Yet another CDI question- AC or DC?

[lingeringstin...]

Since I'm about to start rewiring my bike I was just wondering what the differences were between AC and DC fired CDI boxes for my Chinese engine.

Unless I'm wrong it seems to me like the the AC ones run off an alternator coil and are supposedly push startable if you have a dead battery. In theory that would work but in reality unless you're on a long downhill run I really doubt you can push start my bike with a dead battery and an AC CDI running off the alternator.

For years I've carried a little 9v battery in my MZ side box because the coil I've been using would still work at as little as about 7 or 8 volts and my bike would run off the alternator once started. So in a pinch I could (kick) start my bike with the little 9v battery connected to the coil and then remove it and run off the alternator. I tried it, it works. Nice bulletproof backup system if you need it.

Now, could I not basically do the same thing with a DC fired CDI box? I realize it might not work on a 9v battery (but maybe it would) but to me it seems like it would be easier to start a dead bike with a DC fired CDI as you could just connect up a small emergency battery of whatever voltage it takes to make it fire without needing a major push start (probably not very practical) or jump start with another vehicle (not really practical most of the time).

A wee small battery might not be able to turn the starter but it MIGHT be able to fire up the ignition system easier with a push start, and someday I'll probably add a kicker to my engine anyway.

Am I right or wrong?

[stevo as b4]

I thought nearly all AC powered CDI ignitions, are of the flywheel magneto type, and I assumed that like my KMX so would your MZ are the same?

Obviously you don't need the battery fitted to easily start a bike with self generating ignition, my KMX currently has an AC powered OEM CDI. But I'm fitting an Ignitec programmable CDI and this is a DC powered unit.

If your at the stage of designing your bikes electrical system or building a loom, and you want scope for future changes then I'd personally try and fit a DC powered CDI, especially if you have a decent battery and charging system that'll cope with all the electrical load your likely to run? This is a consideration if your adding extra circuits or electrical equipment, as then running an ignition off the battery too might be marginal?

I think an advantage of DC CDI units is that they are often smaller. This is because they don't need AC-DC rectification and stabilisation circuits to drive the capacitors.

Programmable CDI's tend to more often be DC too as they like a nice stable power source for all the extra electronic circuits and chips etc.

[lingeringstin...]

I was being somewhat confusing there.

I used to run the MZ engine with points ignition and could start my bike with a 9v battery if need be. Now I'm doing away with the MZ engine and fitting a Chinese 250 twin with electronic ignition, so I'll need to do some rewiring.

Battery isn't a problem as there's good room in my battery box for whatever I need.

All I was wondering was whether it would be easier to start this new engine with a dead battery if the CDI was AC or DC triggered.

What I hadn't considered was that the AC one would have to have rectification built into it, which is just another possible point of failure as I see it, so that would imply to me that a DC fired box is a more sensible thing from that standpoint.

[lingeringstin...]

Looks like I'm wrong again according to this:

"AC-CDI - The AC-CDI module obtains its electricity source solely from the alternating current produced by the alternator. The AC-CDI system is the most basic CDI system which is widely used in small engines

DC-CDI - The DC-CDI module is powered by the battery, and therefore an additional DC/AC inverter circuit is included in the CDI module to raise the 12 V DC to 400-600 V DC, making the CDI module slightly larger. However, vehicles that use DC-CDI systems have more precise ignition timing and the engine can be started more easily when cold"


SO... it would seem that the AC one is simpler, which I like the idea of. However, the DC one apparently takes advantage of the immediate battery power to fire up which in my mind says easier starting and possibly better running with a healthy electrical system.

I'm still none the wiser really. Both seem to have their points.

[Teflon-Mike]

I'd stick with the Magneto CDi.
As said not tried K-Start on a Benly; But they will 'bump'.... usually... if motor not a complete dodo.
I am a tad sanguine about kicker an CDi as they do seem to like a good crank speed to get started; but I suspect retarded cam timing and low compression on an older motor is likely as much to blame for that as the Magneto system.
Bug-bears would tend to be in the starter, when brushes start to go; or the free-wheel clutch on the crank, which on older motors can get a bit sticky and whiney and not always engage/disengage.
Worth noting they seemed to drop the Kick-start on the Benly motors entirely about the time they went 12v and CDi; and that the duel-start CG had CDi ignition using the same mag and CDi.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[lingeringstin...]

Yeah, since the generic Chinese CDI's are so cheap I've ordered another of the AC ones so I can have a spare just in case. After researching them I went with the standard Honda clone type for a CA250 which is probably the same as most of the Chinky bikes use. The ones for the CG125 have a different connector plug but I bet they're the same thing inside.

Generic Chinese coils are cheap and cheerful too so I can always have a spare with me just in case.

As far as the starter goes I don't imagine this little 250 twin is hard to turn over in standard trim. And I suspect many of the starters are the same as well or at least the bits are easy to get should I need them. I could probably rebuild one at the roadside if I had the bearings and brushes. The starter clutch looks simple enough and everything seems really easy to access in a pinch. I doubt the starter chain gives much trouble.

The oil pump gear on this one is metal whereas all the replacement ones I've seen seem to have nylon gears. I'm not too bothered, I could always fit the old metal gear into a new oil pump but realistically I doubt there's much stress on it so a nylon gear will probably be OK.

Waiting for my valve tool to come in the post so I can take the head all apart. If I don't find anything glaringly obvious I'll probably fit new valve guides and use the old springs and valves.

The pistons are tiny! I've never had such a small four stroke engine on a bike. Back in the day all my courier hacks were like GT550 and CX's and stuff. It's a dinky engine but seems built well enough. Not a lot to go wrong.

The 300cc pistons are easy to get but I don't know if it would be worth the effort.

[Teflon-Mike]

Lol.. the pistons are tiny! Open a 125 twin! They are like a thimble!
When the Head-Gasket went on Snowie's Bluddi-Guzzi, (And I'd convinced her she didn't need to take the frame and transmission off to take the engine to bits!) The look on her face when she lifted the head and saw the size of slug in 750 V-Twin, was like she'd had a big black Chippendale drop his gegs in front of her!

Starter chain's are 'OK'.. pay heed to the adapter plate between the crank-case and mag case and the chain guide on it; it's only doweled into location and not clamped up until you bolt down the mag cover, and can be held 'off' by the tight fit around the starter knck.

Whilst the generator is 'inside out' with the stator and pick-ups in the cover; if it's not quite flat or lined up, you can have the starter chain ratling in the guides, but more pertinently, the ignition trigger may not be aligned with the trigger lug on the rotor, and in worst case, if the dowels are omitted or damaged, stator can sit skew in the rotor and possibly rub.

Head & Guides? Valves are tiny, & included angle awkward. I have had to use a small socket on the clamp to get enough 'reach' to get the compressor to bear just on the valve head, and the collet slot had to be thinned down a tad to stop it slipping off to one side or other. Magnet- wand is a real boon to get the collets out/in.

Valve-Guides I believe are non service replaceable; I have had a couple that have been cracked, from piston-valve contact; but I haven't yet had one that has failed the vacuum test and not bee held in with thumb over the seal end or not popped back when pulled against it.

More of a concern has been worn/pitted seats. Local M/C shop don't have cutters that small to re-cut or re-face the seats, and I haven't found one small enough on e-bay! Lots and lots of lapping has been required!

Watch the packing washers on the head end of the valve springs that stop springs eating into alloy of the head; they like to 'hide'! Easy to get a spring swapped and have one with two washers under it, and one with none!
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'