Mystery misfire

[Napoleon Boot]

Apologies for length - I wanted to provide all the information I could as briefly as I could - there's just a lot!

You know how everything can be going smoothly, and then several things go wrong at once?

A couple of weeks ago I was about 20 mins into my 40 min drive home, when I noticed a slight misfire under load (driving up a hill around 28 mph). It seemed to get worse, but by anticipating the traffic lights, avoiding braking, and trying to keep cruising at a steady pace, we got home.

The next day I had a look to see if I could see any damage to the exhaust. Covering the tailpipe revealed a slight blowing above the rear axle. I couldn't feel any leaks in the engine bay and the exhaust clamp was tight, but as the manifold gasket looked a bit charred, I thought I should investigate. Sure enough, the gasket was blown in a couple of places and once I'd scraped off the old gasket, I could see that the faces of the exhaust ports were pitted, there was some soot around the exhaust end as well:

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I can't get the old manifold skimmed in a hurry, so I ordered a replacement from ESM, which I cleaned up, painted, and installed with a new gasket and nuts, all torqued up correctly with new gaskets on the carb as well.

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Having installed everything, I was confident that Peggy would be back to normal, but starting up, the idle was very lumpy - almost like a cylinder was missing - and when I tried to drive, a touch on the accelerator just led to misfiring and the odd backfire, the engine beat just broke up. It was all I could do to limp back to my parking spot.

My first theory was that if the carburettor had been adjusted to cope with a blowing manifold, presumably it needed to be adjusted back again. I checked all the linkages, the dashpot, piston, needle, dashpot oil etc, which had been disturbed when I replaced the manifold. The car was worse than ever - now she would barely move.

I decided that the only thing to do was to work logically through everything, returning to factory baselines, and eventually, surely, she would run again. Even if it was a bit lumpy, at least I could get to a garage who could set her up.

Over the past few days I've spent over 12 hours working methodically through the following:

Ignition:

HT Leads (tried two sets - checked for continuity with a multimeter)
Distributor cap (again, checked with a multimeter, I have two and I used the one with the least resistance between the centre contact and king lead)
Rotor arm - tried three known good, red ones from Powerspark, cleaned up and resistance tested with a multimeter
Spark plugs (clean, properly gapped, tried several sets)
Coil - I checked the low tension connectors for voltage, voltage drop, cleaned up/replaced any tired looking ones. It's a Viper Sports dry coil from Powerspark. I tested for spark from the king lead, I also tested for spark from each lead in turn. All good. I added neon spark plug testers between the plugs and the leads and span the engine - all firing in the correct 1,3,4,2 order.

Compression:

I was worried that the head gasket might have gone, but checked with a compression tester and all four cylinders are around 170 psi.

Today I moved on to the carburettor. First I slackened off the throttle cable and choke cable, backed off the idle and fast idle screws and made sure the levers were in the home position. I removed the air cleaner, dashpot, piston. I checked the butterfly valve, looks normal, not loose. I checked the needle - straight, properly inserted with the shoulder level with the piston body.

Following the SU manual and the videos by Blind Man Morris on YouTube, I got the jet level with the body of the carb and checked it with a rule.

Reassembled. Checked the piston moves smoothly up and down. Checked it rises with the lifting pin. Refilled with oil. Rechecked the action. All good. Next, I turned the mixture adjustment nut two complete turns downwards.
I set the idle screw to nearly touch the lever, then gave it one and a half turns clockwise.

I checked the choke cable is raising and lowering the jet and it was working as it should. I set the fast idle screw so it had 15 thou clearance with the choke knob pushed fully in.

I started the car, with the choke knob out about 1/4", and a light push on the accelerator pedal, and she ran, but as soon as I lifted my foot, the revs dropped and she died. I tried starting again with the choke out further and no throttle, and she wouldn't start at all.

I tried giving the idle screw a further 1/4 turn clockwise, the engine would start and run with the pedal to the floor, but died as soon as I lifted my foot.

At this stage, as the engine was running and sounding more normal, and the carb was set up in accordance with the manual, I thought that the timing might be slightly out. The distributor, for reasons unknown to me, is installed 90 degrees out, so number one is closest to the bulkhead, and the carb can hardly be twisted any further anticlockwise as the vacuum advance fouls the oil pressure sensor. So I twisted it a couple of degrees clockwise, probably only half an inch. This time the engine would start, without choke or throttle. It would only maintain it for a few seconds, but it would run unaided, backing up my theory that the timing was too retarded.

I tried turning it a few more mm clockwise, but she would then only start with full throttle, and die as soon as the pedal was released.

I returned it to the previous position but by now the plugs were wet from all the attempts and I couldn't start.

I'd previously checked the vacuum advance hose has no leaks and is fitted properly, and it worked when I sucked on it (urgh).

I found TDC with the starting handle, and made a note of the position of the rotor arm (pointing to one) and made a mark on the top of the bottom pulley (There is no notch on my pulley, and it's impossible to see the timing marks in any case)

At this point I was exhausted. All these tests seemed to show that the timing is out, but I can't accurately measure it because the engine won't run, and I can't see the timing marks and there is no notch on the pulley. Moving the distributor, there's a sweet spot where the engine will actually run, in a fashion, but it's impossible to get a decent idle.

My theory: having tested and replaced everything in the ignition side and reset the carburettor side, the only thing I've not changed is the electronic ignition module - an Accuspark kit for Lucas 25d distributors. It's six years old now. I've checked for spark, and there is a good spark, I've checked for fuel and air. I think that the timing of the spark is erratic, but there's no way of checking it with a test light like there is with points. I don't have a set of points but at this stage I'm wary of introducing yet another variable!

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I would be hugely grateful if anyone can share their thoughts or their own experiences on this. I hate leaving her parked up immobile, and I've tried everything I can think of to get her running.

Thanks in advance,
Seb

[mobylette]

I would definitely swap out the electronic ignition module for points and see how things are. I can't see how the timing could have moved unless the distributor clamp was loose.

[Napoleon Boot]

Thank you - yes, points are cheap enough that it's worth buying some so that I can use a test light to see exactly when the points are closing. I have a strong suspicion that the electronic unit may be doing something strange.

Seb

[svenedin]

Excellent post with lots of detail.

I think, due to poor routing of the electrical connection to the electronic ignition module that the rotor may have clipped the wire and damaged it. See your picture which I have arrowed. Remember that the whole plate that the module is mounted on (or the points are mounted on) rotates with the distributor advance mechanism. Sufficient slack must be allowed in the wiring to allow for the advance but the rotor must not chop into the wire. You can check this by moving the plate with your fingers. I would suggest examining the wires very carefully for damage.

Another possibility is a bad earth from the distributor baseplate to the distributor body. There should be an earth wire connecting the baseplate to the distributor body. Two-wire in distributor modules rely on an earth via the base of the module to distributor baseplate via link wire from base plate to distributor body. The module only has two-wires, one is the live feed to the module and the other is the feed from module to coil The module must have a good earth for it to have a proper power supply.

Regarding the distributor position, this is due to the distributor drive dog being fitted in the wrong position. If it was all working well before, then the timing is not suddenly going to alter so I would leave this alone for now. Once everything is working as it should, this can be put right.

Stephen

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This is a Lucas 25D4 fitted with points but it shows the earth wire which must be intact.

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[simmitc]

Have a look at viewtopic.php?t=49906&hilit=average+misfire

[svenedin]

There are a few other things to check. Did you use new gaskets either side of your carb spacer block? You may still have an induction air leak as it sounded like in the first place before you changed your manifold and manifold gasket.

You have a breather system where the oil separator is connected to the carburettor by a hose. Any crack in the hose will lead to air being drawn in causing a weak fuel mixture. Some times a crack can only be seen by carefully examining the full length of the hose and bending the hose back to stretch it slightly. A leaking tappet chest gasket on that oil separator plate will also cause an induction leak that could be quite severe. There should be either a cup washer and o-ring or just a fibre washer but whichever type there must be a seal on the tappet cover securing bolt as well. Lastly the hose to the servo might be cracked. If it were me, I would do a test by blanking off the breather inlet to the carb (disconnect the breather hose and block off the breather inlet on the carb with insulating tape) and the same at the spacer plate (disconnecting the hose from spacer plate inlet and block by wrapping with masking tape). Then start the engine and see what happens. It is a simple thing to do and would not take long.

My own experience of induction leaks is a failed PCV valve diaphragm. As the vacuum is strong, a tiny split in the PCV valve diaphragm, which was difficult to find, caused severe rough running and misfiring. So much so that the car could not be driven. PCV valves are uncommon on Minors but it illustrates that a small split or hole can cause very rough running.

Stephen

[Napoleon Boot]

I think, due to poor routing of the electrical connection to the electronic ignition module that the rotor may have clipped the wire and damaged it.

Wow, that's really eagle eyed of you Stephen, thank you! I have to admit that I didn't notice that at all, but looking at the image now I can see what definitely looks like a damaged wire, which would explain an intermittent fault between the distributor and coil. When I first fitted the electronic ignition, I remember thinking that it looked a tight fit, and I was careful to route the wires carefully before replacing the cap. It's quite likely that the wires have been inadvertently disturbed at some point, and ended up in harm's way.

Have a look at viewtopic.php?t=49906&hilit=average+misfire

Thank you for the link to an interesting thread Simmitc, it made me feel better reading about the lengths you went to diagnosing your misfire! Unfortunately my issue happens with the air cleaner on and with it off, and the filter is clean. I tried two different sorts of spark plug, both of which I've used without issue in the past - Accuspark AC9C and NGK BP6ES, so it's not the spark plug issue another poster had.

I'm now crossing my fingers that it is the damaged wire to blame - I have ordered a set of traditional points, as well as a replacement Powerspark electronic ignition (the old one worked faultlessly for years), so I should be covered one way or another.

There are a few other things to check. Did you use new gaskets either side of your carb spacer block?

Yep, I used new gaskets either side of the spacer block, and the bracket that the throttle cable goes through, three in all. I thoroughly cleaned all the faces before remounting the carb, thought they were pretty clean to start with. The carb appears to be squarely mounted and I tightened the nuts snugly.

Any crack in the hose will lead to air being drawn in causing a weak fuel mixture. Some times a crack can only be seen by carefully examining the full length of the hose and bending the hose back to stretch it slightly.

That's a good point. I checked all the connections were tight before doing anything else, and I inspected all the hoses for splits/leaks, but I suppose it's worth another look as I can't see them in their entirely without taking them off.

A leaking tappet chest gasket on that oil separator plate will also cause an induction leak that could be quite severe.

Yes, I had to remove that tappet chest cover in order to get the manifold off, due to the oil catcher can. The gasket looked good, and I made sure it was nice and tight once I put it back on. There was a rubber o ring on the inside of the bolt hole which I believe should still be sealing it properly. There's no sign of any oil leaking but I could have another look at it as well. The problem started before I touched that cover though so I'm not convinced it's the primary issue, if it is letting in any air.

Thank you all for reading my long post and for your invaluable responses, this forum is absolutely essential and without it I could never have enjoyed Minor motoring for the last seven years!

Seb

[svenedin]

Hello Seb,

It’s quite a common fault with in distributor electronic modules but if you hadn’t posted the picture it would not have occurred to me. If you are good at soldering that wire could be spliced back together and sealed with heat shrink insulation but it will make that section of wire stiff and it must be able to move with the advance. Have a look at the wire with your own eyes as I had a slight suspicion that what I thought was damage could be a photographic artefact given the obligatory low resolution of the posted image.

Stephen

[Napoleon Boot]

Thank you Stephen,

Yes, I've been squinting at the original photo on my phone trying to work out whether it's a tear or a reflection! I'll have a look in the daylight and let you know. At least it's a possible lead (pardon the pun) as I was really at a dead end until you spotted that.

Best wishes,
Seb