BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: jamieelliott on 10 October, 2017, 10:37:41

Title: BSA B50 MX issues
Post by: jamieelliott on 10 October, 2017, 10:37:41
Hello All,

Im looking for some help on some issues with a B50 MX i have only just purchased. The story goes like this...

On having found a b50 MX i decided to buy it, the advert said all good things about it , starts if you know the knack, and runs and rides well.

I met he guy selling it and he started it with the help of some easy start and a good kick. I was having less luck, but was persuaded that i just didn't have the practice. he rode it down the lane and back, I rode it down and pushed it back.  besides this i paid him.

Firstly the battery was pretty knackered on the bike. I tried many times to get the starting right and, it just wasn't working. I could get it to start with Easy, and a few kicks, and then it would idle but on opening the throttle it would just drop rev and die, there was no chance of pulling away.
I replaced the plug and fitted a new battery and it now starts without easy start . The battery that was in it was dead and wouldn't take charge even when on a charger overnight. With the new battery and plug, it will idle and also rev properly , and so pull away.

So after fitting that and getting it running , i took it to the field and did a few laps. All seemed OK. then a few times it cut out and would be a total b* to start, you would have to leave it and try and try to get it back to life, i was surprised as now it was hot i thpouhgt it would start ok. twice i took the plug out for a check and it was pretty clean, (and only half an hour old)  it wasnt wet or flooded either. 

After a few laps there was a bit of a knocking sound fro the front, i thought it was the mudguard on the frame as after closer inspection the aluminium mudguard bracket was broken.

I took it on the road up a hill and after pulling away at a junction, then dropping into second the big revs dived fast and the wheel locked and it came to a screeching halt. I couldn't move it in gear at all, and i couldn't kick it over. i feared the worst and that the engine had seized. After rolling back to the house and leaving it for ten when i got there, the bike kicked over again. I checked the oil and it was under the dip stick. I filled her up with new oil and drained some old out, there oil had quite a silver pearl to it which i thought wasn't good.

So new oil in and left to rest and she starts again, take it to the field and a few more laps, the knocking noise doesn't sound like the mudguard as its more of a pinking hammer sound from the front of the engine, and its on the revs. the pinking was mainly pulling up the revs and not so much at speed, ie coming out of the corners and not on the straight.  I dont know what the noise it, or if its connected to the revs dropping and failing of the bike.  I di thought maybe it could be the cams had they run dry, but it isnt constant, although it does seem to be in connection the the engine speed.

the bike did the same thing 2 more times, pulling away after changing up into 2nd, it drops revs fast dies and locks up skidding to a halt.

I have no idea why its doing this , first i thought it might be heat and seizure as it seemed to happen after about 8-10 min riding OK. but you can kick it over after, but it wont start.

I spoke to a friend who knows a bit and he said the pining hammering noise might be pre combustion as these bikes should be running on a higher octane fuel. He said it would make that kind of noise.

I was happy to buy the bike  thinking that it was just the starting knack that i didn't have ! but as you can see from all of the above, im not that happy as i thought i was buying a bike that runs fine.

Can anyone shed any light on what is going on? Im really loathed to start spannering this bike as my intention was to get something to just ride and not open up.

I get that it can be hard to start with the 10:1 head, but i expected that when its going it just goes. it will idle fine, but its just dangerous to ride at the moment with the fear of it locking up.

Title: Re: BSA B50 MX issues
Post by: JulianS on 10 October, 2017, 11:33:26
With seizures and the colour of the oil I think you need to look inside.

Check timing, carb settings, oil circulation and make sure the right plug is fitted.

You dont say which year, but the 1973 models gained a compression plate under cylinder to give 8.5:1, no doubt some earlier ones were also modified.
Title: Re: BSA B50 MX issues
Post by: MICKREEVES on 10 October, 2017, 16:49:34
Where are you?
Title: Re: BSA B50 MX issues
Post by: jamieelliott on 10 October, 2017, 18:38:16
The bike is in Mid devon.

Although the bike dies and the rear wheel locked, i could then kick it over after, so it obviously didn't totally seize (if that is what it is?)
Does anyone know anyone in Devon who is good with these, as id be happy to not do this work myself. I have a million jobs to do and dont want this to be added to the list.


Title: Re: BSA B50 MX issues
Post by: jamieelliott on 10 October, 2017, 18:46:27
this is the bike ,  1973
Title: Re: BSA B50 MX issues
Post by: DEREKANDERSON8 on 10 October, 2017, 20:01:44
that looks cosmetically a very nice bike!

is it only in 2nd gear it locks up?

other than gearbox i would say piston clearance, was the bike recently rebuilt before you bought it?