusion that this 'valve' is nothing more than an oil trap & drain idea that he
discusses in the article. The idea behind it's success being no more than the use of a large diameter
vent which he states should be routed well up above the unit, & the fact that any oil picked up remains
in a chamber & large capacity outlet tube of relatively static, though pulsating air, from which it drains
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