Classic cars forum & vehicle restoration.

[1935Hillman]

Back on a well-worn path I'm afraid, the causes of poor running.

My almost complete Aero Minx streamlined saloon as featured on another page on this forum is running badly. She starts well, runs quite well but with small blips and stutters but runs very badly once on the road.
I once again had the original and newly rebuilt Stromberg downdraft carb fitted and as I am not actually a fan of this carb and don't entirely trust it, I swapped it for a small SU downdraft just like the one I have successfully fitted to my Aero tourer but the problem persists.
I had a distributor bracket that was frankly too short and did not properly support the dizzy and was lucky enough to find a taller one on ebay recently that held the dizzy better just in case the problem was a poorly fitting dizzy. Made no difference at all.
I had previously had a problem with poor HT lead ends so remade them very carefullyand they are all very good. Made no difference.
Coil is new
Wiring harness is all new
Dizzy is a Distributor Doctor rebuild item
Fuel supply is good
Really boring now, almost at the end of another nice Sunday and the car still not running so can't go anywhere in her.
Idiot proof process of elimination would be greatly appreciated or anyone in Newbury area who may be able to help in a practical way.
Many thanks,
Tim

[Vulgalour]

Admittedly on much newer engines, when I've had blips and stutters at idle and a choked or stumbling engine under load I've found it to be one or more of the following:

Cracked/damaged distributor cap. Hairline cracks can be difficult to spot but as things warm up they can cause problems.

Fuel filter blocked or degrading. Debris in the fuel lines or carburettor causing a similar issue.

Loose spark plugs and/or leads. A weak thread in the head, loose plug, loose leads can all cause issues like you describe and be a nuisance to locate, especially if it's temperature sensitive as to when it shows up.

Failing coil. Had this once. Again a nuisance to discover because there was nothing obviously wrong. A new coil did fix it.

Failing condenser. Much like the coil, another irritating one to find. Usually manifested as a hot starting issue, but sometimes randomly while driving.

It's really easy to be snowblind when you're frustrated with a job like this so it can help if you've an inexperienced helper who you can give a checklist to. They'll then tell you what to check and you'll be able to check it without convincing yourself you already have. You'll feel a bit silly doing it this way, but it really can help and often when you find the problem it's something so daft you feel even sillier.

[Rick]

I've been caught out in the past with a fuel tank, or cap, not venting, causing fuel supply issues once moving. Vulgalour's list contains a few good possibilities also - if you run the engine in low light conditions, check for any random sparks under the bonnet that aren't usually visible in daylight perhaps.

RJ
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[1935Hillman]

I have now recruited the help of a more knowledgeable friend who is going to help me out. I had planned to swap out the condenser on the diz but have found it to be a soldered-in type which seems a bit of a pain to replace if stuck on the side of the road.
Any advice on that particular subject? Has anyone replaced a soldered-in type for a screw-in?
Ta very much

[Bitumen Boy]

[1935Hillman]

That's a great tip and something I will add. A crash course in slightly more advanced auto electrics would also be a handy addition to my mental kit. I can think the problems through at a very slow speed with a piece of paper and an internet connection but on the side of the road or at a show when she 'wont go' its often a different matter.

[alanb]

Make sure the distributor has a good earth to the engine by adding an additional wire between the base plate screw and the engine, I had a similar problem last year, I went to a local show the car ran perfectly spent the day at the show and won an award, then the car would not start, spent half an hour trying to start it with the help of several others including a retired mechanic, eventually after pushing the car it started but ran very rough coughing and spluttering all the way home, I tried everything, coil, condenser, points, everything seamed ok but I still had a weak spark at the plugs or nothing at all then by accident as I removed the the distributor cap clip I noticed a small spark as it touched the cylinder head, I added an earth wire and it worked perfectly, the problem all along was the distributor not making a good earth in the engine.
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old tourer


Morris 8 two seater

[kevin2306]

Had exactly the same scenario when i first put my Singer back on the road following its long layup.
I am quite methodical and chased the poor running through each element, swapping items out for replacements to see if any improvement occurred.
the symptoms suggested a fuel issue, the car had a prefitted (by previous owner) 1970s Lucas electronic ignition system.
managed to source a brand new, old stock Carb, rebuilt it with new orings etc, fitted and ..no difference.
so, after changing the plugs, leads etc we were no better off.
after one particular run out, i managed to limp home, put the car in garage, closed doors opened bonnet, switched lights out and observed all the little shorts emanating around the Lucas distributor!.
I have a show coming up the next weekend so looked to source a new old stock dizzy, as a temp solution, I bought an accuspark unit, for £59 delivered, fitted in a few mins and 2 years later is still fitted.
Result of such a cheap fix the running and starting has been transformed, I have the NOS dizzy sat on the shelf but cant bring myself to swap things over.

Kev

[1935Hillman]

Hours spent methodically going through everything under the bonnet with my expert sparky. Lots of intermittent readings and variable strength of spark on coil - dizzy lead, sometimes OK, other times sporadic. Noticed also coil was pretty warm after only a few minutes of work. Then we noticed that the acorn socket at the dizzy end was 'wet'. On dismantling so it was at the coil end revealing that the coil was leaking - possibly the answer. Swapped out for an old coil which helped significantly but not entirely solved the problem (we were also aware that we had fitted three different carbs and was likely that we also had a poorly tuned carb). New condenser arrived and was installed and she ran quite nicely in the shed on tickover. Acid test would be going up the road which she did pretty well, again not perfectly but significantly better and hopefully due to aforementioned carb swap. So, looks like both coil and condenser at fault. Added complication also that the red ignition light has packed up but this is something I've experienced before, they are pretty fragile and can suffer damage being pressed into their housings on the dash quite easily. New coil due any day so will fit and re-test once done. Seems like a lot of coincidental failures but others have told me of similar experiences so not unprecedented. My sparky is very good and the work is very very neat. I'll be back with more.

[kevin2306]

Dependent on where you've ordered the coil from, make sure it isnt a counterfeit example.
A few years back I bought a Bosch model from eblag and it was quite obviously a fake once it arrived, I went to a VW specialist after that and bought one off the shelf.
Good luck, you seem to be making good headway.

Kevin

[peter scott]

I've just recently had misfiring that I believe was caused by an in line interference suppressor. It has been trouble free for more than 20 years but appears to have now gone excessively high in value.

Worth checking if you also have one fitted.
https://youtu.be/_5lk0XtJMV4

Peter
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1939 SS Jaguar 2 1/2 litre saloon

[1935Hillman]