B50MX Ultimate Build

[BJJ]

I believe this is an original factory photo I have here, Dated by Kodak on the back (OCT '70)
More photos to come of the first import arriving into NZ.

[BJJ]

Arrival & Road Test @ Ruapuna, ChCh, Canterbury, NZ (Aug '71):

[BJJ]

Fully sponsored and ready to race (Kaiapoi, ChCh, NZ, 1971):

[BJJ]

A Sabiston,  Kaiapoi, Canterbury, NZ, August 1971, 499cc BSA. Followed by Paul Doecke on a 380 CZ.

A Sabiston, Clarkville, Canterbury, NZ, Sept 1971, 499cc BSA.

A Sabiston,  Halswell, Canterbury, NZ, Oct' 1971, 499cc BSA

A Sabiston, Kaiapoi, Canterbury, NZ, Dec' 1971, 499cc BSA

[flower pot racing]

I love my B50.  What is your budget?  You can go completely bananas if you wish......

Stock wheels weigh an absolute ton.  Loose them.  Forks are short and there are better available.  Shocks, straight in the bin.

Motor will make you smile in std trim, but the sky is the limit if you want F1 technology.  A modern style clutch will work wonders and is a good upgrade, but the first thing I would do is change the ignition.  Basic electronic is fine, but you can spend big bucks on Interspan if you wish.

Frame is heavy, loose that, replace with a faber works B50 one or a cheney, or a CCM frame, hang on a minute where did the B50 go.....

If I could only do one thing it would be to fit electronic ignition.  Got and forget.  I run PVL which is fine, fires the bike up easily and has never let me down...... 

Don't for get to add a drop of Castrol R in the fuel, then it'll even smell good to!

[jerry]

So Ben replace everything except the seat,handlebars and tank? J

[big mac]

Thanks for the great pics and advice. ignition was high on the list as I like my bikes to start off the kicker.

[FourstrokeForever]

I love my B50. It's a pleasure to ride and gives the 2 stroke pre75's a run for their money. Mine is pretty stock in the motor department, apart from an Electrex ignition and a bit of head/barrel fin removal. Bigger diameter header pipe. Pay close attention to cam timing and tappet adjustment to make sure they are both spot on. They also need to breathe so there are extra breathers on it. I saved nearly 1.5kg of unnecessary weight from the fin trimming. It runs the standard B50MX cam which by all accounts is the best of the cams. Carby is a Mikuni 34mm. Ditch the Amal carb.
The frame back bone has been shortened and a new rear loop has been made to meet the back bone. That mod saved another 2.5KG! Shock mounts are in the standard position. It runs a '74 CCM airbox with a decent size filter.
Aluminum rims. 21" front and 18" rear. Front hub/brake is '74 Honda XL350 mated to Betor forks. Rear hub/brake is '73 Bultaco. Rear shocks are 14.25 eye to eye Betor's. The longer shock gives a sharper steering angle while staying within the 4" travel limit. Honestly, there is no need to alter the rake of the headstem. The B50 turns exceptionally well and goes where you point it.
I will post a few pics when I get the time.

[flower pot racing]

So Ben replace everything except the seat,handlebars and tank? J

Depends how far you want to take it.....  They are a good bike in std trim.  A bit of Castrol R in the fuel could be the easiest way to have more fun as it will smell nicer.

[tony27]

A Sabiston,  Kaiapoi, Canterbury, NZ, August 1971, 499cc BSA. Followed by Paul Doecke on a 380 CZ.

Paul is still going strong on a CZ250, not an easy rider to finish ahead of even at his advanced age

[Bamford#69]

Hi, if you look closely you can see me stuck in the Gorse bush , one lap behind of course,
cheers

[big mac]

Am interested in the specs for a 34mm mikuni did you get it from the USA with an inlet manifold. How did it effect the performance was there still plenty of grunt.

[FourstrokeForever]

Am interested in the specs for a 34mm mikuni did you get it from the USA with an inlet manifold. How did it effect the performance was there still plenty of grunt.

No, I didn't get the carb from the USA with an inlet manifold. the original port inlet was cut and shut to allow a hose type fitting to connect the carby. There is way more useable power with the Mikuni. basically the bike pulls from zero to hold on with very little noticeable hesitation. 

[Michael Moore]

I've got some B50 info on a page of my website

http://www.eurospares.com/bsaunit.htm

including some detail photos of Cycle Factory and Dick Mann-framed bikes.  I got to race the DM bike though unfortunately the whole day was a case of "bastard stopped again" and I was lucky to get 3 laps in a race, it never ran long enough to complete any of them.  The DM frame was friendly and didn't want to push the front end the way my B50MX did.

I never had much luck with starting with the ET magneto installed.  Converting to regular points/coil/full-circle stator as in the T or SS gets you almost double the amount of static retard, and as mentioned above a Rita or other electronic ignition makes things very nice.

I recommend the rearset footpegs like Dick Mann used to sell.  You can also use the pegs off of a TR5T Triumph 500 as they bolt on and are more rearset than the single parts.

The 932 Amal worked fine but you need to have one that isn't worn out, and they start wearing out as soon as you use them.  I sent mine out to have the body bored and fitted with a hard-chromed brass Mikuni slide, which should fix the wear issues.

The kick starter flops backwards over jumps and tries to engage and also gets in the way of your leg.  Getting a bottom-pivot lever is a big improvement and removes a significant distraction.

I've got a long Carillo rod/JE Piston installed and I['m looking forward to getting the engine built with that and a PES/CCM-UK clutch and an NEB 3 speed.

cheers,
Michael

[sa63]

I have used a Dellorto 34mm at PES recommendation, and im very happy with it. Im getting life within a couple of kicks with a HPI ignition. Have gone down 3 speed, PES splined clutch route.
The clutch totally disengages if you push in gear, and using one of ballards yam. clutch levers the pull is like a modern bike