I have a 1962, 3.4 MkII. Every now and again, the front carb floods at start up. Fuel drips from the float chamber overflow pipe and also appears at the top of the starting carburettor air intake tubes. It usually clears itself when left standing for a short time. I have inspected and cleaned both float chambers and replaced the float valve assemblies. The float chambers appear spotless and both overflow pipes are clear. Both floats seem perfectly free to move. The fuel filter bowl is clean and I have fitted a near filter element. The fuel in the filter bowl looks clean. What can be happening?

 

Wow ! that is a strange issue. You've done all the normal checks it seems already. Issue is that the gas into the carb bowl doesn't shut off when it should, to state the obvious...I assume that you have checked and re-checked the float lever to make sure its at the proper bend ( something like 11 mm ?). Have you replaced the float lever hinge pin or otherwise determined that the lever moves freely ? Have you checked the alignment of the fiber washer that sits under the dran tube banjo , I seem to remember that this needs to be aligned correctly that it will allow "breathing".
Best of Luck !
Schmitty

 

normally my first thought would not be excessive fuel pressure...but it's very odd to be happening on all carbs, including starter carb.

 

I have no evidence that the problem is also affecting the back carb. Whenever it has happened when I have been in a position to see what is happening, it has been the front carb. The starting carb is involved because it shares the front carb float chamber - if that float valve doesn't shut off the fuel, the pump pressure will send the fuel up the starting carb air inlet tubes. I will recheck the alignment of the drain tube banjo fibre washer. Thanks guys. Ian

 

I have a "T" shirt with this written on. lol.
When I had this problem I was as perplexed as you. I had rebuilt my carbs myself and set all the floats to factory specs (don't ask what they were) but still I was getting fuel just as you bubbling up. My solution was to lower the float on the front carb a little bit more. If you look at a cut out diagram of the carbs the float in the front carb is below the level of the top of the valve in the mouth of the carb. So when I had set mine originally I could see that fuel was just spilling out of the hole that the needle valve sat in showing me the level was just to high. By lowering the float level in the front carb (not by much) so fuel was not showing in the hole it not only reduced fuel leakage but also stopped my engine running on when I switched it off.

 

Are you aware of the special washer that fits beneath the overflow pipe connection to the float chamber lid? This allows air to escape the chamber as the fuel enters. If not fitted, the float may not rise fully and fuel continues to enter. Many times, this washer is incorrectly replaced during maintenance with a regular washer or the special washer is refitted on top instead of beneath.
I'm not an expert on this but came across the topic on a forum.

 

I suggest checking the overflow/vent line for partial blockage.
This sort of flooding problem is very common in my area due to lots of mud wasps and other grubs getting into the overflow/vent line and not allowing the float chamber to breathe.
All of my SU carb vent lines are fitted with wire gauze at the bottom outlet.
By the way that special vent washer fits between the top of the float bowl and the vent line.

 

Thanks everyone. I have ordered the special fibre washers. Ian

 

Another tip is to use unsinkable floats, and newer needle valves with viton tips.
This will make the carbs more reliable.

 

last resort...maybe a fuel pressure regulator???

 

The serrated fibre washers did not work. I read in an old forum that a leaking seal on the ball-jointed on/off valve in the starting carburettor can cause fuel to be sucked through and appear near the top of the intake tubes. Has anyone knowledge of this? Ian

 

First off, Cass has already explained, but I'll try be more specific.
Look at the diagram below and note where the drawing puts the fuel level (red arrow).
When you set up the float level, the manual will have you set the float fork at 7/16, using round stock.
This puts the float level just below the jet bridge (green arrow points to the jet bridge)

I have found that setting the floats at 7/16th are too high, and this can allow fuel to percolate over the jet bridge and cause flooding, this gets worse on a hot day, or even when the car is simply at full operating temperature.

I want you to do something for me, remove both dash pots and carefully set them aside with their plungers.
This will leave the jet bridges visible on both carbs, looking down into each carburetor.
Now turn the ignition on, let the fuel pump fill both float bowls, and note where the fuel level is in relation to the tops of both jet bridges.
It should be at 1/16th just below the bridges.
If it's above that, or even nearly spilling out, then you have to reset the forks in both float bowls.

I set mine at 1/2 inch, it won't hurt for the fuel level to be a bit below spec, or even lower, maybe even what the diagram represents where the red arrow is pointing.
There's tremendous drop in air pressure right where the main jet is, even if the fuel level was at the red arrow mark, I don't believe it would effect the proper running of the car.

I'm betting that when you do my check, the fuel is nearly at the point of spilling over the jet bridge and trickling into the engine.

Another problem is the fuel tanks breather line is too small, and on hot day, the pressure built up in the tank can't be sufficiently released.
This in turn makes the flooding problem worse by adding air pressure on the pump and forcing un-wanted fuel into the carburetors.
My theory is that poor fuel venting in the tank raises the fuel pressure beyond the pumps 2 to 4 lbs. and the needle valves in the float chambers are easily over come with too much fuel pressure.
I have made my fuel tanks vent larger to deal with this problem, or what I believe to be a problem, it didn't hurt anything, and I don't have a fuel odor.

Do my jet bridge test first, and don't worry about the tank vent for now.

 

Thanks, JeffR1. Only just seen this. The fuel levels of both carbs have already been confirmed to be at the normal level of just below the bridge (1/8 to 1/16 in). The problem is very intermittent, usually only happening after the car is thoroughly warmed up. Once cooled down, all is well again.

Yesterday I spoke to a classic Jaguar engine specialist recommended by Burlen fuel systems. He said that this is typical of a Mk 2 running on modern fuel and is caused by the fuel boiling (i.e. vapourising) in the hot engine bay. Not sure what to make of this suggestion. Hard to believe that more owners don't experience it.

Ian

 

Do you have the isolators between Carb an inlet manifold?

 

When did you last fill up with fuel? Winter blend will have more lighter components. And some suppliers use as much light and heavy to maximise the amount of gasoline they can refine from a barrel of crude. Have fun using what's in the tank, refill with premium fuel (BP. Ultimate or similar) and see if it ameliorates the problem.

 

It has standard isolator spacers at the inlet manifold.

 

The problem has been with the car since I acquired it in April 2024. Probably 10+ tankfuls of fuel. Shell, BP, Esso, various supermarkets. Always use E5 (E0 not being available!). Makes no difference.