Blowing Back through the carburettors only when cold!😖

Hello, a little mystery. When starting the 3.4 MK2 from cold it spits back through the carburettors until it has warmed up. The timing is right (ignition and valve) and the car runs like a dream when warm. Starting carburettor switches on and seems fine. Thoughts?

At idle or under load? If you are trying to accelerate even moderately on a cold xk engine with SUs, you will get some coughing. Mine does that. You might try manually switching the starter carb solenoid so that it can stay on longer if that is the case. The engine may be sputtering because it’s too lean when cold if the otter switch is turning off the solenoid too soon. I am very gentle with my engines until at least the halfway-to-warm mark on the temp gauge is reached. The oil in your crankcase probably isn’t fully hot until you’ve driven 10+ miles. If you are having the problem at idle and you are sure the hisser is working, I don’t know. Maybe lean for some other reason.

I know you say the ignition and timing are right, but that’s what was causing the exact same symptoms on my 3.4S. Foolishly, I had set the points gap with distributor on the bench, thinking I could do it more accurately, failing to account for a small amount of play in the bush, which closed up the gap when installed. Naturally I discounted ignition as cause for a long time. Once set correctly, car started and performed flawlessly.

Is it…
a) only while the hisser is on
b) until the engine has warmed up and not related to the ASC or
c) only after the hisser turns off?

Hello Kevin,

Thank you for the tip re the points. I’ll recheck them tomorrow. I did happen upon another thread of yours about the white MK2 which you bought for $500 after a few drinks - LOL! You said that the float had a hole in it which was a possible cause of the blow-back. I suspect that the carburettors might need a little attention. I have rebuild kits for them but not had the time nor inclination to do it with Covid. I might have a go at them soon. Given that it only happens when it’s cold, then runs beautifully when it’s up to temperature makes me suspect the carburettors. I don’t know very much about the Starting Carburettor but it might have something fouling its operation and perhaps is consequently running lean? It does make the hissing sound so I know the solenoid is working and then the Otter Switch turns it off. Perhaps it’s not allowing enough fuel in when first started? It didn’t happen before the ‘lay-up’ of over 12 months while Covid was raging.

I would first analyze if it is connected to a lean mixture and if so, why. The only way I can see the ASC running lean is when the brass needle was stuck which is easy to test. That or it gets no fuel but in both cases it would probably have a harder time starting.

Hi Ron,

I too and very gentle with my engines while going through a start up. Even after it has reached temperature I drive very conservatively for the first 10 miles or so. Even after that I prefer to ease her along rather than push her. As I said to Kevin, I don’t know a lot about the Starting Carburettor but I suspect it might need stripping down, cleaning and having compressed air passed through its passages. I guess the same goes for the carburettors? Just writing these responses out and hearing what has been kindly offered, I guess it might simply be that its too lean? What fun! :grin:


Thank you David. Does the needle pop up when the Starting Carburettor is activated or just under manifold vacuum?


The Burlen website has a good write up on how it all works. As I understand it, the air passage is opened (by the solenoid) and then the needle is sucked down. How far it is sucked down depends on the vacuum the engine produces. A higher vacuum means less fuel.

The device can only be adjusted when the main carburetters are correctly set. Hence my a-b-c question; if it runs well when hot and you don’t want to enrich it… and as it says in some manuals, convince yourself that the ignition is working well before touching the fuel side, especially if it can give those symptoms.

Thank you David.

I’ve got the Service Manual out now.


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Which is why I ditched the points and replaced them with a Pertronix electronic ignition 19 years ago.

I agree with the others, check the points gap in situ

In addition, the ASC can easily be adjusted to run leaner or richer via the brass nut, and I would try this (that would be my number one suspect)

check for any vacuum leak

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Thanks, Tony.

I’m leaning in that direction (just noticed the terrible pun :crazy_face:)

Points gap perfect.


Hi Timothy
I would try just enrichening the ASC by winding the brass nut anti-clockwise
This will make the engine run richer when the solenoid is activated.
I would be cautious of revving the engine when the mixture is lean as it has been known for the fuel to spit back and catch fire to the bonnet underseal and sometimes you can see burn marks on the underside of a bonnet. (Not Yours!) Did you know that when the car has started on ASC you can avoid excessive smoke by quickly turning the key on and off and the ASC solenoid will disconnect? and then come back on when you press the accelerator gently

Hello Brian,

Thank you for the suggestion which I will try tomorrow. That being said, I still think that it might be the float levels or faulty needles and seats (or both). I need time to pull bits off, so here’s hoping. I think that I might change the oil in the dashpots and put in some genuine SU oil. If the oil is too old, low or not the correct viscosity that might cause it to run more lean.

I did know that but the the solenoid cuts out with the otter switch in a timely fashion, so that’s not an issue. My MKV doesn’t shut off until the car reaches 50C. The changes in cars over ten years is quite astounding. The MKV is like driving a lorry while the MK2 is like a modern car, until you drive a modern car! :smiley: They are all great driving experiences.

Best wishes,


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Does it stutter when you press the throttle?

You still haven’t answered my a-b-c questions, those would be helpful. If it is mostly at idle and mostly while the asc is still on enriching that might be all you need. Can’t hurt.
I see warmed up as thermostat open and fully warmed up as oil pressure at idle is as low as it will go.

It is at idle and equally when I press the throttle gently until it is warmed up with the ASC ‘hissing’ and also for a while after the ASC has cut out.

a) No
b) No
c) No


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If it still does it when you have the ASC off, that suggests the ASC is switching off too early (should have a manual switch fitted imo)…or the carbs are tuned too lean

I never liked the ASC/carb switch over period, it is a compromise that can never be correct, as the engine is not fully warm when the ASC is switched off

I fixed this issue with full manual choked HS8 carbs and the situation was entirely eliminated. I acknowledge this is not really an option for most other owners

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