Jensen heritage for the next generation


From the Jensen FF Mk1 sales literature: “Mechanical pump. Filter in fuel ine. Flashing low fuel-level warning light operative below 3 gallons ( 13 litres). Tank capacity 16 Imp. gallons ( 72 lires).”


Photos of the right hand vent pipe as it exits from the top of the fuel tank and joins a plastic pipe (photo 1). The plastic pipe finally exits at the bottom of the rear wheel arch. Blow down it to make sure it is clear and is properly venting the petrol tank. Mine wasn’t. There is one on either side.

I had a problem with what I thought was fuel vaporisation and I asked the list:

“The car just stops with what appears to be fuel starvation and most likely due to fuel vaporisation. It happens when the engine is hot (at normal temperature). It has happened whilst test driving after 10 minutes or so and also whilst sitting on the drive and running at a fast idle. In the latter case just before the engine stops there is a rise in the revs. For a few seconds before it dies out. I fitted a new fuel pump a few weeks ago. Question? What measure do you take with the Jensen FF/Interceptor to cure this problem? Pipe re-routing and keep the mechanical pump or are electric pumps and re-routing the answer or just shielding the existing fuel pipes.”

My second posting to the list was:

“Thankyou everyone so far. The consensus seems to be that if the car engine dies out whilst running it can’t be fuel vaporisation because the fuel is moving and wont heat up sufficient to vaporise. I should have known that myself really!So my problem is most likely fuel starvation Thanks John, as per your suggestion I have checked the fuel filler cap for a vacuum. There doesn’t appear to be any. I will check the vent pipes though. I recently replaced the fuel filter a couple of months ago, not long after getting the car (as part of the recommissioning work). At someone’s suggestion, via a direct e-mail, I ran the engine whilst looking underneath and observing the clear plastic fuel pipes/filter. There connections. Hopefully, if I can find where the air is being drawn in, this will solve the problem. Anyone else had anything similar

The responses were:

1 Have you tried opening the filler cap around the time this is happening??? Could you hear a hiss??? It could be vacuum in the tank. On the Mk1 there are two fuel tank vent pipes, which both must be fitted and not kinked & properly routed. Welding repairs to the rear arches often destroy the pipes, blocking them, or causing fuel vapour to get into the car. Another thing to check is the inline fuel filter near the drivers floor pan. I don’t think that this is fuel vaporisation, because fuel is moving too quickly when driving to vaporise. Fuel vaporisation symptoms occur when trying to restart a hot engine.

2. BTW, have you fitted an auxiliary fuel pump? Most Int./FF owners fit an electric fuel pump in the fuel line which primes the carb before starting, especially helpful when starting a warm engine. This means that a turn or two of the engine and it fires – you just need to remember to turn the pump off. This may help your fuel vaporisation. Apols if I’ve got the wrong end of the stick.
3. The rising revs may be due to a beginning fuel starvation if you are running too rich from the outset. It sounds to me as if you have a blockage somewhere, probably in the tank pick-up or maybe the union at the tank. On Mk.1 that is usually a bayonet type which clogs up all too easily.
4. Bremar: Your fuel problem sounds, at least to me, as if you have a clogged fuel filter. I’d check that first.
5. As a complete novice in motor mechanics I am reluctant to offer advice but I thought I would pass on something that was written in the Tech Tips Manual put together a few years ago by the JIOC. It might pertain to your problem.

Steve Brenneman writes:

“A common problem with Jensens is that of percolation in the fuel bowls of the carb after shutting off the engine. This problem manifests itself by making your car hard to start after relatively short (5-20 minute) periods. Because of engine heat coming up to the carb from the intake manifold, the fuel in the carb bowls often boils over and down into the intake manifold passage. This excess fuel tends to flood the engine, which is then best started by flooring the accelerator once – and holding it there until it catches – and cranking the engine to clear the excess gasoline. When an engine starts that has been flooded, there usually is a significant cloud of black smoke from the exhausts. What to do? Tell your friendly Chevy dealer that you need a GM part #3969835 (carburettor heat shield for 427-454 Corvettes). This is a large flat aluminium plate with proper mounting holes and carburettor baseplate holes to fit your Chrysler manifold and four-barrel Holley or Carter carburettor. If you run a Holley, install the plate as is: the gaskets installed on the plate are intended for a Holley four-barrel. If you are using a Carter AFB,AVS or TQ discard the GM gaskets and obtain two new gaskets from your local autoparts store. I recommend a thin gasket (app. 1/32″) between the intake manifold and the heat shield, with a thicker
(app.3/16 – 1/2”) gasket between the heat shield and the carb. In any case you may have to perform some minor trimming on the GM shield to clear obstructions such as ignition coil, vacuum lines, etc. I’ve been using a Corvette shield on a V-8 Ford for five years with good results, and have just installed one on my ’71 Jensen. The Jensen was prone to percolate on hot days.

6. Bremar, I observed bubbles as well, but that was turbulence combined with underpressure drawing air form the fuel and collecting it in the filter. I didn’t get false air flow just died out as the partial and increasing blockage manifested itself. And as mentioned in my previous mail, it was fine while the engine was cold.

7. I would be interested to know more about the air in the petrol line. I have recently replaced my fuel filter which means I can now see into it, only to notice that the upper part seems to be air. Since I’ve only ever looked when the engine is stopped and cold, I cannot say whether there is a flow of air or it is a stationary trapped volume. Since the engine runs very well I’ve never had cause to investigate further.

8, If I were You, I would change all rubber hoses between the tank, and the engine. You will maybe see it pays off Keep me posted….
9. On one of my interceptors, I found the label from the float assembly was floating in the gas tank, sometimes clogging the system!!

Finally, having got over the problem I posted the list with:

“The bubbles of air in the fuel line that I observed was solved by replacing the filter and doing a better job on the filter/ fuel line connections. Air was obviously being drawn in. The filter still has air in it and this has not effected the supply of fuel to the carb. (On my Triumph Vitesse there has been air in the filter for 10 years and it never seemed to make a difference). The fuel starvation problem wasn’t due to either air in the filter or air being drawn in, because when I sorted this latter problem the cars engine would still die out after driving for a short distance. Off list a contact said that they had had a problem with something? floating in the fuel tank. They/it would occasionally get sucked upto the intake and block it. After the engine stopped, and without the suction from the fuel pump, it would float away allowing the engine to be restarted. This would repeat itself intermittently . It was evidently worse with low fuel levels. He solved the problem by cleaning out the fuel tank and putting in a petrol tank sealer. The symptoms exactly fitted my problem. As I had an MOT due I blew compressed air at 40 psi back down the fuel line from the fuel filter location) and bubbled it into the fuel tank. This seems to have done the trick. Permanently I hope, but probably not?.

Actually three weeks later the car again stopped about 4 miles from home. The fuel gauge didn’t work then so I emptied the spare can in but to no avail. The engine would start then just die out. Eventually I discovered that it would idle but not accelerate. I made my way home very slowly driving on just above idle.I removed the banjo bolt at the rear of the fuel tank and when I inspected it, it was almost completely blocked.


Removing the fuel tank sender unit

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On the Mk. 1 FF as part of the fuel gauge, there is a flashing low level warning light operative below 3 gallons (13 litres). There are therefor three wires attached to the sender unit. I understand that the sender is basically a variable resistor. As the float moves up or down in the tank with the level of the fuel, it alters the resistance in the circuit which is measured by the gauge on the dash. The easiest way to verify that it is the sender unit and not the gauge is to remove the lead from the sender unit. This should give you 100% resistance (with the ignition on) and the gauge should read “FULL”. If you then ground this wire you get 0% resistance and the gauge should now read “EMPTY”

The Unit is easily removed on the Mk. 1 FF by locating the metal flap in the boot floor, unscrewing the self tappers holding it in place and lifting the plate. This reveals the top of the sender unit. I used a flat tipped blade screwdriver, inserted into one of the lugs and then tapped it round anticlockwise until it releases. About a quarter turn I think. This ring lifts off (on the right in photo 2). The sender unit then just lifts out. In my case it had seized in the quarter full position. I sprayed it with freeing oil, left it overnight, and with just gentle pressure the next day, it started to move again On reassembly the fuel gauge worked perfectly.