Intermittent Misfire (distributor rotor)

[svenedin]

Yesterday whilst out on a drive my car started to misfire. I thought it was going to cut out but the engine recovered and drove normally afterwards. It has had a few tantrums like this and I have also noticed that whilst not what I would call difficult to start, it has been more reluctant.

Anyway, I set about diagnosing the issue and found the distributor rotor with a burnt appearance and also with the plastic with chips out of it. It looks like it has got too hot. I changed the rotor for a Distributor Doctor red rotor and the engine fired up eagerly. I shall test drive later.

I find it slightly ironic as I am always going on about getting the best ignition parts. However, I thought this particular rotor was NOS Lucas and it has been on the car almost exactly a year. To look at it I wouldn't had thought it was necessarily faulty but it appears that it is.

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[myoldjalopy]

Well, I suppose anything can fail at any time, and some NOS is very old now - the rotor material may have gone brittle. But if you think it got 'burnt' from being 'too hot', then the question is why did it get too hot? But I'm thinking it was just too old and brittle and started breaking up a bit.

[svenedin]

Well it was not the rotor........

After a short while, the car started to misbehave very, very badly. Misfiring, smoking a lot, running incredibly rough. So rough that there is no way it could be driven.

I spent all afternoon trying to trace the fault. Spark=yes to all cylinders (I have those neon testers which are handy). Petrol=yes. Pump pumping happily into a container with the flexible pipe removed from carb. I then looked at the PCV valve diaphragm and it seemed fine.

A long list of totally useless and waste of time manoeuvres and increasingly frustrated. Removed float lid: no debris, fuel in chamber. Carb piston sticking=no, damper oil topped up. Air leaks? Could not find any. Still ran extremely badly. Changed HT leads, distributor cap, coil. No change.

Did a compression test: 170 psi all cylinders.

Last thing and ready to give up..........I checked the Smiths PCV diaphragm again against a light. Sure enough it was split. Changed it for a new one and the car runs absolutely fine!!!!

PCV valves are rare in Minors. I think only 1969 cars. I did know about this because my car has one but I did not check carefully enough.

Stephen

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[myoldjalopy]

Well done! An annoying series of checks but at least you solved it in the end. Happy motoring!

[svenedin]

Well done! An annoying series of checks but at least you solved it in the end. Happy motoring!

Thank you. I really was quite worried that something catastrophic had happened to the engine. I could not work it out and because I had checked the PCV diaphragm once I did not see why I would look again. I won't be caught out next time! I have ordered 2x new diaphragms and I will definitely be putting them in my tool bag for Minors on Tour to Holland May 2024. If that had happened overseas I might have ended up being towed to goodness knows where......

Incidentally, the distributor rotor WAS also at fault. I tried the old one back and the car didn't run very well after I had fixed the main problem which was the PCV diaphragm. Two faults, one of which I think occurred after I had fixed the first one.

Now I've missed the sunny day today and tomorrow it's tipping down with rain. A shame but it's fixed now.

Back to why did the distributor rotor got too hot. I do not know but arcing excessively I suppose.

Stephen

[Bill_qaz]

Is the dizzy cap tightly located, if moving on clips it changes rotor to cap terminal clearance. Or excessive side play in dizzy shaft.

[svenedin]

Is the dizzy cap tightly located, if moving on clips it changes rotor to cap terminal clearance. Or excessive side play in dizzy shaft.

I think it's Ok. Dizzy cap looks fine inside, no arcing marks. Distributor was professionally rebuilt less than a year ago. Just a bad rotor I think.

Stephen

[firedrake1942]

Where did you get the PCV diaphragm please. Mine in my 1957 car may well need changing after 40 odd years.

[svenedin]

Where did you get the PCV diaphragm please. Mine in my 1957 car may well need changing after 40 odd years.

The part number for the diaphragm for the later Smiths PCV valve is 27H7758. I ordered from a company called Rimmer Brothers. However, the earlier Smiths PCV valve has the diaphragm fused to the top plate and those parts are no longer available. You would need to check if your valve has the replaceable diaphragm.

Stephen

[firedrake1942]

Thanks. Mine was fitted by BL in Coventry to a 948 cc gold seal in 1979 so likely a later Valve.

[svenedin]

Took the car out for a 20 mile drive today and all was well. On an earlier test drive there was still a slight misfire at times. This was due to the spark plugs becoming fouled when the PCV valve diaphragm failed. When the diaphragm failed there was an odd combination of the fuel mixture being too weak (air being drawn in through the split it the diaphragm) AND a load of oil ending up in the bores (crankcase ventilation not working). With that split in the diaphragm the car would only run with choke which makes sense because the fuel mixture was being made too weak. It ran very badly though. The strange thing was that if the choke was pulled out more than about half an inch the revs rose enormously high presumably the air leak being a bit like having the throttle open.

Finally, the car has its MoT renewed (with one day to go). I was taking it for the MoT when this fault occurred! Of course it doesn't strictly need an MoT but since I do almost all of the work on the car I do like an inspection of safety to check my work. The garage were so sympathetic to my tale of woe and liked the car so much that they did the MoT for nothing! There are some very nice people around. Car passed with zero advisories.

I even managed to drop the hood down and enjoy some (chilly) sunshine. I am having an operation tomorrow and will not be able to drive for a while so it was good to have a final blast.

Stephen