Rough running

[Piston broke]

Hello all. It’s been a while..

Ursula the convertible came out of hibernation this weekend and we had some running issues.
She’s a 948cc Morris minor 1000. 1958. Only notable upgrades- servo assisted disc brakes.

in 2024 she had a new AUD13 SU HS2 carb fitted. She ran lovely. After a while, it took her a short time to warm up, as using the accelerator before this would lead to Rev drop and misfire. Once warmed- lovely runner.

In 2025, after several months not being used, she would not run well at all. Foot down on accelerator led to poor running and rev drop. It was a very busy year and I had no time to tinker.

Fast forward to 2026. She fired up well but had poor idling again and even when warm, she would splutter when the accelerator was depressed. I decided that the carb needed a tune as it sounded like she was running lean. Using the lifting pin to aid this, I finally got her to a point of being very stable ie, the revs stayed the same when the pin was pushed up a few mm.

The test run- she ran sweet! Zoomed along. Half a tank of gas. But then- a mile later- she conked out. The fuel pump tapping away like a one fingered typist on red bull. Once pushed back into a carpark, I tapped the fuel tank which echoed…. Empty. Fuel gauge issue- I’m blaming the resident mice on this…. Anyway…

About 30 minutes passed and my wife rocks up with two cans of unleaded from our barn.

Car fired up! With the choke out slightly, I pulled away and she spluttered coughed and faltered. Kangaroo! With lots of revs I managed to get her moving but she was misfiring and there were a few stalls. Then- I got to a hill. She was spluttering and hiccuping and ejecting white smoke. I pulled the choke out further and instantly had the full, incredible power of the 948 engine with no smoke. We then zipped home like a good ‘un. I checked the radiator when cold in case this was steam- but there was only slightly less water in it than when I’d set off- but I had just topped it up and they tend to lose a little anyway.

My investigations will continue but my questions are thus:

Is she just running lean and had cooled down necessitating more choke than I gave her?
Is the choke badly adjusted?
Is there an air leak from the servo?
Has she sucked up rust from the empty fuel tank (I have a filter which shows only a few tiny bits)?
Has the head gasket got a leak- and if so- why are the symptoms intermittent?!?
And- who trains mice to chew through cables??!!

If anyone has had similar issues, or a brainwave- I would certainly welcome your help!

Many thanks

Pete.

[mike1864]

First of all, was the petrol, both in the tank and the Jerry cans, fairly fresh?
I have found that six month old is just about OK. But 12+ months won't work at all. Is your in this range?
There's another recent thread that suggests putting fresh fuel straight in the float chamber, to eliminate this possibility.
If this doesn't make it run OK (albeit only for a few minutes!), await the next reply.

[svenedin]

Old fuel is a distinct possibility.

You say that in 2024 the engine ran well with a new SU HS2 carburettor but in 2025 after a period of no use of several months the problems started.

I think it sounds like the carburettor piston is sticking. You have been topping up the dashpot with oil? It is very easy to test if the carburettor is sticking. Push the lifting pin in all the way and quickly let go. The piston should return with a "soft metallic clunk". If it does not, there is your problem.

The white smoke is odd. How bad was this? It does sound like a head gasket issue. I would do a compression test. Why intermittent? Well you say you were going up a hill and so the engine was under greater load and presumably hotter as a result. Sometimes gaskets fail gradually rather than catastrophically.

Let us know. Possibly more than one problem going on at once.


Stephen

[Piston broke]

First of all, was the petrol, both in the tank and the Jerry cans, fairly fresh?
I have found that six month old is just about OK. But 12+ months won't work at all. Is your in this range?
There's another recent thread that suggests putting fresh fuel straight in the float chamber, to eliminate this possibility.
If this doesn't make it run OK (albeit only for a few minutes!), await the next reply.

Duff fuel is possible. I have filled her up with more - before the hill- but forgot to mention that bit of the saga. Maybe it worked its way through half way up the hill…

[Piston broke]

Old fuel is a distinct possibility.

You say that in 2024 the engine ran well with a new SU HS2 carburettor but in 2025 after a period of no use of several months the problems started.

I think it sounds like the carburettor piston is sticking. You have been topping up the dashpot with oil? It is very easy to test if the carburettor is sticking. Push the lifting pin in all the way and quickly let go. The piston should return with a "soft metallic clunk". If it does not, there is your problem.

The white smoke is odd. How bad was this? It does sound like a head gasket issue. I would do a compression test. Why intermittent? Well you say you were going up a hill and so the engine was under greater load and presumably hotter as a result. Sometimes gaskets fail gradually rather than catastrophically.

Let us know. Possibly more than one problem going on at once.


Stephen

Dashpot checked and a nice clink heard- but I will top it up to be sure.