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Stalling Sceptre
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peter scott

Joined: 18 Dec 2007
Posts: 6175
Location: Edinburgh

PostPosted: Tue Dec 22, 2009 11:06 am    Post subject: Stalling Sceptre Reply with quote

I was chatting to a friend who has a very nice collection of Rootes cars the other day and he related the story of his Humber Sceptre that mysteriously stalled whenever the engine was hot.

The problem would certainly have had me scratching my head for a while so I thought you might be interested to read the article that Hugh wrote for "Old Faithful" the Humber club magazine.


"I’m sure you probably recall previous articles about the saga of my stalling Sceptre, which involved me in several embarrassing incidents, most notably the occasion when the Humber stalled at the toll booths on the Forth Road Bridge in the rush hour and had to be towed ignominiously in front of waiting queues of commuters. Once the engine got hot, if it stalled there was no chance of it restarting until it cooled down.

Just recently, my wife and I decided to go for a short run on a fine spring day to cross the new Forth Bridge at Kincardine. As we stopped at the lights in said village, the engine faded away and wouldn’t restart. Eventually we pushed it into a parking space and walked around for nearly an hour before the Sceptre would restart. My wife declared that she wouldn’t go in that “Old Wreck” again until I’d got it sorted!

Not that I haven’t tried previously on many occasions over the last 10 years, including replacing all the ignition components and leads as well as upgrading to a Flame Thrower coil specified to give 40kV. I thought I might have a slightly overheating engine, so I fitted a Kenlowe fan. Other enthusiasts have said it is fuel vapourisation, so I re-routed the fuel pipe to be further away from the hot manifolds.

None of these ideas solved the problem, though I did find substituting a cold ignition coil giving a slightly higher output would sometimes restart the engine after it stalled. All this pointed to an ignition problem, and when the engine is hot and it stalls, the spark at the plug is quite feeble. I also made many measurements on the low voltage side to ensure I've got maximum voltage at the coil and a good earth via the circuit breaker and minimum voltage drop when operating the starter motor.

So I've focussed again on the ignition and made up a spark-gap tester - two nails in a wooden slide arrangement so I can safely adjust the spark gap and measure it. The idea is to slide the spark gap apart until the spark starts to become intermittent. With this, I've been able to get relative measurements of the HT voltage.

When the engine is cold, and being cranked by the starter motor, I got a spark length of 12mm from the coil lead and about 10mm at a spark plug lead. Once running, the spark at the plug lead went up to 12 to 13 mm. Then as everything warmed up, I occasionally checked the spark on No.1 plug lead and it gradually dropped to about 4mm just before it stalled. By now, the coil was pretty hot, however the spark at the coil lead was still about 11mm when cranking the engine.

I was obviously losing a lot of HT somewhere in the distributor. I had replaced the rotor arm some years ago with a new Lucas-branded item, but it seemed the most likely problem. I bought a new rotor arm from a local factor, this time made by Commercial Ignition of Nuneaton. I put this in the distributor and the engine started immediately and there was a large spark of about 12mm at the plug lead. I thought I'd fixed it, but half an hour later the same thing happened again with a weak spark at the plug lead. Swapping back to the original rotor arm, the engine started immediately with a large spark restored. I did try holding the HT lead from the coil close to the contact on the rotor arm and got a slight discharge, indicating some leakage in the rotor arm. There is a very high voltage-gradient across the insulation.

I was pretty well stumped until, searching the internet, I came across a specialist firm called Distributor Doctor ( The proprietor, Martin Jay, can rebuild/test classic Lucas distributors and also supplies improved rotor arms. This extract from his website seemed to explain my problem:

“Recently manufactured rotor arms have been failing because the typical "mix" used in the injection moulding nowadays contains more carbon blacking and is therefore more conductive. Still more importantly, the rivet which holds the brass inlay into the moulding is slightly longer than the original, bringing it too close to the spring clip on the underside. The high tension current, averaging 30,000 volts, is always looking for the easiest route to earth and shorts out from the tip of the overlength rivet, through the reduced thickness of more conductive plastic and the spring clip on the underside of the rotor arm, to earth out down the distributor shaft. Result - no sparks at the plugs. The situation sometimes rectifies itself on cooling, but then reoccurs with increasing frequency until the rotor permanently short circuits.”

I ordered one of his recently introduced Red Rotor Arms.

According to the website, these are modelled on the original, dependable 1960’s design, but made from modern materials on the latest machinery to very close tolerances. The integrally moulded brass inlay dispenses with the troublesome rivet used on most versions which is known to short circuit. The increased insulation properties of the body makes them suitable for use with high voltage coils and electronic ignition kits.

Fitting the Red Rotor and with everything cold, the spark at the plug lead was now even bigger, about 15mm, presumably as there is less leakage in the rotor arm. I took the Sceptre for a good run, and when I got back with everything hot including the coil, I still got 12 to 14mm at the plug lead. The engine would restart immediately. As my wife said, we haven't seen that for years. Brilliant! She might even go out in that “Old Wreck” again.

The frustrating thing is that I've spent so much time on this problem, assuming that by replacing the rotor arm I had eliminated it as a possible problem. It’s very confusing. It was only when I started doing the spark measurements that I realised where the problem was located, but I was still stumped by it as I replaced the rotor arm without fixing the fault. The Red Rotor Arm seems to have solved the problem.

Another interesting twist in this story is that I, like many others, have substituted high-energy coils or electronic ignition systems to improve reliability, but the higher HT voltage has ironically made rotor breakdown even more likely as a result.

I wonder how many other people, afflicted by stalling engines and roadside breakdowns, are on a “wild goose chase” because of faulty rotor arms?"

1939 SS Jaguar 2½ litre saloon

Last edited by peter scott on Thu Jul 06, 2017 7:23 pm; edited 1 time in total
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Phil - Nottingham

Joined: 01 Jan 2008
Posts: 1254
Location: Nottingham

PostPosted: Tue Dec 22, 2009 4:22 pm    Post subject: Reply with quote

The poor quality of most so called new Lucas rota arms has been raised a number of times on here and other forums. The Distributor Doctor has received many endorsements as to qulaity of parts and service too.

Been proved now
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Site Admin

Joined: 27 Apr 2005
Posts: 21007
Location: North-west UK

PostPosted: Tue Dec 22, 2009 5:25 pm    Post subject: Reply with quote

Interesting, I'd heard of duff modern repro rotor arms, but didn't know why they were failing.

thanks, R
Rick (OCC Admin)
Various 1920s-1960s - Austin, Morris, Commer, Dodge etc.
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Joined: 18 Oct 2008
Posts: 1157
Location: 100 miles from Melbourne, Victoria, Australia

PostPosted: Wed Dec 23, 2009 12:11 pm    Post subject: Reply with quote

Back in May this year, I returned from a monthly trip to Melbourne (200 mile round trip) and parked the 1982 Rover SE, a fuel injected V8, at the back gate to unload it. When I tried to start it to put it away, it just refused to go.

After some fiddling, I isolated the problem to distributor area, and finally replaced the dissy cap and rotor with known good second hand ones, and she fired and ran well. I further experimented and decided that the rotor was the culprit.

In Oz the firm of Repco is an old and respected supplier of parts, and I get on well with the people at the local branch, so I asked them to order in a pair of new rotors- one for running, one for a spare. They got in two in Bosch packages.

I duly fitted one of the new units and voila! all went well, however on the next Melbourne trip, we started to have interesting problems. It was as if I had turned the ignition off for a second or two, then back on again. The engine would seem to die, then with a resounding explosion from the exhaust would pull away again. We struggled home leaving a string of alarmed motorists and bystanders in our wake.

Next day I took the Bosch ones back to Repco, and they ordered Champion brand ones. The Bosch ones had no markings on them, but the Champion ones were branded 'Made in Italy'. Fitted a Champion and perfect running.

We left for a trip to Queensland just after that, and took the spare Champion rotor as well as the good second-hand one just in case. There are four posts about that trip starting here-

Nearly 5,000 K's and no more problems on the trip, and now many more K's later, still no further problems.
Dane- roverdriver but not a Viking.
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Joined: 18 Oct 2009
Posts: 20

PostPosted: Sun Jan 17, 2010 10:15 am    Post subject: Reply with quote

I have stumbled on this post quite by chance and I'm so glad I did.

My B made a visit to London recently and stalled on Hampstead High Road, in the one-way system at Wandsworth and in the middle lane at the traffic lights in Streatham Vale. On each occasion it would restart only after resting for at least ten minutes.

Having replaced both the fuel pump and the distributor I was wondering why the problem still occurs from time to time with a hot engine. I was thinking of investing in an expensive insulated manifold believing that vapour-lock might be the cause. I was also considering replacing the coil and fitting breakerless ignition.

I will now concentrate all my efforts on the rotor arm!
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PostPosted: Sun Jan 17, 2010 8:15 pm    Post subject: Reply with quote

Further to above, I e-mailed Distributor Doctor this morning to obtain a red rotor-arm and received a reply within minutes. On a Sunday too, how's that for customer service?
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PostPosted: Wed Jan 20, 2010 4:20 pm    Post subject: Reply with quote

New Rotor-arm arrived by first class post the very next day, how's that for service? and all for less than £9.

As you can see from these links, it's much more workmanlike than the old black one.
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