Hi,
I have just bought a 1980 V50 Monza as a part-finished restoration, and the air filtration and crankcase breather apparatus are clearly non-standard.
Is there a diagram anywhere which shows what the system should look like?
If I decide to get individual cone filters for each carb, what hardware do I then need to install to complete a crankcase breather system that works?
Hello and welcome to the forum. I know the air filter box is a pig to get in and out of the frame and so many have changed to individual filters.
There has been discussion about this recently and I am sure someone will come along shortly with the answer!
Best wishes Chris
It is very tempting to fit individual filters the original filter gives a lot of filter area and the air box recycles blow by gases but as a result it is enormous and a real skill changing them without bloodshed lol
Back when i used to work on these we fitted Uniflo foam filters as we reasoned that you can see through a K&N so it is a strainer rather than a filter and not a good idea on an engine you want to keep! However we found that many bikes with on-carb foam filters of any make started and ran really badly after rain. It appeared that as that they sit just under the front corners of the seat rain runs off the seat onto the filter effectively clogging it up until the engine sucks it dry, giving poor starting and running. So if you do fit them i advise making up a cover to deflect the water that will drain forwards out of the seat!
Hello again,
If I go for the individual filters, what do I need to do with the two rocker cover breathers and the crankcase breather?
The bike had a cannister to which these three pipes were connected, but this was clearly not a Guzzi item, and I don’t know what it was for. It was also full of rust, so I’m inclined to bin it!
Well, as you know, the original method runs the blow-by oil mist from inside the engine back through the intakes to burn it.
That is really hard to do without the filter box so you have 2 choices.
(1) run it to a catch tank or bottle which you empty when it is full
(2) run the three pipes to ground between the swing arm and gearbox
That’s great. Thank you.
A friend of mine ran the breathers into a Guinness can that matched his black and gold colour scheme. 
I run a V50 with pod filters . I made this up using a Y branch l got from an aquatic shop. You can get internal Y branches, but l thought the bigger bore would be better . Cheers.
That looks good. I’ve ordered some tube and a Y-piece much as you describe.
One question though: The breather that comes from the back of the engine sump: I took the tube off, and a significant quantity of oil then drained out of the spout left behind on the engine. Is this normal? Is the spout below the surface of the engine oil in the sump, and if so, how does this outlet act as a breather for the crankcase? I’m well confused!
The return to the sump ? Think it’s below the normal oil level , so there will always be some oil in the tube .
Yes the connection low down back of the sump is not a breather it is for the return of oil that has separated out of the blow by in the air box.
You definitely don’t run that to ground lol
Many thanks for all your help.
Just to be clear then: There are probably three options here, assuming that I am using individual filters:
Which would you recommend?
This thread could become a science project all of its own, which means I don’t really understand what the breather system is for, and what the anatomy is supposed to look like. I assume that the crankcase (i.e. below the pistons) must be able to vent excess pressure (blow-by), so how does it achieve this on the Guzzi engine?
Thanks again.
As far as I am aware Guzzi let their engines breathe from the top of the cylinders to allow for air movement caused by piston movement and blowby. In the small block engines this is is from the rocker box.
The breathing goes to some kind of reservoir to allow gas and liquid to separate (In V50 that is the air box). Liquid then drains back to the sump.
A sweet project would be to mount a transparent container in the space between the filters with 4 pipe fittings. Run pipes from the rocket box to the middle, the drain to the sump from the base and a vent tube from the top looped down to ground. Bingo your own ‘seperator’ and you can see if the engine starts breathing more oil.
What would cause an engine to throw lots of oil out of its two breather pipes?
Having just ridden the bike for the first time since fitting my home-made catch can, I was lucky to survive the trip, because there was oil getting out of the air breather pipe, which then got all over my back tyre.
Not good!
Pressurisation of the crank cases would push oil back up the return pipe that goes into the bottom rear left corner of the cases. I wouldn’t think much oil would come from the vents in the top of the engine.
Have you run a pipe from what you call the “catch can “ back to the sump?
As i said above Guzzi are designed to push oil vapour up the push rod tubes to the rocker boxes where it breathes into a separator gases then are separated off to the air filter and oil separated off to drain back to the sump via the long tube
My catch can has three connections.
At mid-height to the two breather tubes from the rocker covers (should be oily vapour only).
At the top, to atmosphere via a plain tube routed behind the gearbox to low level.
At the bottom, connected directly to the engine sump return point.
I suspect that liquid oil is being pushed UP the return line into the catch can by crankcase pressure, where it is overflowing down the vent pipe in some quantity.
Perhaps, I should plug the return point on the sump and just take the rocker cover breathers through a filter to atmosphere. No catch can at all.
Sorry i clearly did not understand your design. Sounds like too much gas leakage past the rings 
Hi,
Would it be wrong to simply block the breather oil return by fitting a suitable bolt in the threaded hole in the sump? I’m concerned that excess crankcase pressure is forcing liquid oil up this pipe, and throwing it out through the catch can air vent, all over my rear tyre! The engine will still be able to vent crankcase gasses through the usual pipe from the rocker covers, which I will tee together and terminate with a filter.
Its worth a try but i expect the breathing from the rocker box will carry a lot of oil and fill your catch tank quite quickly.
By the way have you checked that the oil drain back to the sump is not blocked? If that was blocked the oily mist from the rocker boxes would go every where, Try blowing down it
