B31 spluttering and flashing back through carb

[john.k]

Supressed ignition componenets?.....a mag should have a copper wire cored spark lead,the plug and/or plug cap must not be supressed,or the mag windings will break down from overvoltage..................I would also point out the well known fact that its virtually impossible to accurately locate TDC,with stick in plughole,and hard enough with a dial gauge .......therefore ,the full advance point is used as a reference.

[Rex]

True, but then we're talking about a mass-produced rudimentary iron-head commuter than was designed 70 or so years ago. So many manufacturing variables and so many rebuild variables since that that the figure is only ever a guide or starting point.
The point is that accidentally moving the mag lever too far would make it too advanced, and that's harmful.
I have read on the Net about riders who claim to  time to TDC and then jigger with the lever constantly to keep it in the advanced "sweet spot", but I don't think I've ever met someone who advocates that on an old British work-a-day bike.
Dial gauge? A little OTT on a manual mag-equipped B31. This ain't a GP racer...

[john11668]

Supressed ignition componenets?.....a mag should have a copper wire cored spark lead,the plug and/or plug cap must not be supressed,or the mag windings will break down from overvoltage..................I would also point out the well known fact that its virtually impossible to accurately locate TDC,with stick in plughole,and hard enough with a dial gauge .......therefore ,the full advance point is used as a reference.

There is a suppressor cap  so will remove that to eliminate. Not sure what core  the lead has  but will check , cant imagine that it wont be copper. As you say TDC is hard to find accurately   so the logic in using 7/16" figure is easy to accept . 

I fully concur with what you have to say too Rex  Even if you are a bit out on  timing then the lever gives you about 35 degrees of "adjustment" but using  that adjustment does not seem to improve the problem, and as you say it is an old iron work a day bike .
Will check through the valve gear again for  any  mishap and maybe see where tappet clearances disappear ( relative to tdc ) check for soft springs,  and see that the crankshaft pinion woodruff key is in good order and not allowing any movement .

We dont have a lot of experience with old bikes, but I was pretty adept with british car engines in the sixties  so just have to dig in the annals for the skills,   but have to say that this is being a bit of a trial 

[john11668]

Just as a follow up:-
HT lead is copper  but did not have non suppressor cap so will get one and replace .
Checked drive to valve gear and all sound. Keyway good and timing marks allign so valve  timing ok  ,  clearances fine at 0.003".  No noticeably soft springs  so not much further on really.

Did find a misdemeanor on the carb we removed . Fuel in the float so bouyancy reduced , but not flooding so I presume still holding level OK . But as I said before , new carb behaves just the same .
Not quite sure where to go next.

[R]

If the float has fuel in it, then the fuel level will be higher than it should be, and the mixture will be richer than it should be.
Not sure how that sits with not being able to run without the choke on.
Does it (exhaust) smell noticeably of petrol ?
Did you try someones suggestion of spraying a bit of petrol down its throat while reducing the applied choke. ?
(watch that it doesn't flash back while doing this, so keep an extinguisher or garden hose handy - and be outside)

Not quite sure where to go next.

Magneto.
You can work the advance/retard lever back and forward, and hear the difference ??
It revs up a bit when you advance it ?
You can pull the decompress lever, and it kills the engine ?

If the spark isn't strong enough to spark under compression, then that may be your trouble.
If you had a spare maggie on the shelf, now would be a good time to test it !
Know anyone you can borrow one from ?


When you examined the carb, the needle was lifting up with the slide, all the jets were screwed into place
and the choke slide was lifting up clear out of the way ??



Its in there somewhere, and it should be fairly obvious with such blatant symptoms...

[iansoady]

A sticking valve does sound like a possibility. You usually get increased tappet noise when that happens.

[john11668]

""If the float has fuel in it, then the fuel level will be higher than it should be, and the mixture will be richer than it should be.
Not sure how that sits with not being able to run without the choke on.
Does it (exhaust) smell noticeably of petrol ?
Did you try someones suggestion of spraying a bit of petrol down its throat while reducing the applied choke. ?
(watch that it doesn't flash back while doing this, so keep an extinguisher or garden hose handy - and be outside)

I thought that  but the carb is not flooding so I am presuming ther is enough bouyancy to close the needle valve . We will change the float though

Magneto.
You can work the advance/retard lever back and forward, and hear the difference ??
It revs up a bit when you advance it ?
You can pull the decompress lever, and it kills the engine ?

yes  yes and yes


If the spark isn't strong enough to spark under compression, then that may be your trouble.
If you had a spare maggie on the shelf, now would be a good time to test it !
Know anyone you can borrow one from ?

there is a growing wealth of opinion on this being the main suspect . We have a recond  mag on an ariel red hunter but  it is MO1 (rather than MO1L) Not sure id this would fit.  Dont seem to have a local lending library for mags    and they seem to be as rare as RHS.
One guy wondered abour electronic conversion so am exploring, but gona try to find a safe way of testing spark strength without the jolt up the  arm which I dont enjoy.


When you examined the carb, the needle was lifting up with the slide, all the jets were screwed into place
and the choke slide was lifting up clear out of the way ??

  All working as they should , needle in mid groove , new carb has a removable pilot jet which appears to be adjustable but no info or guidanceIt is a Wassel 928 similar to this one .    https://www.ebay.co.uk/itm/113753033071




Its in there somewhere, and it should be fairly obvious with such blatant symptoms...""

We must be getting close

[john.k]

Unfortunately,no other mag will "fit" a BSA ,due to the unique base location setup.... the Ariel mag has the standard" Bosch " four bolt mounting......There used to be lots of WW2 surplus NOS BSA M20 mags around,cant imagine they ve all been used up.

[john11668]

Looks like the going rate is about £300-400 and in some cases condition is in doubt so looks like a minefield.
If we dont seem to be getting a decent and consistent spark out of this one we may send it to the refurb shop.

[chaterlea25]

Hi All,

Unfortunately,no other mag will "fit" a BSA ,due to the unique base location setup.

A small amount of work will make it fit
Make some short dowels with 3/8 Whit threads and the other end turned down to the BSA dowel size, (5/16in from memory) screw them into the mag base to replicate the BSA dowels

John

[R]

Doesn't the magdyno on BSAs have the 4 pegs to locate it, and the strap clamps it down.
https://i.ebayimg.com/images/g/PCwAAOSwdIZdrrhu/s-l1600.jpg

BSA's were numerous enough that this doesn't exactly make them rare.
Enfields used the same locating/clamping system ?
Always watch the rotation direction....

[R]

If we dont seem to be getting a decent and consistent spark out of this one

That can be very difficult to judge - apart from taking it for a run, and seeing how it performs.
Yours doesn't currently seem to be doing too good.

(Difficult hot starting is often the 1st sign of trouble, condensor wise)

If you don't know the history of this maggie, that may be a wise move anyway.
Reliable motoring starts with reliable ignition....

[john.k]

Where I worked as a kid,the old foreman had a simple way of checking timing and spark........remove plug,hold in your hand,place thumb over plug hole,kick motor over.......you can judge ignition timing ,spark,and  compression by the bite at the same time as your thumb blows off the plughole.

[Rex]

I thought the Lucas MO1 series of mags were all essentially the same. If the DoR and spindle height is compatible then you should be good to go.
I wouldn't jump too soon on the mag conversion though...snap diagnostic "repair by replacement" can be expensive and futile.
What was the starting and running like before all this started? For a mag to fail electrically at the same moment that the sprocket came loose stretches coincidence a bit..

[john11668]

Prior to adding our electric start, the bike could be a pig to start hot, which was awfully inconvenient if it stalled at a junction. Electric start overcame this but I can see how it might mask a basic problem.
Are we saying that the addition of some screwed dowels  into the base of our Ariel Mag might make it fit ? Nice if it would as that would be a relatively low cost way for us to  prove the mag..

Not much chance of taking this bike for a run untill we can get it to run better though.