'Slipping' timing on 57 Huntmaster

[TimOC]

Help please. Bit new to the old bike game and definitely new to magneto ignition (and 4 stroke, twin cylinders and Huntmasters) My last 'old' bike was a D7 Bantam 40 years ago. Also fairly new to posting here, tried a couple of days ago but it's not appeared, so hopefully I'm not duplicating this post.

Problem started a month ago.

I'm now something of an expert on resetting timing. I do it with the marked stick down the left hand barrel and a MiniMag magneto timing aid to tell me when the points open. I move the cylinder forward (so any slack in system is taken up) and remember to set timing when fully advanced and check it after tightening the magneto up again. Bike starts first kick very happily, warms up and ticks over OK (still some work to do on carburation I think.)

Increasingly rapidly the timing just goes. Bike starts to stutter and misfire after a few km. I check timing and find it's way out. Set it again, starts perfectly, run it and it loses timing. Last time I tried I made it 200 metres before bike started misfiring, backfired and stopped. Won't start now.

I haven't checked the timing again yet (I put the bike away and took two days to calm down) but I will expect to find it's gone again when I do.

So... is there something classic I could be failing to do, or doing wrong? Or is there a common fault/dodgy part/etc. that I should be checking?

Couple of things I've noticed but don't know if they're normal.

When I slack off the Auto Advance retaining nut the auto advance system operates smoothly, returning to non advanced position under it's own spring power easily. When I tighten it back up it seems to operate less smoothly and not always return to non advanced position straight away. Is this normal? The tighter I tighten, the stiffer the mechanism gets.

When tightened up I can move the whole auto advance mechanism side to side and up and down. Seems to centre itself OK when I release it, but I don't know if that much play is normal or not.

Any suggestions or insights welcome!

[simon.holyfield]

What does the taper on the magneto shaft look like? I'd put some marking blue on the taper, and look at the fit to the female taper in the advance mechanism.

[Steven.Carter]

Where about are you?
If you weren't too far away from me you'd be welcome to borrow my Auto Advance to see if that the cause of your problems.

Steve

[TimOC]

Thanks Simes... bikes in France at the moment, will see if they'll let me bring my engineers blue back in my hand baggage when I nip back to UK for a couple of days next week.

Steve, that's a great offer. As I've just said bike's currently at our home in France. Our home in UK is just outside Banbury. Plan is to put the bike on a trailer and bring it back to UK in October and see if I can get time (and resources) over the winter to turn it into a reliable runner.

We pass Eastleigh on our regular commute to France via Portsmouth and Brittany Ferries!

Would be good just to see a working system in position on a Huntmaster just to give me a reference point. Never having seen or heard a properly running bike yet means it's hard to judge just how many problems I've got to solve!

[Steven.Carter]

Your very welcome to drop buy Tim, just drop me a message nearer the time.

Steve

[TimOC]

Bit more info now...

I've checked the timing again, retarded itself significantly. Firing a while after TDC. That's checked with auto advance jammed in advanced position. So the fact it's sticking isn't affecting that measurement. If it's sticking in use (not tried dismantling the clutch side to put on a disk and try a strobe on it so can't be sure it is) the only affect would be to leave ignition advanced at lower revs. My problems are at all revs, so there's something else going on.

Have removed the magneto from the bike. First thing that strikes me is that the taper end can be moved 2.5mm side to side. Is that a lot of play on the bearing there, or normal? Seems to affect what's happening at the points end fairly significantly.

Thanks for the help so far.

[Dave.Barkshire]

I have had a damaged taper which was cured with some very light fine grinding paste.

Also, I remember someone having a very mysterious problem which sounds a bit like yours. The center bolt was loose so the points plate was mobile.

[johnwalsh]

Hi Tim
If I understand you correctly, it sounds like the bearing is very worn. Like any bearing, there shouldn't be any discernable play.

[TimOC]

Thanks John.

That's very useful information, confirms what I feared. Also near 2mm of end float.

It's sparking well, don't think there's any problem with the windings or condenser. Points need changing, I can certainly do that. I've no idea whether changing bearings is a potential home job but I recognise it's probably worth saving up and getting the professionals to test the whole thing and decide what needs sorting?

I've seen recommendations for people who sort magnetos elsewhere on the website... any updates?

Meanwhile I've stripped and cleaned points, advance unit, etc. Reset timing and got the bike running again. That will do for the next couple of weeks I hope (no long trips) and then I can bring the magneto back to the UK for servicing.

[pete.collings]

Another cause is breakup of the insulating washer that locates the bearing outer ring, if it has completely disappeared it would give around 2mm float