Electronic ignition wandering spark

[rhyatt]

https://www.ebay.co.uk/itm/191112866807?_ul=GB&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5339019731&toolid=10001&customid=eb%3Ag%3Avms%3Aeb%3Ap%3A191112866807%3B{GCLID}&_trkparms=ispr%3D1&amdata=enc%3A1JI4DdDIRRFSN2QActaccnQ43&ad_type=pla&loc_interest_ms=&loc_physical_ms=1007061&gad_source=1&gad_campaignid=20747179918&gbraid=0AAAAADmMgiiDHp2qyA3znNTwdmQrS6X-3&gclid=CjwKCAiA24XJBhBXEiwAXElO3xfA4UOrOA5rJkYjpH3H-7qnffxLWc6uh4r8qdXPhVUe16AOXiTOlRoCZSsQAvD_BwE

these are popular hall conversions for Mg cars
whereby you have 4 magnets(I think) moulded into the black plastic rotor to give 4 triggers(same as a 4 cylinder ignition car that it replaces).

so the principle is sound but you just need for a 1 cylinder bike

[PeteB31]

Thank you for the recent responses.

I'm familiar with the wagon wheel (stagecoach not a round chocolate confection) or stroboscopic effect/aliasing. I may not have made it clear, but I was running these tests at very slow speed, under 3 rotor revolutions per second. I could see with the naked eye and hear that odd things were happening. The camera was used to record and retain a record. At 240 frames per second there were no stroboscopic illusions going on.

Rhyatt, we used a sensor very similar to the one you have linked. There's no shortage on Ebay. One magnet with the rotor we made, as it's a 4 stroke single and the magneto runs at half engine speed.

It seems I may have fitted the wrong coil. I'm told it should be a non ballast coil, not a ballast coil. And I thought this was going to be simple!

Pete
 

[rhyatt]

It seems I may have fitted the wrong coil. I'm told it should be a non ballast coil, not a ballast coil. And I thought this was going to be simple!

what voltage are you using on the bike ? 6 or 12 and pos or neg earth

you may find this useful - it says about coils with 1.4 ohms max(12 volt system)
https://www.mgocspares.co.uk/DocumentHandler/72022/g421-accuspark-ignition-fitti-pdf?srsltid=AfmBOormEQtjilVBUB0Y5mNWkh82S9cT0Hmd0Nda_VkQ773xleA2qD7o

[PeteB31]

rhyatt. 6v / positive earth. I will read through what you have sent. Thank you.

Currently looking at 6v non ballast coils. No shortage on the interweb, but they all seem to be the larger (55mm diameter) size. Going to make squeezing one in discretely a tricky job.

Pete 

[scifi]

When you say the spark is occurring at the wrong place, I take it you mean it is jumping from the plug body and not from the spark gap..?
It's at the wrong place, not the wrong time..?

[iansoady]

40mm diameter 1.5 ohm coils are readily available - I have a couple which have been used for only a few minutes (failed experiment!). You could have one for say £12 posted (UK).

[PeteB31]

When you say the spark is occurring at the wrong place, I take it you mean it is jumping from the plug body and not from the spark gap..?
It's at the wrong place, not the wrong time..?

scifi. No, the spark is in the correct place, just not at the right time.

[PeteB31]

40mm diameter 1.5 ohm coils are readily available - I have a couple which have been used for only a few minutes (failed experiment!). You could have one for say £12 posted (UK).

iansoady. Thank you for the offer. Is it specifically a 6v non ballast coil?

Pete

[limeyrob]

No, the spark is in the correct place, just not at the right time.
Ah, the Andrew Preview problem..

[rhyatt]

So to clarify,
The HT spark is occurring across the spark plug electrodes , initially at the correct degrees btdc , but then wanders and changes it's point of timing all on its own

[PeteB31]

rhyatt.

That's about the gist of it.

To recap -

Firstly, the rotor with the magnet is fitted into the end of the magneto shaft. It is not screwed in, but is free to rotate. The rotor is being spun with the aid of a wooden rod. Initially by hand, then with a slow speed hand drill. At no point during the observed test was 180 rpm (rotor speed) exceeded.

At very slow speed the spark occurred as the magnet passed at its closest point to the middle of the sensor. As the rotation speed was increased the spark became further and further delayed. In the picture on the first message the spark occurred approx 160 degrees after the magnet passed the mid point of the sensor. A further small increase in speed and a second spark just after the first commenced. A little bit quicker again and a third spark happened. So now three sparks per revolution of the rotor?

Culprits.

Faulty sensor - Unlikely.
Faulty or incorrect coil - possibly.
Magnet in rotor too strong or wrong polarity - maybe, bench testing this now.

Pete

[iansoady]

40mm diameter 1.5 ohm coils are readily available - I have a couple which have been used for only a few minutes (failed experiment!). You could have one for say £12 posted (UK).

iansoady. Thank you for the offer. Is it specifically a 6v non ballast coil?

Pete

Yes. It has a screw in HT lead connection. It's one of these: https://www.classicbikeshop.co.uk/6v-screw-top-ignition-coil.html

[rhyatt]

ok

most car systems use INDUCTIVE on their crank sensors and front wheel ABS sensors and these drive modules that are capable of dealing with the Amps that an HT coil needs.

a crank sensor has its own inbuilt magnet that just needs a metal trigger whizzing past it to make it work(it generates an AC voltage as it  whizzes past the crank sensor and this triggers the module that triggers the HT coil)

if you find your magnet it causing the issue , maybe consider a small inductive sensor

your initial picture , looked like this untit ----
worth speaking to ACCUSPARK and all you need maybe is a single trigger built into the black circular slip over cam

[limeyrob]

I'm thinking two things:
Magnet too strong
Ally rotor - its not a full circle and its conducting.  So it has edges passing the sensor.
If eddy currents are the problem then they would increase sharply with speed.
The retarding is a serious issue, the Lucas RITA I'm more familiar with automatically advanced with speed as the inductance increased and triggered the spark earlier so it was inherently reliable.

Aside: I must ask "why".  I much prefer magnetos over coil ign, there's a reason most light aircraft flying today are still using magnetos and I notice that when they raced A65s in flat track many were swapped from coil to mag - a significant mod.  Mags fell out of favour because they cost too much, not because they did not work.

[PeteB31]

Thanks all for your comments.

Iansoady - That's exactly the coil I bought for this exercise.

rhyat - You may be on to something. We didn't buy the sensor direct from the supplier, so it seems a bit of a cheek to quiz them.

limeyrob - My feeling is that it's something to do with the magnet installation in the rotor. Testing currently happening, hopefully that will spill the beans.

The bike runs fine with the magneto. The only issue being that it was difficult / impossible to get a low steady tick over. Anything other than fully advanced and the engine would stall. I'm fairly confident that the timing was correct, so deduced that the problem may have been a weak spark when the magneto was not in its best flux position (fully advanced). A contributing factor is the very low quality of replacement points available.
The up side of all of this messing about is that nothing is spoilt and I can go back to the original magneto / points system.

Pete