B31 (1954) - no spark

[chrisb]

I'm just trying to get my uncle's B31 going again.  I've only done some cosmetic work on it and changed all the fluids.   It ran fine when he last used it (3 years ago) so am assuming all the timing etc is ok.

I can't get it to produce a spark.  When I lay the spark plug on the top of the cylinder (next to its hole), with the side of the spark plug in contact with the cylinder and kick it over, there is no spark. 

I have put a new spark plug on (to rule out a dodgy spark plug) and cleaned the contacts (with 600 grit paper slid between them - lots of black muck came off).   The spark plug lead looks in good condition.

Does it need the battery connected to 'energise' it somehow?

What should I investigate next, and in what order?   I'd be grateful for any advice.

Many thanks

Chris

[Rog1]

The first thing to try is to slide a piece of thin card moistened with petrol between the contacts. Move it gently back and forth and see how much dirt you can clean off. Repeat with a dry bit of card; this will ensure the contacts are clean. Don't, of course, try getting a spark again until you've given enough time for the petrol to evaporate. Did you remove the contact breaker to clean it? If so, check the gap.

If you are still not getting a spark, remove the plug cap and see if you can get a spark from the bare end of the HT cable to the cylinder head. If you can, the problem is either the plug or the cap, so maybe try the old plug. Magnetos are self energising; they don't need a battery.

[chrisb]

Many thanks Rog1.  Cleaned as you suggested (the contact breaker hadn't been removed and the gap seems ok).  I can now get a light spark between the bare HT cable and cylinder head - so I guess I need a new cap!  It is a weak 'white' spark with no sound, not the strong "crack" and flash I'm more accustomed to from car engines when testing for spark.  Does this sound okay? 

[idie]

No! You should get a nice blue spark that is able to jump about 1/4 inch. If the bike has been standing for some time there is a good chance that damp has got into the magneto windings. A rebuild maybe the answer.

[Gadorey]

Before rebuilding take the pick up attachment out of the mag, insert a clean rag soaked in spirits and turn the mag. This way you clean the pick up ring. Then deglaze the carbon brush (sticking out of the pick up body). Run a jewler's file a few times over the brush. Then check and set the breaker points gap to their correct gap.

If you still can't get a proper spark, you might have to go deeper. Usually the condensor is gone.

[chrisb]

Thanks everyone.  I'll have a go at cleaning the brush and mag ring.  I've heard changing the condenser is really for experts so hoping it isn't that!

[chrisb]

Well, it sparked weakly first of all.  I then left it for a day, tried again and no spark at all.  I have tried changing the lead, dressing the carbon brush, cleaning the contact ring (best I can with a rag with petrol on a pencil), cleaned the contacts and tried to get a spark with and without a spark plug.

I'm now at a loss and I think that diving any deeper into the mag (eg condenser) is beyond my capability.  It is quite gutting really since I've spent over 2 years working on it steadily, mainly doing cosmetic things and tidying it up.

Does anyone know of a good honest bike classic bike repairer in the East Midlands area who might collect it and get it going for me?  I am in the Rutland area near Stamford (Lincs).   I don't have a trailer or towbar so they would need to collect.

Many thanks

[Pete A10]

There is a guy called Peter de Kremer, trading as Rutland Dynamos who does magneto and dynamo rebuilds. My A10 was giving exactly the same symtons as your bike. After the mag rebuild it would send a spark over a quarter of an inch. The rebuild cost for a mag is £310 - not cheap but he gives a five year guarantee on it. Fits new condenser and new windings. His phone numbers are 07928 875844 or 01572 724873 or 07973 326796. If you google Rutland Dynamos it takes you to his website which gives more information.

[chrisb]

Thanks Pete.  I'll give him a call!

[Gadorey]

I also had a very good experience with Brightspark magnetos - www.brightsparkmagnetos.com

This was 4 years ago with a MDB1 for my 1929 Sloper, so I am not sure if they are still around.  They also replaced the condenser with a solid state one, which I would recommend.  Since the overhaul, I never had a problem again and the bike starts reliably with one kick, cold or warm.

[berniej]

Quite possible that there is a breakdown in coil, condenser or both and it needs a rebuild.

However, worth checking that the ignition timing is spot on first.

Magnetos only produce their maximum discharge at a precise point in the rotation and if the timing is too far off the spark energy will be less as the contact breaker will open either too far before or after peak discharge (i.e. outside the acceptable range of the intended advance/retard).

From memory it's 7/16" BTDC fully advanced but hopefully someone will correct me if that's wrong for your bike.

Cheers,
Bernie

[Pete Gill]

I can recommend Peter de Kremer at Rutland dynamos too.
He refurbished my B31 magneto and did a really good job. He phoned me after he'd stripped it and discussed what it needed and what was okay. He is busy and so it can take time but he puts a lot of personal care into it.
It came back with new bearings, new lead and cap, all blasted clean with points set up all ready to fit. The winding on mine was okay so he didn't recommend that it needed rewinding. The main issue on mine was that the bearings were worn out.
He tests the mags on his 'rolling road' set up where he get the mag running over an extended time to get it hot, as its when they get hot they start to fail in performance.

Pete