Modifying the BSA A10 breathing system with Bunn valves

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The BSA A10 has a timed breather system as standard, which, although it has its strengths, doesn't work well with higher tune engines - particularly so if being ridden hard (as mine always is!).
I've replaced the timed breather with two one-way Bunn valves that let fresh air in the top of the engine and then out of the bottom. Here's how I did it. If you have any questions, leave a comment below.

Опубликовано:

9 апр 2019

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Комментарии : 47

@philmuskett265 11 месяцев назад

Very snazzy, mate. A good idea that really works!!! Well done.

@t20racerman 10 месяцев назад

Thanks. It really did make a difference 😊

@zepelin-on-road 2 года назад

I like your photo album at the end 👍, remember me younger, some (many ...) decade ago with my A10 ...

@t20racerman Год назад

Thank you :-)

@thomaswykes3647 Год назад

I fitted a larger NRV to my wifes 2CV years ago. The opposing pistons cause the crankcase to pant and the NRV meant a slight vacuum was maintained. This stopped the oil leaks dead.

@t20racerman Год назад

Properly breathing makes such a difference to an engine 😊

@caiowainbirch7750 2 года назад

Great video, I've used your video as a guide for fitting a breather to my A10 sidecar outfit. Just drilled and tapped the crankcase breather hole to accommodate a 3/8" spiggot and cut off the end of the timed breather as shown in your video. Hoping this will relieve the oil pissing out. It's been a real problem, especially with the engine under the extra load from hauling a heavy sidecar around. Thanks for the informative video!

@t20racerman Год назад

Sorry for the late reply. Hope it worked for you. It certainly did for me. You do need a valve either end of the breather sysyem though. Bunn valves are no longer available, but there are others on the market that do the same job.

@441rider Год назад

Glad I did this, will try the bun valve too. My unit single gearbox vent tube goes right to my chain and seal around main shaft..

@t20racerman Год назад

Bunn valves are unobtainable now, unfortunately, but there are similar products on the market.

@xvrays 4 года назад

Hi Adrian. Very informative video on your Bunn valve breather system. I remember reading your article in last years Classic Bike. I've just finished restoring my 56 Plunger A10 after 2 1/2 years. Now I'm having trouble with the oil drips (same old story British bikes...). There's even oil forcing its way through the camshaft cover gaskets. I think I need to take some drastic action.

@t20racerman 4 года назад

You have to get the timed breather clearance spot on. If its too loose you'll get problems. Also, worn bores and/or worn valve guides don't help. The Bunn valves make a huge difference, but aren't being made any more. There are other similar valves around though - definitely worth looking into.

@xvrays 4 года назад

Adrian Baker. Thanks Adrian I’ll keep that in mind. I’ve another problem now. Was out on the bike when the engine started banging and misfiring badly. Fortunately I wasn’t far from home. Back in my garage it was only firing on the left cylinder. Took the rocker box off and found that somehow the exhaust pushrod had jumped out of the rocker arm cup and off the tappet so this little problem will keep me busy for a few days.

@stuartosborne3013 2 года назад

I’ve used the Ali Fuel NRV that are cheaply available. My current head scratcher is oil leaving one exhaust pipe. I’m hoping the new rings will address that 🤞.

@ogrithetrog Год назад

A bit of a pause at 5:17 and I think we were both at the same rallies around that time - provided I've recognised the "Nob-rot" badge correctly!

@t20racerman Год назад

Indeed we were. I was based in Coventry and loved going to the Nob-rot. Great Rally. Happy memories 😊

@chrisstaylor8377 Год назад

I ran the breather on my A10 into the primary chain case chain was allways lubricated

@t20racerman Год назад

Good idea 👍

@lamprenen 4 года назад

Hi, I have had a Bunn set on the shelf for years, intended for a Triumph, and comes with hoses for that. It has no privision for it venting to the oil tank, and is intended just to vent to atmosphere. Do you think it is necessary to vent to the oil tank on the A 10? And then attach a hose to the froth tower? Do you think it would leak lots if I just vented to atmosphere, maybe with longer hoses?

@JohnSmith-yv6eq Год назад

The vent to atmosphere wastes the oil drops that would otherwise return to the tank and be recirculated. A lot of blowby with a lot of oil mist will inevitably cover with oil wherever you allow the pipe to exit. It's why cars had the "road draft tube" turned into the PCV system... American highways had an oil slick down the middle in the 1950's from all the drips of oil blasted out of the tube pointed down to the road....

@steveking4966 2 года назад

okay, this sounds like a good mod, my question is , the existing breather port on l.h. crankcase adjascent too the cam is it threaded if so what thread please?

@t20racerman Год назад

Sorry I never replied to this. I can't remember what I used, but it was quite a course thread. It doesn't really matter what it is as it if you use a bit of thread sealant, it wont come out. Mines been in there for a decade or more.

@spottycat22 Год назад

Hi Adrian. Thank you so much for the video.. Before undertaking such a job I want to be sure a) I know what I am doing and b) it is the right thing to do to my bike. You say that bunn valves are no longer available but other one way valves are. I can not find specifications for the bunn valves for comparison but I have found some aluminium ones with the specs below - I wonder if you know the spec of the bunn valves and if these would be suitable? If so, what I/D of tube would you recommend? Size: 6mm (1/4"), 8mm (5/16"), 10mm (3/8"), 12mm (1/2"). Material: Aluminu+NBR Rubber . Opening pressure：0.2bar Working pressure:0.2-6bar Operating temperature:-30 ℃-130 ℃ You also mention that you made a threaded stud to fit into the breather. What thread did you use? and did you have to make the female thread in the casing? Lastly you said that you would make a some kind of collecting cam for the oil tank breather in case there is more vapour than you expect. Can you clarify what you did for me please? I really appreciate you sharing this mod. Thank you so much. Chris

@t20racerman Год назад

Hi there Any low pressure valve would work. The one you mention seems OK. I used a Whitworth tap in the breather hole in the casing, then made a threaded tube connector to screw into the casing. The breather tube then fits on that. I didn't need a catcher can on the breather as any oil drops back into the oil tank. The breather tube on the oil tank remains dry 😊

@larwatersports 5 лет назад

Hi there, loved the vid, i saw there were lots of chromed parts on the bike which i did the same on my old bike back in the day so was thinking could that be my old bike. The reg was 886 HBH. 1959. i would love to know.. cheers lee

@t20racerman 5 лет назад

Sadly not - sorry. Glad you enjoyed the video though.

@jimager4021 5 лет назад

Great isn't it. Revs quicker and no acidic gas stays in the cases. Almost the same as mine!!! Cheers. Musky

@daveco1270 Год назад

whats that piece of metal attached to your carb? It looks like a heat shield or something, but seems like it would cut down on air flow over the carb when you're moving.

@t20racerman Год назад

The big angled piece between the carb and the head is the drip tray. When 'tickling' the bike to start it from cold, petrol floods out of the carb and drips down. The tray guides it away from the magneto and it's sparks... 🔥🧯

@kafkastrial8650 4 года назад

Hi Adrian great mod .where do you get a bun valve from? thanks .Rich.

@t20racerman 4 года назад

Hi. Unfortunately they are no longer being made, but I'm sure that there are similar very low pressure one way valves out there. This really is a mod worth looking into - it made a huge difference to my A10

@jcerullo5015 2 года назад

fish tank use a low pressure one way valve. very cheap and it will work

@JohnSmith-yv6eq Год назад

Surely the earlier PCV valves are exactly this... they used to be pushed into a rubber grommet on the rocker cover on lots of engines and quickly replaceable. So putting a rubber tube on one end and another rubber tube on the other does the same job.....

@krrrazy 4 года назад

Hey that's a neat mod . Do u restore British bikes . How can I get in touch

@t20racerman 4 года назад

I used to restore them full time - only occasionally now. Currently doing a D1 Bantam for a customer. Contact me via my website www.t20suzuki.com

@johnmcdougall4149 4 года назад

I was very impressed with what I saw, Adrian but when you replaced the timed breather, did you actually remove the timing disk altogether prior to fitting a two one-way Bunn valves? I also noticed that a small air-filter was attached to the end of rubber hose but you didn't have an engine air cleaner on, how come? OK, I currently own a Dominator and I was contemplating of doing the same thing but instead of fitting a small air-filter, I could instead join that end of the rubber hose onto an inlet fitting I could make myself on a lathe where it can be fitted somewhere on the air-cleaner which attaches the carby. Is your BSA a road rocket, Adrian?

@t20racerman 4 года назад

Hi there. I machined the end off of the timed breather so it still located against the cam, but it remained open all cycle. You have to do that if you fit breather valves. The air comes in through the small filter at the top, through the engine and out the bottom breather to the oil tank. As the air is going into the engine internals, it needs a filter. No air filter on the carb though as its tuned for speed, & you get a better induction sound too. Raced two-strokes for years with open carbs - small increase in wear, but increase in power, pick-up and sounds. Bike is a Bitza. It had a 9 page feature in November 2019 Classic Bike magazine, and made the front cover too! 😁

@johnmcdougall4149 4 года назад

@@t20racerman When I dismantled the Norton Dominator's crankcase, I noticed there was a sharp edge on the inside of bronze bushing where the camshaft rotary valve used to spin whilst the engine was running. Also the two lugs were apparently broken off at one stage but subsequently brazed back onto the rotary valve leaving a very rough finish. However, I purchased both new rotary plate and valve for re-assembly but decided to leave out the rotary valve because I didn't like that concept. The reason for this is that the Norton's engine does about 7,000 RPM at 100 KPH which is twice as many revolutions as the VW Beetle. I could never appreciate a hideous design as the rotary valve because when I used to own a BSA Golden Flash, the camshaft had no provision for a rotary valve at the end and neither did the VW Beetle. Hence, there's less wear to deal with when those parts are omitted. I'm surprised that you leave the air-cleaner off the bike for racing because in Australia, an engine can wear out very quickly without an air-cleaner due to dusty conditions.

@t20racerman 4 года назад

@@johnmcdougall4149 Just thought I'd mention that the Bunn valve conversion has transformed the bike. After 30 years of rockerbox oil leaks, with oil pissing out if you held the bike at 4000rpm (80mph) it's now oiltight. One of the best improvements I've made to it. 😊

@gerry343 3 года назад

5:00 Finchingfield.

@t20racerman 3 года назад

It is! 😁

@oldbatwit5102 Год назад

Just leave a tappet cover off.

@t20racerman Год назад

Why didn't I think of that? 😁

@oldbatwit5102 Год назад

@@t20racerman The simplest solutions are often the best. Make sure you always wear full waterproofs though or you might get a little oily.

@Cobra427Veight 5 месяцев назад

I have a bsa b33 that's bored and stroked to 600cc it once had a rev counter drive on the side cover , so it has a cover plate on there , I took it off the other day , wow talk about windage , it has also has a breather tube on inlet rocker cover but looks way to small about ⅛ I think , I'm going to make a breather tank and add bigger hoses , might help all the gaskets, under pressure . Cheers

@t20racerman 5 месяцев назад

It should certainly help 👍