Mystery misfire

[geoberni]

Anticlockwise Berni, 1,3,4,2

Stephen

Dohhh, trying not to confuse myself I did exactly that. I should have just stuck with calling them positions A, B, C & D

[svenedin]

Well done. That is almost certainly your fault all along.

Thank you Stephen and 59Elva, apologies, I didn't see your comments when I made my last post as my browser didn't reload the page properly.

I think you're right that the rotating shaft must have worn through the insulation over time, hence the problem appearing seemingly out of nowhere. Hopefully this means that I now 'just' need to look at the timing or the mixture to get things running correctly.

In fact - I think I remember when this might have happened. I had to leave the car away from home a month or so ago and I took the rotor arm out as an extra precaution. Presumably that's when I dislodged the wires

Seb

A valiant attempt at fixing the lead but damage done unfortunately and Accusaprk unit appears to be fried.

Stephen

Let is know the results with a new unit. Fingers crossed.

Stephen

[Napoleon Boot]

Hello all,

Just back from playing a gig up in Edinburgh, and returned home today to find my replacement parts waiting for me.

I'm delighted to report that after fitting the replacement Powerspark module, the car started first time and the misfiring and so on had completely disappeared. Such a nice change to be able to conclude a problem with a definite cause and solution!

The only niggle was that the idle was now much too high, with revs up to 2000 when creeping along at jogging pace. I brought it down a bit with one turn anti clockwise on the idle screw, but it was still running very fast, so I checked the fast idle screw. Although I thought I'd adjusted it to 15 thou with the choke knob fully returned, now it appeared to be screwed in a long way, and it took a lot of turning to get it back to having a small gap. I then found that returning the choke knob once the engine was warm led to the revs dropping and the engine stalling.

I've left things as they are for tonight as it's been a long weekend and I'm tired, but a huge relief to have got to the bottom of this. I will look over the carb again and try to get it as it should be - as soon as I've found a screwdriver the right size and shape to get to the fast idle screw, which is really hard to access. It's a shame it's not hex headed so that I could use a socket.

Seb

[svenedin]

Good news! I suspect you would have replacement the module anyway at some point but it is good that you know why the module failed and so it shouldn't happen again. Not by wires being chafed at any rate.

Stephen

[Napoleon Boot]

One last addendum to this thread and another quick question. I got out yesterday and reset the carb according to the SU manual, and to my surprise I got everything working well very quickly! A session with a timing light will follow at some point but for the time being, I have a happy sounding engine and a lot more power.

However! The oil pressure light didn't go out, and on investigation, all the oil has dribbled out through the forward tappet chest cover (which I had to remove in order to get the manifold off). Before I refill it with 20W50 (which seems to have increased hugely in price since last time), I'll replace the seals which have been on the car longer than I've had it, and may be past it.

The engine is a late, HC 1098 engine and the tappet chest cover has the cylindrical 'can' oil catcher. I believe this is the concave version? If that's correct, I think I need a neoprene seal for the inside flange, and a rubber seal held in a cup washer for the outside, is that correct? I've looked through the various parts diagrams but with all the different variants, and the fact that ESM don't always use the BMC part numbers, I'm unsure of whether I'm getting the correct items?

Screenshot 2026-09-13 at 11.08.32.png (216.71 KiB) Viewed 258 times

I believe the fibre washer may only be for earlier engines?