Happy Easter everyone!
I'm having some running problems with my B31. It starts and idles perfectly but stutters and won't accelerate when you add throttle. Initially, I thought it was an air leak in the carburettor. However, after rebuilding the carb with an Amal stay-up kit, it still doesn't perform as expected.
The B31 has a Thorspark electronic ignition kit with manual advance/retard and I'm wondering if the ignition might have wandered. I can't find any instructions for the Thorspark unit online so I'd appreciate any information anyone has.
Many thanks.
I phoned Thorspark directly when I was having issues with a unit on a bike I'd just bought. I spoke to the owner and he emailed the fitting instructions etc. Nice bloke, but I wouldn't consider buying a Thorspark kit for any other bike, I'd just get the mag rebuilt.
I have the fitting instructions for a magdyno system from Thorspark. Send me your email by DM and I'll forward it to you
Cheers
Pete
I can't see how it would be ignition, even if the advance / retard was out by 10 degrees.
If it cuts, as in no fuel, try lifting the needle a notch or two. Have you altered the air-cleaner.?
Thornspark only eliminates condensor problems, the mag can still be faulty
Quote from: paulm on 04 Apr, 2026, 11:49 Thornspark only eliminates condensor problems, the mag can still be faulty
Hmm
My understanding that with Thorspark the ignition becomes coil/trigger with battery required.
The magneto is not used in the conventional sense, all you need is the magneto spindle to be true and spinning in its bearings. It links the magneto drive pinion in the timing case to the electronic trigger in the magneto end plate. Because the trigger replaces the points the trigger can be rotated to advanced or retarded positions by the ignition lever and basic timing is set by the user with pinion/mag taper as usual.
Given the symptoms it sounds more like fuel/carb settings.
Has the carb had a winter lay off with modern fuel drying out inside it?
If so you need to strip and clean thoroughly, drain and wrinse tank and full with fresh petrol.
2c 😉
BTW- what carb (concentrics are notorious for blocked needle jets that are a pain to clear properly
Quote from: Pete C on 04 Apr, 2026, 08:55 I have the fitting instructions for a magdyno system from Thorspark. Send me your email by DM and I'll forward it to you
Cheers
Pete
Thanks for the offer Pete.
I have sent you a DM.
Many thanks.
Thanks all for replys.
To add more info...
I'm running a 376 Monobloc Carburettor and I've already lifted the needle a notch without any improvement. Trying another notch to position 4 might be extreme.
My bike has been running lumpy since I picked it up. I initially thought it was an air leak as it ran better with choke. so I flattened the warped flange face, ran it through an ultrasonic cleaner and gave it a full genuine Amal kit refurb.
Upon inspection it turns out the main jet was incorrect and would have been very lean at 200 instead of the 260 it should have had, so I rectified this with the rebuild. Even at this rate, it's not getting past the needle jet.
I'm going round in circles trying to figure out the root cause. One minute I think it's an air leak the next fuel starvation and then the ignition. It's driving me mad!
Has it run well before and this issue is recent, or has it never run right?
I think I've identified the problem!
I removed the plug and turned the bike over to see where the spark occurs when the piston is at top dead centre. Instead of sparking at the magnet on the rotor and the black mark on the pickup, it sparks well after at the opposite end of the pickup.
However, if I roll the engine back with the magnet shaft rotating anticlockwise, it sparks at the black mark.
As the magnet rotates clockwise on my bike, I suspect the thorspark has been set up incorrectly and is actually sparking well after top dead centre.
Can anyone confirm my suspicion/thought pattern?
Quote from: V500 on 04 Apr, 2026, 17:33 Has it run well before and this issue is recent, or has it never run right?
No idea. I only picked it up three weeks ago. It's always run at idle but would never pick up. That's why I assumed fueling.
So it could indeed be something fundamental like the ign timing way out.
Its not unknown for people to set up electronic ignition for the wrong rotation. The B31 mag rotates the opposite way to the engine. Same for the advance and retard, easy to get those reversed too and time at full retard instead of advance.
Best thing is to assume nothing is right and set it up from scratch.
Quote from: limeyrob on 04 Apr, 2026, 20:13 Its not unknown for people to set up electronic ignition for the wrong rotation. The B31 mag rotates the opposite way to the engine. Same for the advance and retard, easy to get those reversed too and time at full retard instead of advance.
Best thing is to assume nothing is right and set it up from scratch.
Yup!
Quote from: B31Thumper on 04 Apr, 2026, 18:21 I think I've identified the problem!
I removed the plug and turned the bike over to see where the spark occurs when the piston is at top dead centre. Instead of sparking at the magnet on the rotor and the black mark on the pickup, it sparks well after at the opposite end of the pickup.
However, if I roll the engine back with the magnet shaft rotating anticlockwise, it sparks at the black mark.
As the magnet rotates clockwise on my bike, I suspect the thorspark has been set up incorrectly and is actually sparking well after top dead centre.
Can anyone confirm my suspicion/thought pattern?
I don't know where my instructions are, but I think they say that just turning the bike over can produce a spark, but not at the same point as when the engine is running. However, I think you are on the right track.
The mark is an initial setting, accurate enough to start the bike so it can be dynamically timed. You'll have to remove the primary cover and Blu-tack a degree disc to the crank, to check the timing; setting it up from scratch as limeyrob suggests.
Just a thought, is the wiring for the Thorspark sound? Many people have a blind spot when it comes to electrics...
I have just fitted Thorspark ignition to my B33. Any sparks you get by turning the back wheel or even twiddling the rotor with a stick in a power drill will not be in the correct place or even anywhere near it.
Make an initial setting to the black line on the sensor if still visible, this should be close enough to get the bike going. Then use a strobe and timing disk or similar.
It seems unlikely that the position would change once set correctly, unless the screw holding the rotor to the magneto armature is lose.
Pete
The triggering mechanism on these electronic ignitions is usually offset from the main encapsulation. If your Mag turns backwards, you need a pick-up on the other side.. Do Thorspark make a different pick-up for your rotation..?
The same pickup is used irrespectve of the magneto's rotation. It is fitted to the points backplate which is specific to the type of magneto. So the pickup postion is correct for both rotations. This a picture of the Thorspark fitted to an anticlockwise rotating mag.
Thanks Pete for sending the Thorspark instructions!
However, I seem to be mistaken. As mentioned by others and in the instructions, manually turning the engine might create a spark elsewhere on the pickup but won't start the bike. Consequently, I've had to return it to its original position.
Now I'm back at square one spinning in circles, wondering if it's the carburettor. This is really getting to me.
I've stripped the carburettor again and tried the original float and float needle. I thought the new float might be more buoyant and closing the inlet valve. Unfortunately, the same issue persists; it idles but won't pick up when the throttle is applied.
Where are you positioning the Advance/Retard lever on the handlebar for 'revving up'? If your Mag is Slack advance, you push the lever away from you to advance, but pull it towards you to advance if a Tight advance Mag.
Regarding rotation. While the pick up and rotor are not rotation specific, my experience with another type of electronic ignition is that the timing is. I don't know about Thorspark but if its magnetic or induction it may have a natural advance due to the increase in rate of rise of magnetic flux at the speed goes up.
Quote from: Ian C on 06 Apr, 2026, 17:04 Where are you positioning the Advance/Retard lever on the handlebar for 'revving up'? If your Mag is Slack advance, you push the lever away from you to advance, but pull it towards you to advance if a Tight advance Mag.
Mine is tight wire advance.
🤦�♂️ It turns out the battery was flat. Thorspark was lacking power so while it could start and idle it didn't have enough to sustain a spark under load. Everything's sorted now. Thanks for all the information.
Quote from: B31Thumper on 07 Apr, 2026, 14:34 🤦�♂️ It turns out the battery was flat. Thorspark was lacking power so while it could start and idle it didn't have enough to sustain a spark under load. Everything's sorted now. Thanks for all the information.
Excellent.
Quote from: Rog1 on 07 Apr, 2026, 18:20 Quote from: B31Thumper on 07 Apr, 2026, 14:34 🤦�♂️ It turns out the battery was flat. Thorspark was lacking power so while it could start and idle it didn't have enough to sustain a spark under load. Everything's sorted now. Thanks for all the information.
Excellent.
Sorry I am late to this party. And I am so glad you sorted out the problem. I was getting concerned reading through comments from the beginning that the Thorspark was not good, as I have just ordered the Thorspark for my 1950 B31, its in the post as I write this.
But I was thinking, if your bike started with the poor battery why did it not pick up power as it charged..
Is it still ok now?
All the best Steve