MK2 Starting carburetor flooding

Hi fellow Jag-lovers,

I’m having trouble with the starting carburetor of my 3.4 mk2. The engine starts very good but when the engine gets warmer and the solenoid switches off the level of fuel in de starting carburetor rises quit quickly resulting in flooding petrol. It only drips out from the two outer pipes.

I’ve already checked the float chambers. The valves are working good and everything is set as it should be. Also when the engine is running, I can switch off the petrol pump so the level of fuel in the float chambers will drop but even then the fuel stays high in the starting carburetor. See pics attached.

Whats happening?

If the float floats, and it’s set at the correct height, and the needle valve on the float chamber is actually closing against the pressure from the pump, then it’s likely a clogged vent pipe.

But there’s a bunch of "IF"s chained together there.

Is the fuel actually flowing out of the starting carb, or is the level just high enough that it spills out when you go around a corner?

The fuel level in the auxiliary starting unit should be established by the fuel level in the float bowl. They’re intended to be one in the same (per the attached diagram). When the thermo switch de-activates, the valve (11) closes and vacuum is discontinued so the auxiliary feed of fuel/air stops. The fact that you begin to overflow once the draw of fuel/air from the auxiliary unit stops makes me suspect that the float, float height, needle or some combination of the three may be at fault. As Andrew suggested, these “if’s” should be thoroughly vetted.

Good luck!


I checked all te IF’s already a few times. I don’t think the float is the problem. Also because, when I manually turn off the Fuel pump, after a few minutes the engine stalls and the float bowl is empty. But at this moment the starting unit is still full (not overflowing). Is this because maybe a clogged vent pipe or because of the weight of the needle?

It’s dripping out of the starting device, but too much to be normal, im getting a petrol puddle quite fast

What fuel pressure does your pump develop?

What order of washers etc. do you have at the vent banjos?

Did this start out of the blue, or shortly after you fixed something?

@Andrew_Waugh, Maybe you are on the right track!

I bought the car in a not running condition. So after I overhauled the engine I noticed the fuel pump was stuck. I opened it and, as i can remember, bent the spring on the end (V) a little bit to get it running again. After that the pump is doing a good job, but maybe the fuel pressure is too high? Causing the starting carburetor to overflow?

What is the recommended pressure for SU Carbs?

Between 2.5 and 3.7 is fine. Absolute max 4 psi.

You can tell if the pump is pushing past the float chamber valve because it keeps on ticking (Ignition on, but engine not running) after it has filled the system.

If your pump doesn’t stop ticking after 20-30 seconds, absolute latest 60 (disconnect the dizzy wire so you don’t burn the ignition up), then either your pressure is too high (measureable) or your float valve isn’t sealing.

If it were the chamber vent then fuel would also pour out of the carb jet: If both carbs do that, then your pressure is too high, if it’s only one, then it’s either a float valve not sealing or the vent tube on that specific carb.

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I was thrown off the Channel Tunnel for the same thing!
To be honest the petrol was dripping out of the auxiliary carburettor
This is how I fixed it temporarily! Eventually I cleaned everything up and it stopped, never did really work out why it was leaking.

When I finally got on the tunnel train they came round with the sniffer device and the nice man said " perhaps next time you should take the ferry sir!"

Ingenious! When my engine was spitting up (lean rear HD8 due to stuck float valve) the hisser would neatly deposit a bolus of petrol on the wing (just outboard of the bonnet), which always seemed to catch fire. Older models had an anti-blowback valve but it’s absent on the Mark X setup.

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To add to the questions… I saw the same thing - the two outside tubes full of petrol, but they do not seem to be connected to anything?
I assumed that the petrol was flooding up the center tube, hitting the cover plate and filling the two tubes on the sides. As they are blind tubes the petrol stayed there until it evaporated (setting off the channel tunnel alarms …).
Am I correct in this assumption - if so what are the two outside tubes there for?

Is that Teflon tape or toilet paper? Would this not create a venting problem?
Could one use a cork plug for easy removal or do these come out easily?


I don’t know either… I cannot find anything in literature, and don’t know a reason why they use 3 tubes. Maybe to get the correct air/fuel mixture in the starting carburetor?

I’m not certain, but I think I remember a cross bore at the bottom of the 3 tubes. Below is a picture of the hisser body with the 3 nubs which I think were for bores to adjoin the different passages circled in black.

I remember that there was at least one nub in the hisser body which had been bored then plugged off - presumably that would be the one at the bottom left, but if the one at the upper left was also bored and plugged then it would join the two outer passages to the middle one.

It was Teflon tape, but was just a fix to stop the leak/back flow out of the intake and keep me moving.
I dismantled the carburetor and cleaned everything up and the leak stopped, so it is back to normal now. I did have one more incident like this, but it was very short term, so I think something is sticking when not used for a while.


Hey, I just started working on my Dad’s '53 xk120, hasn’t been running for about 10 years… It’s having this exact problem. This has starting carb on the rear. The fuel is only coming out of the Starting Carb. When I got the car, the Starting Carb was left off and it ran pretty good, but I noticed a bit of fuel sitting at the air intake on that one Carb. I put the Starting Carb on, and this started. The exact symptoms are that it only overflows while the ignition is on and the pump is pumping. It only pumps for about 5-10 seconds, but in that time it overflows, then once pumped up the pump stops, and the overflow also stops. I checked the float height and it is within specs at 1/2" exactly. It wasn’t clear to me where the vent is, perhaps that’s the problem. Thanks for any replies.

Dale, the vent is on top of the float chamber and has a line that runs down towards the ground. Blow through it.

David, Thank you. Mistake above, my starting carb is on the front carb. I did take the vent off and it was almost completely plugged so I cleaned it out well. I also noticed that on the starting carb the spring that goes on the plunger that is moved by the solenoid is missing so I’m trying to get a replacement and will get it back together. Thanks again.

Good news! Cleaning the vent resolve the overflow on the starting carburetor. Making progress. The jet on the front carb was stuck but cleaning out the entire chamber and jet got it moving again. slowly getting things tuned up. Thanks for the help.