Hi All. In Australia a number of people have been having intermittent flooding of the SU Carburettor out the overflow on startup after a long period of being left idle. What happens is the car starts then fuel leaks out of the carburettors and the engine the floods. If your leave the car for a few hours to dry out you can then start the car without issue and it will be fine. If you use the car frequently you do not get this problem - so the short answer is to use the car, but not always possible.
The Carburettors on my '66 3.4 S-type have been rebuilt several times, a fuel regulator added, in-line filters on both tanks etc. The fuel causing the problem is Shell V-power and other 98 Octane (RON) fuels. It is happening across multiple marques with SU carburettors and also causing problems with motorbikes.
It is almost as if the modern additives for high octane fuel designed for fuel injected cars are precipitating out and blocking the needle and seat valve. Then the carburettor floods and cleans out the needle and seat.
Keep in mind there are different ratings for octane. Here in my area (Oregon USA) there are three octanes commonly available 87, 89, and 91. All with 10% ethanol. I find my 9;1 compression ratio 3.8S runs satisfactorily on 89 but it gets a treat once in a while with 91.
Hi John. Was aware the fuel rating system was different between Australia and USA - have no idea how to compare or reconcile, however! We only have one grade with ethanol - 93/94 in our system. Also 91, 95 and 98 have no ethanol, as I understand it. Happy to be corrected by locals! Paul
There are multiple float bowl seat needle seatings designs in use in SU carbs. There are specific SU choices for combinations of fuel pump pressure in the line and the float chamber specification.
Incorrect needle for a combination may lead to:
1.) engine flooding or running too rich,
2.) fuel overflow out the overflow tubes
3.) fuel pump clicks correctly and comes to stop prior to starting, then engine at idle speed gradually runs rich and floods due to needle jitter on seat not quite closing as needed.
The Carbs have been serviced by a period jaguar specialist so I am pretty sure that the right needles and seats were used. Most of the time the car performs flawlessly, its only after a period of a month or so that I am having problems.
My only other thought was to upgrade to Grose jets. Any thoughts about this.
Also keep in mind there is a upgrade for E10 fuel.
The cork seals won’t last long using these fuels.
There is a new Part for the spimdle seals WZX966 SPINDLE SEAL KIT HD CARBS
I’ve no comments to add on pondering switching to Grose.
My experiences have been that once fuel pump pressure and float bowls bits are proper, a common cause of overflow problems is grit in float bowl (needle seat, float shaft rough, silt in bowl). Modern fuels clean well, it is easy to imagine solvents in fuel cleaning the old tank and line with more grit ready to come down the line after a storage period. Additionally, I also remember these problems happening in the 1960s and '70s on cars infrequently driven, before widespread ethanol adoption.
One step I’ve found helpful is to install a modern fuel filter in the line before the fuel pump.
I am quite sure you will find all the SU Carburettors you mention are suffering from incorrectly adjusted float arm tangs or, are still fitted with float arms with no tangs.
These tangs are positioned on the float arm adjacent to the pivot pin. The tangs are bent and set to prevent the float arm over stroking up or down. The needle valve and float can only move vertically, whilst the float arm pivots and swings on a radius thus forming an acute angle with the needle valve in the lower position. Often the result can be seen with a wear depression at the arm to needle contact point.
When the engine has not been started for some time the arm is at the bottom of its stroke and is trying to push the needle sideways as the float tries to rise. The needle jambs and flooding commences, this problem also causes intermittent flooding.
Set the tangs so that the arm drops just and only just enough to open the valve.
Could you just have the classic sticky float problem? When the car sits for an extended period, the gas evaporates out of the float bowl and then the float can stick to the bottom of the bowl. Just rapping the side of the float bowl with a wrench (not too hard) will usually loosen the float and then all is good.
This is a problem I have had with the old H type carbs on several cars. I’m not sure if it can happen on HD and HS carbs. New fuels are even more volatile so may evaporate more quickly.