A working Bunn valve breather system on my A10.

[t20racerman]

Hi all

The topic of crankcase pressure and the BSA timed breather system has been discussed quite a lot on this site in the past (eg here - https://www.a7a10.net/forum/index.php?topic=721.0 ) and I joined in a few of these discussions as I was interested in fitting a Bunn valve (one-way flow) system to my A10. I did eventually do this to my bike, but I had to take it off the road shortly afterwards due to a few unrelated problems including a failing gearbox and (sadly!) it was off the road for nearly 18 months...

Anyway, its back on the road, running lovely, and that inspired me to make a YouTube video of what I did and how I did it for those who are interested. (The video is of the latest set-up as I've improved/changed it twice)

You can find it here:
https://www.youtube.com/watch?v=P1gGEYJd-A0

Hope you find it interesting.

If you want to see the bike being taken for a thrash through the countryside, here's another video that I did some time back.
https://www.youtube.com/watch?v=Wg8_lj2q4ac&t=1s

All the best

Adrian

[muskrat]

G'day Adrian.
That looks familiar.
Shame their unobtainable now.
Cheers

[BSA_54A10]

G'day Adrian.
That looks familiar.
Shame their unobtainable now.
Cheers

The whole kit & cabbodle is for sale if you want to make Rex an offer
You could sell them as Musky-Bunns, I can see the packaging right now.

[t20racerman]

Hi muskrat

Yes, you were one of the original commentators that piqued my interest in Bunn valves! So thanks for that :-)

Although the original Bunn valves aren't being made any more, there are numerous similar products on the market that would be just as good. Search for 'check valves'. Here's a good site:

http://www.smartproducts.com/check_valves_series_100_cartridge_specialty.php

Plenty of others too. Industry use these a lot.

Cheers
Adrian

[berger]

T20 racerman what size did you tap the crankcase and what is the bore of the fitting cheers, at the moment mine is tapped 3/8 whit cheers

[t20racerman]

T20 racerman what size did you tap the crankcase and what is the bore of the fitting cheers, at the moment mine is tapped 3/8 whit cheers

I really don't remember the thread size - I originally tapped it back in the early 90s when my bores were badly worn and the bike was blowing out a lot of oil - I did it to put an oil collection tube and container on!

Re the hole in the middle, again, my memory eludes me. I reprofiled the outer end, and then re-drilled it recently to make the hole bigger, and went as big as I dared before making the walls too thin. 7.5mm rings a bell. As wide as possible seems the way to go.

[berger]

ok racer man the hole in crank case is near to 8mm and my tube outlet 7mm , I will be looking into this before I build it  up  cheers

[chaterlea25]

Hi All,
Some years ago I fitted a Bunn breather to an AMC 650 twin, they originally a flap valve breathing into the primary case

I fitted the inlet to the rocker box and an outlet stub welded into the timing cover
It works very well    Rather than oil gathering in the bottle water vapour condenses in there and needs emptying out every so often

On another Notrun that had oiling problems I tried a catch bottle as well, it filled with snot like emulsion

From those experiences I think piping the breather back to the oil tank is a bad idea 

John

[t20racerman]

Hi All,
Some years ago I fitted a Bunn breather to an AMC 650 twin, they originally a flap valve breathing into the primary case

I fitted the inlet to the rocker box and an outlet stub welded into the timing cover
It works very well    Rather than oil gathering in the bottle water vapour condenses in there and needs emptying out every so often

On another Notrun that had oiling problems I tried a catch bottle as well, it filled with snot like emulsion

From those experiences I think piping the breather back to the oil tank is a bad idea 

John

A fair point, and I have noticed some condensation coming out the breather - but only a very tiny amount. It's something I'll keep in mind, but when I originally routed the outlet pipe down towards the swing arm, rather than up to the oil tank, I did get a lot of oily vapour blowing out and it made a right mess on a long run (no catch bottle originally...). Now its going upwards, there will be a tendency for any oily 'mayonnaise' that was going to form to stay engine side and hence get hot and so not last for long.

Also, I tend to use the A10 for decent distance rides, and ridden quite hard too, so it usually gets pretty hot when run, and any condensation should be disposed of via the breather pipe, rather than contaminating the oil. Thanks for your thoughts, and I'll monitor the situation, but everything is working well, and as I'd hoped for, at present. Hopefully it will remain so! 

Adrian

Edit: Another thought - collecting condensation and a little oil vapour in a small cold collecting bottle would definitely encourage the emulsion you saw wouldn't it? Whereas a hot oil tank wouldn't?

[chaterlea25]

Hi T20,
The catch bottle is sited well hidden away up high beside the iol tank on the AJS, that one only shows water
The Notrun has an alloy oil tank that might cool the oil a bit?
I tried lots of permutations on the Norton with routes and places to feed the breather pipe to
including the oil tank, I had transparent tubing from the crankcase to the snot was easily visible 
(The eventual cure for it was to go back to the original 3? start oil pump gears and breathe into a bottle)

A long time ago I built a tuned engine for a BMC mini, I added a 3/4in breather into a catch bottle, it gathered water too
The engine oil stayed like new in it and none of the usual snot in the rockerbox

John

[muskrat]

G'day John.
I don't see much "snot" at all. Must be the hotter, drier weather down here. I run the breather tube to the oil tank then vent it through the Bunn to atmosphere. The "snot" is an emulsion of oil and water (humidity in the air) which evaporates out when the motor warms up. That's why I always bring a motor up to operating temp before I shut it down. Good excuse for a few laps of the block.
G'day berger.
1/4 BSP for a 3/8 hose on the inlet (front tappet cover). Drill and tap same at the outlet hole in the case.
Cheers

[Rocket Racer]

on my methanol (very hygroscopic) a10 road rocket  which also runs a bunn set up, my catch bottle when emptied typically includes a quantity of water! presumably attracted by the methanol contamination from bore wash.
So on a race machine the bunn set up is very good for clearing water absorbing fumes.

[ironhead]

G'day John.
I don't see much "snot" at all. Must be the hotter, drier weather down here. I run the breather tube to the oil tank then vent it through the Bunn to atmosphere. The "snot" is an emulsion of oil and water (humidity in the air) which evaporates out when the motor warms up. That's why I always bring a motor up to operating temp before I shut it down. Good excuse for a few laps of the block.
G'day berger.
1/4 BSP for a 3/8 hose on the inlet (front tappet cover). Drill and tap same at the outlet hole in the case.
Cheers

G'day Musky
You got a reverse head on that engine ?

[Rocket Racer]

with methanol, my theory is I want to introduce fresh air to the bottom end and extract the contaminated air from the top end, so my exhaust rocker cover is also the outflow for my breathers. Works for me. sounds like the opposite of Musky's set up.

[muskrat]

Yes I have the inlet Bunn in the exhaust tappet cover with the little air filter sitting just in front of the frame down tubes. I figure that's where the cold fresh air is , cooling the hottest part of the motor.
Exit is the normal breather exit in the crank case and piped to the oil tank. The oil tank is vented through the exhaust Bunn to atmosphere.
Cheers