battery drain

[RDfella]

The Everly bros used to sing 'problems, problems, problems all day long'.
Was humming that this afternoon when I ran the project to test the smaller starting jets I'd fitted. Bike ran OK for five mins then rough as hell before stopping. Suspected ignition, so checked voltage at coils - 3v. Battery is 6V 4AH and the coils draw around 8A. In theory that's 1/4hr to half voltage except, as electric car owners will discover, as a battery gets used / recharged, it slowly deteriorates and you never regain the full quoted figures. Hence why it stopped after around 5 mins. I've now added another battery in parallel to give me a bit longer. At present it is total loss (no charging) because I'm designing an electric starter for it and that'll include a 12v alternator and battery.
However, bearing in mind charging output varies with revs, even that would struggle to maintain a starter battery plus two ignition coils. I suspect I'll be going back to magneto if the starter motor plan works out.

[Bsareg]

You could try reducing the dwell angle. I think you said its V twin so the ign cam only needs to allow the points to close long  enough to saturate the coils.

[RDfella]

Agreed, Bsareg. Cam I'm currently using is half of a standard Triumph cam. I probably could improve on that with, as you suggest, less dwell angle but really can't be ar**d to start making another special cam. Problem I hadn't anticipated is the fact I've now got two coils instead of one.

[chaterlea25]

Hi RD
Even so that sounds like a lot of current going to the coils?
When you go to 12v those small Japanese style coils take a lot less current than the Lucas style canister types..
Those 4Ah batteries would lose voltage very quickly if asked for 8 amps??
You will need a decent battery for your electric starter, which should run coil ignition without issue
The starter kit I just bought for my A10 came with a 14AH 220 CCA AGM battery. The 60w dynamo is enough to keep this charged by all accounts so your alternator should be fine (Have a look at startyourbsa.co.uk)

The other day a friend rang me to say his Vincent had stopped charging, some investigation revealed the chain had sawn its way through the leads from the Alton alternator!!. the ammeter had stopped working some time ago and had been bridged out, it was when he fitted a new one it did not show charge.
This Vin has an electric starter and coil ignition, using a twin lead coil rather than a distributor
The only battery that will spin the Vincent starter properly is the Odyssey PC680. He thinks he has been running total loss from the battery for quite a while and did not get into trouble due to the batteries capacity
he connects a battery tender when at home so again missed the problem

John

[RDfella]

What I appreciate about this forum is the fact others'input often highlights issues one has overlooked.
Despite working with old-fashioned ignition systems (points / coils) for many decades I never gave current draw a thought. It was only this present issue that made me investigate and the first thing I found was that most coils (6v or 12v) draw 4A. I was surprised it was that high but apparently that's optimum as above that points will burn and below the spark becomes weak. That's 8A for two coils, though obviously only when on 'dwell'.
Thinking about this last night, I concluded a more appropriate cam (say 45* dwell) could cut the current draw down to under 2A averaged as only one coil would be earthed at a time and then only for a short period. At present I'd guess dwell is nearer 200*.
My aversion to making another cam was because the last one was a real fiddle - not only did I need to accurately machine cam lobes at 150 / 210 but concurrently ensure the rotor arm was close enough to the distributor cap's HT pickups spaced at 180*.
Of course, this time there is no rotor arm or pickups to worry about, and only one cam lobe to machine (or more accurately one recess in an almost full-circle lobe). As the 150 / 210 cam is only a light press fit on an original cam body I should be able to get it off and press on something more appropriate. Should only take a couple of hours to make a new, simpler cam so it's definitely worth doing.
Re John's comment re current draw of modern, small coils - I added one alongside my 'standard' coil when going to a twin points set-up. Have measured their resistance and they're both around 1.7 Ohms.

[RDfella]

Well, a successful day today. Took a couple of hours to remove the old redundant 60* cam, machine up a single lobed one with a 50* dwell, fit it to the base and then fit it to the bike. Only very minor issue was that hardening the new cam made it 'grow' a tenth or two (always a possibility) meaning that the light interference fit was now a sliding fit. But Mr Loctite kindly lent a hand, so all's well. Hopefully now the battery will last a lot longer!