Joined: Mar 2019 Posts: 39 Threads: 9
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Location: QLD Australia
Car type: Austin 7 1936 tourer
Started up the old lass (1936 tourer) so as to check a possible gearbox problem. Usually starts on the first or second swing of the crank handle but then I had to use the starter and the engine backfired and would not accelerate smoothly and lacked power so straight back into the garage. A bit strange since it was only a day or two earlier I did a 14 mile run with no problems. Took the plugs out and found some hard deposits which were interfering with the spark gap and cleaned them and checked the gaps. When taking the plugs out Nos 1 and 4 were hot to the touch but Nos 2 and 3 were only warm. I have been running the engine in since the last overhaul (260 miles ago) and I thought that perhaps the carbon deposits were extensive and causing pre-ignition so removed the head and the manifold. The deposits were not particularly bad and were fairly soft but I cleaned them off. Valve seatings looked good and clean. Checked valve clearance and found that several valves had slightly more than the proper clearance by a few thou. Everything looked good so reassembled the head etc. Started the engine and, as before, it did not start easily and backfired through the carby again. Did not sound quite right and did not rev nicely when the accelerator was pressed. Shut it down after a few minutes and again noticed that that plugs 1 and 4 were hot but 2 and 3 only warm. Having taken the plugs out and laid them aside, the chances of the same plug going back into the same hole is pretty remote so I think the plugs are ok. Ignition timing checks out, points gap is ok. Wot, i ask, could be wrong.
Joined: Jun 2019 Posts: 17 Threads: 2
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Location: NZ
Car type: 1933 Austin 10/4
I have an Austin 10. Same problem after a rebuild.
Would start but ran rough with little rpm increase hand blipping the throttle.
Discovered 3 and 4 head area was warm, 1 and 2 cool.
Took 1 plug lead off, no charge to rpm. Same for plug lead 2.
Good spark to head bolt from both 1 and 2 HT leads..... mmmm.
Since the plugs were recent I discounted them as a problem and fitted a second Dizzy I had.
No change. Then thought about it a bit more and decided to pull the plugs.
Checking resistance of centre electrode to the plug outer body on my digital multimeter, 1 and 2 showed about 60 k ohms, 3 and 4 showed infinity. Put in two new plugs and all is good.
They were new NGK AB-6.......
Check you plugs Peter.
Cheers
Gary
Antipodes
Joined: Aug 2017 Posts: 629 Threads: 19
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Location: Sheffield South Yorks
Car type: 1932 RN saloon
Just had same problem with my RN. Had fairly new NGK plugs (say 500 mls). Engine been in and out of garage for me to alter garage wall when last week quite rough and no power. One plug was not functioning. Replaced with brand new NGK and now OK. Is there a quality control problem with NGK. I think they were purchased from Green Spark plug co.
Joined: Mar 2019 Posts: 39 Threads: 9
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Location: QLD Australia
Car type: Austin 7 1936 tourer
Thank you I shall check the plugs. The other thing that has occurred to me is to check the advance/retard mechanism. Could be a combination, but I think the plugs are most likely.
Joined: Jun 2019 Posts: 17 Threads: 2
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Location: NZ
Car type: 1933 Austin 10/4
In my case all the NGK’s were purchased from a reputable supplier in Auckland. Must be a quality control issue. Testing plugs with a multimeter, digital or moving coil type is only good at revealing plugs with low resistance.
Some plugs may still be leaky but not revealed by a multimeter as the range of Ohms does not go high enough and the voltage the meter uses to provide a resistance measure is quite low, think 9 volt small battery.
Very high resistance, insulator grade like a spark plug, is properly measured with special equipment like the Megga we used in my Airforce days. Not sure what is used these days.
It was hand cranked to generate a high voltage but low current source.
Gary
Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
A butt joint inside a plug could show infinite resitance but be of no consequence to the spark. Pugs which incorporate a radio resistor read quite high ohms. Not always satisfactory on simple coil ign systems.
Incidentally very wide gaps as moderns only necessary with weak mixtures, this undesirable on Sevens as need all power and to minimise valve recession. A normal mixture is fired by a very small gap as the magneto cars. The wWsp engin as DC3 ran .012 inch.
When running my Seven everyday and very familiar with its characteristics, I convinced myself that wider gaps had the same effect as slight spark advance.
Joined: Aug 2017 Posts: 622 Threads: 19
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Location: Hampshire UK
Hello Peter
If you had reported a general misfire and hard starting I would have advised checking the condenser (capacitor). When this is on its way out the available HT drops, and you get a misfire especially under load.
However, a problem only on 2 cylinders suggests something specific to those cylinders. This could be plugs, plug leads, distributor cap or lack of compression. The difficulty with a misfire from any cause is that it tends to soot up even an initially good plug, and eventually the conductive path resistance drops so low that the ignition system can't reliably produce a spark. In my experience this happens when it gets below about 100 kilohms. A sort of vicious cycle of misfire, plug fouling and worse misfire.
For this reason it sometimes helps to have a spare set of clean plugs, so that you can swap them all out before faultfinding further.
Having said all this, don't discount the possibility that the condenser HAS gone faulty, and this is just highlighting the fact that two plugs just happen to be more sooted than the other two.
Joined: Mar 2019 Posts: 39 Threads: 9
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Location: QLD Australia
Car type: Austin 7 1936 tourer
Thanks for all the advice and comments. I always thought a spark plug was just a spark plug so I have learned a great deal from your replies. Anyway I managed to buy a set of NGK AB-6 plugs which appear to be Japanese in origin. Having installed them and checked and cleaned battery terminals the old girl started on the crank handle without the choke.
Cheers,
PeterA
