Puzzling failure to proceed

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

Bentley 4.5 litre, 1928.

I've had this happen twice in the past 5 years or so. The car starts and runs very well. After about 10 miles or so, while driving, the car splutters and dies.

Attempts to restart fail - the starter and flywheel spin normally, but the car does not fire. Float bowls in both carbs full and fuel can be felt down in the jets. It feels very much like an electrical issue - so I disconnected the grounding wires of both magnetos (in case they were grounding through a fault) but still it would not start. The magnetos both usually work well and the car will run well on either. The car was not hot.

About 2 hours later, the car starts and runs normally.

any ideas/suggestions?

Iain Warner · Posted: Tue Jun 15, 2021 10:38 am Post subject:

It seems very odd that both mags would pack up at the same time. If when you restart it runs well on either magneto I would have thought the problem is fuel delivery related.
Did the misfiring last for long before the engine died or was it a cough or two then stop?
Could the carb jets have some debris in them to restrict the flow?
Iain

Stephen Blakey · Posted: Tue Jun 15, 2021 2:59 pm Post subject:

Dan,

Thank goodness you weren't on a "smart" motorway at the time, or you wouldn't be writing about this experience.

I'm with you on the fuel/sparks thing. It seems to me that the fault is electrical. Electricity is not to be trusted, of course, as you can neither see it nor hit it with a hammer to fix it. Probably just a passing fad.

It is unlikely that both mags would fail simultaneously. What if they fail sequentially? Say one routinely packs up ten minutes into every journey? By way of diagnostics, you could temporarily fit a couple of these;

Would it reduce your breakdown anxiety if you were to to find another mag and convert it to coil ignition so you've got one of each, as it were, like the eight litres? There must have been a reason W.O. came to that arrangement.

John Murch · Posted: Wed Jun 16, 2021 4:13 am Post subject:

Years ago I had a lot of trouble with mags, both failing. They had previously both been rebuilt by a very reputable firm. Subsequent investigation revealed that the wire/insulation used in rewinding them was fractionally thicker than original resulting in slightly reduced clearance at certain points. When hot they would short, but after cooling start working again (but not for long!).

John Murch · Posted: Wed Jun 16, 2021 11:53 am Post subject:

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

Stephen - I've been driving old cars long enough not to have "breakdown anxiety". It's just part of the charm of these cars, and I expect it.

I would never convert or alter from original. At that point, I might as well put in electronic ignition. I buy the car I want because of what it is, not what I can make it.

Robert Craven · Posted: Wed Jun 16, 2021 3:22 pm Post subject:

I note you say your engine was not hot so this may be inapplicable, but my 3 Litre once proved very difficult to start again after a short stop on a hot day. At first neither mag was producing a spark but luckily the engine eventually started.
On stripping them the garage found the mags very oily and decided the oil seals were failing. Replacing them and cleaning the mags cured the problem.
Maybe more oil would leak into the mags when the engine was switched off but the oil was hot than when the engine and the mags were rotating. Or maybe not. Anyway, the cure worked.