E: Engine BTH Electronic Mag Timing Comet -Q?

[Jez Nemeth]

Morning all, hoping that there might be a pool of knowledge on this question - have a 12v BTH FM1C Electronic ignition /Magneto, on a Comet, and looking to time it for good all round road use.

Series C. Without ATD, all advance/retard is down to the chip inside the device, so that's taken out of the equation somewhat. 8:1 Piston, 105 Cams, 30mm Concentric/Premiere Carb -What might be the consensus on setting the timing? -I'm puzzled between 4 Degrees BTDC, and somewhere in the region of 34-36 degrees BTDC (which would be right for an ATD on full advance I believe)? -Help, advice, levity and satirical comments welcomed providing they get us closer to usable answers! Thanx in advance - jez

 

[davidd]

Jez,

Forget, for the moment, about the 4 degrees BTDC exhaust, because that is for cam timing. For ignition timing, you can use 36 degrees. I have used many of the FM1Cs on engines with good results no matter what the retard was. These were racing engines, but I never had starting problems. Follow the directions. Make certain that the 6mm rod is inserted all the way to the bottom of the magneto. Make sure that the arrow or mark on the magneto pointed "up" when you installed the gear and self withdrawing nut. Do not "bundle the magneto wires tightly together as they may "cross-talk."

David

 

[timetraveller]

I think that the four degrees before TDC he is talking about is the fully retarded ignition timing figure to give good starting

 

[Simon Dinsdale]

I think that the four degrees before TDC he is talking about is the fully retarded ignition timing figure to give good starting

Which you can check on a standard magneto with the original ATD, but on the new electronic BTH you cannot check the fully retarded figure unless you have a way of strobing the bike. The BTH only allows for the full advance figure to be set when stationary.

 

[Jez Nemeth]

Jez,

Forget, for the moment, about the 4 degrees BTDC exhaust, because that is for cam timing. For ignition timing, you can use 36 degrees. I have used many of the FM1Cs on engines with good results no matter what the retard was. These were racing engines, but I never had starting problems. Follow the directions. Make certain that the 6mm rod is inserted all the way to the bottom of the magneto. Make sure that the arrow or mark on the magneto pointed "up" when you installed the gear and self withdrawing nut. Do not "bundle the magneto wires tightly together as they may "cross-talk."

David

Hi David, your advice is good enough for me -thank you very muchly -Sound advice, appreciated, 36 deg it is.

 

[Jez Nemeth]

while we're here...strobing done on the primary crank side I'm guessing? Running a belt -

 

[BigEd]

If you use Davidd's advice you shouldn't go wrong. David has successfully raced with these units and I have done 10's of thousands of miles using one on my Rapide twin.
Fit the magneto with the studs in the middle of the slots. That way you can slack the nuts and rotate the magneto backwards or forwards to try slightly different advance/retard without having to remove the magneto gear each time.
Your BT-H Magneto is independent of your battery/charging system so it is not 12v.

 

[Jez Nemeth]

So with a dead electrical system it'll get me home on a bump I'm assuming? - That's good to know -Thanks Ed, this is all good stuff to know -checking, making adjustments and timing it up now, this evening's task with a fair bit more confidence than I had about doing it a few hours ago

 

[Martyn Goodwin]

Jez,

Forget, for the moment, about the 4 degrees BTDC exhaust, because that is for cam timing. For ignition timing, you can use 36 degrees. I have used many of the FM1Cs on engines with good results no matter what the retard was. These were racing engines, but I never had starting problems. Follow the directions. Make certain that the 6mm rod is inserted all the way to the bottom of the magneto. Make sure that the arrow or mark on the magneto pointed "up" when you installed the gear and self withdrawing nut. Do not "bundle the magneto wires tightly together as they may "cross-talk."

David

David,
Jeze has a 8:1 piston fitted. My experience with that compression ratio in my Comet was that 36 BDTC full advance resulted in heavy detonation when under load - I now run 32 BTDC and seat of the pants, the engine is both smoother and stronger.

Martyn

 

[vibrac]

And my starting point is 34 btdc and this is what I do make sure the mag sits in the middle of the slot on its 3 mountings and mark with a scriber on the flange and crankcase then best idea is spend some money get it on a rolling road and try adjusting position and look at results. Second best vary it on the road save money loose time