11 replies to this topic

[Jetpacktoby]

I’ve got an F reg 1989 mini 998cc. Just since the weekend it has been running roughly. Changed the dizzy cap and rotor arm and ht leads but it’s not running on cyl 3 and 4 correctly.
I’ve pulled the ht leads off while running and it doesn’t change its roughness when 3 or 4 is disconnected bit 1 and 2 makes it even worse so they are working ok.
It’s got electronic ignition of an unknown type as it was in the car when i bought it.
Correct me if I’m wrong but if the coil or ignition pack had gone wrong it would muck about on any and all cylinders so am a bit perplexed.
I took the rocker cover off and all valves seem to be working ok and did a compression test and all were about 170psi
Someone at work said the dizzy shaft could be out of wonk but why would this all of a sudden happen?

[Jetpacktoby]

Video of Engine running (sort of)

Attached Files

•  82a7d7a4-9a25-4469-bf95-1953c39f196f.mov   1.35MB   25 downloads

Edited by Jetpacktoby, 27 May 2026 - 09:16 PM.

[lsto]

Did you change the parts before or after the fault started? Might be something as silly as leads the wrong way round.
It's unusual to lose just 2 cylinders unless it's a head gasket fault but if your compression is even on all cylinders then you've pretty much rules that out.
Any air leaks? Loose manifold? There doesn't appear to be a video link unless it's just my phone messing about?

[Jetpacktoby]

No parts changed before the fault. All was running happily. The only thing on the Mot from a month ago was emissions were a bit high so they sorted that and retested. But I would have thought that if It’s carb related it would affect all cylinders or plugs but all are looking healthy

[Quinlan minor]

Check oil for water and check compression.

[68+86auto]

Can you post some pictures of the engine bay featuring the carb and distributor?

[timmy850]

I’m assuming when they “sorted” the emissions they’ve just wound the screw to make it run leaner at idle? If so it likely needs a proper tune from someone who knows how to tune an SU carb

[Jetpacktoby]

Yes I think just a lean off of the idle nut. But again that should surely affect all cylinders.
No water in the oil either. Compression good.
The spark plugs look fine as well. Not carboned up. It just sounds really rough and stalls as not enough power being created to keep it going.

[timmy850]

Does it run better if you leave the choke on? Have you tried adjusting the mixture to see if this was the cause?

If they have to adjust it to pass emissions to the extent that it becomes undrivable, then it sounds like it needs a proper tune

If you have an imbalance 1/2 to 3/4 it could be something like:
Poor manifold gasket seal
Leaky manifold connection to brake servo
Breather port on the carb adding more air to one side of the manifold

[Jetpacktoby]

Thanks I’ll have a look at leaky pipes and gasketry tonight.

[stuart bowes]

It is single carb, isn't it (i think so, from the video, just checking)

[Jetpacktoby]

I feel a right plonker. It was the vacuum hose to the brake servo that had popped out.
I’d fiddled with the washer bottle at the weekend and must have caught it.
Lesson learned. Retrace steps before exploding and fearing a hefty bill.
I never would have thought that a vacuum hose would cause so much of a problem
Thanks everyone for your help