E: Engine My BT-H Is Not Providing Spark

[ClassicBiker]

I would suggest the answer is, yes. This photo snagged from post #8 suggests to me that there is a relationship between the arm as it is clamped to the shaft and the timing hole that is drilled in the shaft. I don't think there is a relationship to a collapsing magnetic field and the points opening as there is in a traditional mag, this is conjecture on my part. I suspect if you were to open up a brand new functioning BTH and line up the arrow on the end of the shaft and place the 6mm rod in the timing hole that arm will be just approaching the one of the coils.
If you think about Lucas Rita systems, you set the nub on the reluctor back 5mm from the pick up coil and on and Boyer Bransden systems the is a little window to view a red dot as the magnet on the disk approaches the coil and you can choose whether the system is spinning clock or counter clock wise. The system needs time to react that is why I think everything is timed at where you want full advance to occur.
I'm wondering if part of Carl's issue is due to heat and/or vibration if the arms has moved a little or the clamping screw has worked loose.
Just grasping at straws.

 

[Chris Launders]

As I understand it the generator part just charges up the capacitor so wouldn't need timing, it's the rotor passing the coil which induces the capacitor to discharge giving you the blast to the coil.

 

[Comet Rider]

I've got the cct diagram of the BTH somewhere at home.
If I can dig it out I'll post it up this weekend.

From memory the guy who worked the cct out said that the timing over the magnet and the air gap have the bigest influence on spark generation

 

[MarBl]

Just thinking, do electronic CDI "magnetos" need an internal timing for generating high voltage ?

I don't think there is a relationship to a collapsing magnetic field and the points opening as there is in a traditional mag

After mine failed, I experimented a bit with it. The problem was, that it started quite easy when I got it first. Then it failed soon and after repairs it sparked again, but the bike was almost impossible to start, whilst there was no perceptible difference when it was running.

So I first tested, whether the distance between trigger magnet and coil was an issue. It turned out, it was not, half a mm is just fine. Then I tested the positioning of the trigger arm on the rod and that made a huge difference. On optimal position I got a spark by turning the thing by hand whilst on the worst position I needed to double the revs to get a spark.
My assumption is, that on low revs it can be compared to a classical magneto indeed. This seems plausible too, since with hand cranking speeds I would not expect a smooth voltage from any generator.

 

[greg brillus]

One other possible scenario........I had a case years ago where the taper on the drive gear came loose.......this was because the pinion nut bottoms on the outer edge of the taper before it actually pulls up against it........this could work for some time, and then suddenly fail allowing the timing to slip........The quick fix was to undo the four 2 ba bolts on the drive gear, remove the main securing bolt........put it in the lathe and cut a chamfer into the mouth of the thread, thus allowing the bolt to go further home dow nthe thread on the mag shaft.......this then allows the taper to seat fully.......problem solved......

 

[CarlHungness]

Carl
Sorry it may sound like a stupid question but was the BTH actually rotating when you kicked the engine over? Just wondering if there a few stripped teeth on the mag gear as it still uses a fibre or plastic drive gear in the timing side area.

Yes, it turns just fine. I put my spark plug stop in after finding TDC with a spoke/poke and brought the piston
up to the stop, and my very old paint marks on the B-TH gear and the one next to it were on, and the pin
dropped in so it's timed correctly. As noted, I used to be able to hold a plug against a cylinder and see
spark when kicked over. This time I held the plug bare handed, got no shock/spark. Have the unit off
now and a man with mag experience is looking at it.

 

[CarlHungness]

One other possible scenario........I had a case years ago where the taper on the drive gear came loose.......this was because the pinion nut bottoms on the outer edge of the taper before it actually pulls up against it........this could work for some time, and then suddenly fail allowing the timing to slip........The quick fix was to undo the four 2 ba bolts on the drive gear, remove the main securing bolt........put it in the lathe and cut a chamfer into the mouth of the thread, thus allowing the bolt to go further home dow nthe thread on the mag shaft.......this then allows the taper to seat fully.......problem solved......

I noticed I had two thin washers on the nut holding the drive gear in place, installed about 15 years ago.
No rust inside, some condensation. Have a man looking at it now who has mag experience.

 

[CarlHungness]

I

See if you can check continuity with a multimeter from the wire end to a tab on the coil. When you fitted the new HT leads to the new coils did you check that the spike or screw thread in the nose of the coil actually went into the wire? It is possible to miss the wire and screw into the insulation of the HT cable. Poking the point of a scriber into the wire to make a bit of a pilot helps. See if you can check continuity with a multimeter from wire end to tab on the coil.

know what you're talking about and I made sure the point was making contact with the wires..I think I did use a
scriber to 'countersink' the wire, and twisted it in tightly. A mag man has it now and taking a look.

 

[Bill Thomas]

One other possible scenario........I had a case years ago where the taper on the drive gear came loose.......this was because the pinion nut bottoms on the outer edge of the taper before it actually pulls up against it........this could work for some time, and then suddenly fail allowing the timing to slip........The quick fix was to undo the four 2 ba bolts on the drive gear, remove the main securing bolt........put it in the lathe and cut a chamfer into the mouth of the thread, thus allowing the bolt to go further home dow nthe thread on the mag shaft.......this then allows the taper to seat fully.......problem solved......

I just used a drill to the same effect.

 

[davidd]

I installed a B-TH from Peter Nugent in David in the engine I built for David Tompkins, which he installed in his bike in 2019. In late 2020, it stopped working and David sent it back to the UK to Peter. He repaired it and returned it to David. He did charge for the work, but it was not under warranty. I talked David into crank-fired ignition to save a bit of weight and lose the parasitic drag. Here is a photo Peter Nugent took :

Something came adrift and wedged in between the rotating magnet and bent the rotating spindle.

David