Faltering on start up

Hi All

I have an issue with the running of the car when starting from cold.
It normally starts well with the AED (manually operated), and if I set off driving straight away I have no issues.
However, if I leave the car at tick over for a while, the engine will not rev, it falters and “chugs”. Not sure of a better word.
It does not seem to matter if the AED (hisser) is on or off, the engine will not accelerate it oscillates, sometimes back fires, and then all of a sudden it will run normally. The time it takes to run normally seems to be getting longer, so I am trying to sort it out.
I have a video which shows the stages, but I am not sure how to attach here or link - is there a recommended way? (Edit I have tried via OneDrive, link below)

  1. Normal tick over
  2. Chugging phase where car will not accelerate
  3. Then it is back to normal

Any ideas of what this could be?



I wonder if the AED is over fuelling and the plugs are fouling at idle.

Are you getting black smoke from the exhaust when it’s happening?

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Hi Andrew

I wondered about the AED, but if I turn it off, it does the same, and no black smoke.
The car has always produced a little black liquid on start-up, but no smoke.
I might have to have a look at the plugs when it does it next.


The AED is a cunning little device, once its engaged the engine vacuum hold the internal operating disc firmly on its seat to allow the enrichment. its sometimes necessary to "blip " the throttle to get the AED to engage/dis-engage.
Having said that you engine behaviour does indeed look to be only associated with the AED so it should be reasonably easy to sort out. You description sounds very much like its just very rich whilst the AED is on. how many threads are showing on the AED needle?

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So do you have an AED (used after Mk2) or an ASC (used in Mk2)?
Earlier ASC (auxiliary starting carburettor) takes fuel from main carb bowls and is controlled by thermo switch in thermostat housing.
Later AED (auxiliary enrichment device) has its own fuel line and is controlled by inlet vacuum.
Burlen has good techinical documents for both at:

(ASC is under Thermo Carburettors)
Just clarifying to make sure people give you correct instructions.

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Hi Kopik and Steve

You are quite right, I have a standard MK2 ASC or thermo or hisser, my bad and sorry for any confusion created.
The reason I have questioned whether this is the cause is that when I turn it off, and it stops hissing (am I correct to think it has stopped working when it stops hissing?) the behaviour remains the same.

I will pop out and count the number of threads, but it is quite recently rebuilt with a new spring and needle and adjusted as per the SU manual linked above. I think there are quite a few threads showing so maybe I will try and lean it up a bit.




If at temperature, does the exhaust go a bit black, engine still going, and is the needle pulled down to the stop? Then it can’t be far off, not enough to behave like this.

If hissing or not hissing doesn’t change the behavior, but it does make the engine rich, I‘d want to loon everywhere else first. The ASC is so very simple.

As a thought, Make sure your carb dome pistons are full of oil (20w “three in one” brand works well) and that your vacuum advance is connected and that the advance mechanism is working within the distributor. I also get this faltering at times which goes away when warm. It’s annoying and hard to pinpoint.

One can not download videos to the forums. I use YouTube for all of my information. Label them as “private” and then you can share the URL with the forums and forum members can easily view them.

Very important that we all keep accurate videos and photos of any work performed for future reference. I also have a manually operated choke on my MK2. I use the Cigar lighter as my switch. Once I get rolling and am in third gear, I shut it off to avoid washing the plugs in fuel. It will stutter a bit until fully warm, so I avoid any major acceleration until its at the 70C mark. 100 is actually not too hot for these cars and will keep the plugs looking grey and performance good.

I do get a lean stumble and hesitation just when the ASC shuts off on a cold day. It’s gone after a minute.
I can’t tune my engine much leaner without having that stumble across the band. If his was so lean how does it drive at all?

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Is that like a pop and backfire through the carbs; a bucking sensation.Similar to heat soak after restarting the car. I get that if I press to hard on the throttle when cold driving down the road. I don’t give it the beans until I’m almost at 70C on the gauge.

Pretty normal I believe. I give it a few minutes until I drive like other people, at least 10 minutes until I drive it normally.

Maybe his ASC valve is sticking in the solenoid? Time for cleaning?

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Start with some spray carb cleaner and see what that does. Today’s fuel does tend to gum up small orifices. BG-44K is also a magical fuel system cleaner, not snake oil by any means. Not sure about Seafoam?

You don’t just let it idle for 10 minutes do you? The idle circuit on SU’s is pretty rich leading to fouled spark plugs.


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Hi All

Thanks for comments, and a few answers…

This normally happens if has been idling for too long, not 10 minutes, more like 2. If I drive off straight away - no issues, which seams to suggest something coking up? If I turn the ASC off at this time, the engine will stall, so is before it is warm. I don’t want to give it too many beans before it is warm, as this is not a new engine, and I don’t think has ever been rebuilt.

When this behaviour, chugging/faltering, starts, I cannot seem to get out of it - cannot drive the car as it will not accelerate for any length of time, revs just go up and down. The car has an automatic gearbox, so will barely move. Turning ASC on and off does not make any difference (though tends to stall if on zero throttle when cold). I can turn the engine on and off as well, no problem, but will not run much above tickover.

It does seem like there is a pop and a backfire just as this clears, however its not clear to me if that was through the carbs or the ASC. There was a bit of a petrol spill from ASC, but to me looked like backfire (not on video linked) was through the carbs air intake.

I did check the dashpots were full of oil (they were a bit low, but now topped up), and I also checked the vacuum advance pipe - I am running a 123 ignition.

I think I will have a look at the ASC to check for gunk and any dirt in there. The fuel system was all rebuilt recently with ethanol resistant parts, and the petrol is fresh, but worth a check. Might even try a squirt of carb cleaner !!

Regards and thanks again

OK question: Once the engine is up to temp, does it run OK or do you still have issues?

Hi Pat

Once the engine is up to temp it runs just fine.
I just don’t get how to get past this chugging/faltering issue when it starts.


So have you adjusted the ASC per the manual or do you stineed to do it? It may start just as well if it’s tuned a little leaner.

OK. This tells me everything is fine with the engine and it is properly tuned. The issue, therefore, must lie with the ASC. I suspect that, since the engine is acting like it’s cold and not receiving a rich-enough mixture. You say that you can hear the ‘hisser’ which tells us the solenoid is working and the valve is not stuck (or you wouldn’t have heard it). My guess is the ASC is either not getting fuel or is set too lean. Make sure the fuel passage from the carburetor is open/not restricted. I believe you said you saw gas spurting at the ASC. Next, remove the mixture adjusting needle and run a fine wire into the jet to make sure it’s not clogged. Re-insert the needle and adjust per spec. Hope this helps.

Either that (too lean, starts okay but runs rough immediately) or

Too rich, starts well cold, then runs worse heating up, then the plugs foul, and nothing helps

Rhe asc is too lean and the main carburetters are too rich. That makes the asc useless, but the car starts, and gets rougher and dies as there is more and more excess fuel.

Other suggestions?

Maybe a float is sinking or a valve/jet becoming sticky.

Are you doing anything in the video or is that all engine? Is the wiring to the ASC sound?

Hi All

Lots of comments and thanks for all of them.
In the video :

  • First 2 seconds are tickover, no throttle.
  • Then at 2 seconds I press the accelerator to a fixed position - previously I was stuck in this mode for about 10 minutes! This is chugging behaviour or faltering I am refering to.
  • Then at about 6 seconds, after the little backfire/hickup, it seems to clear and the engine runs normally

I adjusted the ASC as per the manual - the workshop manual that is, which has very scant instructions. I had another go today after leaning in the ASC off a turn or so, and all seems to work OK. Unfortunately the behaviour did not repeat, so I am not sure if I have fixed it.

I don’t thing the normal carbs are rich, I have checked spark-plug colour, and also used a colour tune. I had it set up professionally previously, but as this was done with the air cleaner off I think it was too rich and leaned it up a quarter turn. The car will not run when cold without the ASC.

I am hoping that the ASC was too rich, and this will help. Now I will wait until tomorrow, when I am out for a run with the Somerset JEC to see if it behaves or plays up again.