BSA M20 identification and advice

[Harry7610]

Hi guys..... A little background....

Ive always loved board track bikes and since ill never be able to afford one id like to build my own. I only just joined but ive been a member over at motorbicycling.com for a while and built bikes over there, with bicycle frames and small stationary engines. I built a boardtrack style bike using a 40s Villiers 98cc cylinder mower engine. It wasnt a rocketship but was a joy to ride and i couldnt go anywhere without people asking me what it was..... until the police warned me not to ride it on several occasions (ICE motorized bikes are illegal here in OZ).

[Harry7610]

Although these bikes are illegal i have such fun building them that i still have more builds going. I figure its good practice for an XS650 or similar bobber build when i can afford it.

So i planned on just building motorized bikes for a while until i came across this BSA engine at a great price. I believe its a 500cc BSA ZM20 engine. The engine number is "ZM20 4913" and from what i could find on the internet it would be a 1950 model, but some guys over at motorbicycling.com seemed to think it was older because of the carb. I thought id join here also because you may be able to I.D the engine and some advice.

[Harry7610]

Plans change but id like to build a boardtrack bike (unregistered at this stage), obviously with this engine id need to build a droploop frame. Ill find vintage drum brakes and lace them to worksman 26" rims with Simplex tyres, but if its too dear to get worksman rims here to OZ ill just use 21" motorcycle rims. I have an Albion gearbox on its way.

Heres a comparison to another Villiers engine i have (same engine as in above bike)

[Puttete]

Hello

Very nice bike you have there.
Of course, that engine isn´t that strong but hey, it´s running and it looks awesome.

Yes, that engine number says it´s a 1950´s engine. The numbers on that year started with -4001.

My boardtracker got a BSA engine from 1928. It´s not a rocket either but i can keep up with the traffic. And it´s very, very illegal....

Keep up the good work and don´t forget to keep us posted here.

Oh, i almost forgot.
That carb suggests it´s been a stationary engine. I have never seen anything like it but i don´t think it´s for road use



Later // Patric

[Harry7610]

Im fine if this engine is slow, the Villiers motorized bike was a little slower then my other "Chinagirl 66cc" bike i have, but i couldnt stop riding the Villiers. Im more about looking cool (and cool in my book is vintage) and not so much about speed. Besides that actual boardtrack bikes werent all that fast. And this engine was cheap enough........ So now i just have to find a use for it.

Thanks for the headsup on the carb, it had me scratching my head.... obviously ill need to change it then.

Do you have a link to your bike?.... id love to see it.

[Harry7610]

Puttete......AHHHHHHH i found your thread, as soon as you said '28 model engine i wondered if its that beautiful red boardtracker id seen on Youtube............ and YES thats it. Id made a comment on your video...... looks like ive come to the right place. I absolutely love your bike. Any advice you could could give to help my build along would be greatly appreciated

[Puttete]

Thanx for your kind words.
It´s always fun when people like what i do. That´s not what i aiming for but it´s still nice when it happens.

The best tip i can give you is to be prepared to redo something you just did. When you build from scratch it´s almost like inventing the wheel again.

Plan ahead as much as possible and measure atleast twice before you cut anything.

Make drawings, check angles, measure twice, take a break and try to find out what you forgot, then measure again....
I think you get the picture.

You can also check out my build diary for "the Grudge" here on JJ. Search on Rudge racer and you will find....
It´s been a little slow lately but i hope to get it together soon.



Later // Patric

[Harry7610]

Cool i will check out your "The Grudge" build

[Harry7610]

First things first, i will build a stand for the bike and see if i can get it running.

A couple of questions:

1. Is it ok to test run the engine without an exhaust.

2. How can i test fire this bike? A drill or rattlegun on the crank? Providing that doesnt undo the nut.

Im going to have lots of questions and any advice would be greatly appreciated.

[Harry7610]

Just thought of another question (sorry guys, i am a newb to these big engines)....
If this is indeed a stationary engine carb, will this still work, and if not any idea of a recommended carb, something vintage and cool looking rather then a high performance carb would be good.

I obviously need to downsize my pics too :-(

[Puttete]

Originally Posted by Harry7610

First things first, i will build a stand for the bike and see if i can get it running.

A couple of questions:

1. Is it ok to test run the engine without an exhaust.

2. How can i test fire this bike? A drill or rattlegun on the crank? Providing that doesnt undo the nut.

Im going to have lots of questions and any advice would be greatly appreciated.

Fire away, i´ll try to answer as good as i can. I don´t visit this site every day so you might have to wait a bit.

1. It´s okay without exhaust, it will just not run perfectly.

2. Drill to the crank will work but at some point you will probably undo the nut. I don´t have any better suggestion.

As stock, i believe these engines came with Amal 389 carb. I´m not sure about the number but definitive an Amal. The inlet should be around 1 1/8, or 28 mm depending on what system you are used to. You might find a similar carb instead, just don´t use a carb with acc. pump. It will drown the engine at higher rpm.

The carb you have will probably work at a specific rpm. If it´s clean that is...



Later // Patric

[Harry7610]

Cool, thanks for the reply, so after a cleanup I'll try to run the engine with this carb, should it go then I need to find a carb and planning a frame.

I'm more then happy to wait for answers as I'm still active building motorized bikes so this will be a slow build.

Another question is what's the outer pulley on the crankshaft for? Is it for a generator? Is it a stock piece or has it been added for stationary engine purposes?

[unclechop]

Hargansaw engine

[Harry7610]

Originally Posted by unclechop

Hargansaw engine

Sorry but I don't follow, what's that?

[unclechop]

[unclechop]

They are called sniffer carbs the engine is mounted upside down in the hargan saw(not sure of spelling)

[Goldy]

Hi Harry, funny finding you on this site...I just finished rebuilding a WM20 engine...this one is definitely different...weird carb, Mag only with no Dyno and of course the pulley. Whatever it is and wherever it was originally from I suggest you haul it apart before you even bother to try and start it...it it probably will run as long as the mag has some life, but the main and crank pin bearings are probably pitted, spalled and fretted just from sitting for a dogs age...I have found this to be the case with almost every old brit I have overhauled. Good luck with it, it looks like you should be able to come up with an interesting build, keep us posted.

[Harry7610]

Originally Posted by unclechop

They are called sniffer carbs the engine is mounted upside down in the hargan saw(not sure of spelling)

Thanks uncle chop, I'm with you now

[Harry7610]

Goldy, I'm stalking you, I told you I was a big fan of your builds :-)

I'll follow your advice I think and pull the engine down for inspection. Cheers

[Harry7610]

Goldy, do you know of a site with all the rebuild info on these engines or am I best trying to find a manual?