Puzzling fail to start

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

4 1/2 litre.
After about 70 miles great driving yesterday, the car sputtered and died. Would not start, either immediately or after letting it cool for a couple of hours (it was not running hot, just normal hot). It has done this once before, and after a few hours started. But not this time.

Compression 90 on all cylinders.
Fuel flow excellent to the carbs, fuel bowls full, drips from the carb when choke opened and left open.
Magnetos both have normal point gap, both timed to 45 degrees before TDC, perfectly synchronized. Spark appears good visually.

Fuel directly into the cylinders also made no difference. Fuel is fresh.

Valve timing to the flywheel appears normal too.

I'm stumped.

John Murch · Posted: Tue Apr 05, 2022 12:16 pm Post subject:

I would suspect magnetos, often a plug 'sparks' fine when out of the cylinder but once subject to compression fails to do so.
What starter motor have you got? if its not turning the engine fast enough and the mags are failing........

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

John - agree that under compression it might be different story altogether. But both magnetos? Possible, sure. I intend to use an induction analyzer today to measure spark strength. I would have thought though that when cold it might start more easily if it is a weak under compression issue? I also intend to remagnetize them today and see if that helps

John Murch · Posted: Tue Apr 05, 2022 1:36 pm Post subject:

one mag might have been struggling at low rpm for some time, and the other may just have followed a year or so later.
Are they ML mags?
During my WO ownership (48 years now, where did that time go.). I have had 7 mag failures, 1 broken crankshaft, one shaft from the cone to gearbox come adrift and one carb needle drop out! Of the mag failures only once did both fail (rebuilt by the same person) but fortunately I had a spare with me and changed it at the roadside, doing the next 1500 miles on one mag. In well over 100,000 miles only 3 failures that required a tow home. (And I should have spotted the carb needle,)

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Well, the car started this morning. Not really happy about that as I dont have any idea of that this issue is and have no confidence in driving it anywhere. Oner mag does have more trouble at idle but seems ok at speed. The other idles just fine.

John Robins · Posted: Wed Apr 06, 2022 7:14 am Post subject:

Insulation breaking down when really hot, self repairs when thoroughly cooled? Sounds daft but just the sort of thing the varnish insulation might do.

Cheers
John

Stephen Blakey · Posted: Sun Apr 10, 2022 8:31 pm Post subject:

Isn't there a condenser in the rotating part of a magneto?
If those have been replaced with modern components, they may not have the working life of the originals. That seems to be received wisdom with condensers for coil ignition systems (usually expressed in more pejorative terms).
It's quite likely that a condenser in the early stages of failure might function whilst cold, but then play up when hot.