After painting my car and doing some interior work I'd like to drive it a bit. I've replaced or changed nearly everything on the engine. After the car has warmed up and the choke turned off it idles nicely for maybe 30 seconds or so and then suddenly slows down to almost dying and then slowly will increase the revolutions again. I noticed at the point the revolutions slow down the fuel in the glass sediment bowl drops to about 3/4 full. When the revolutions increase the bowl fills again? I have no idea of the cause. Can anyone help?

 

The fact that the fuel bowl level drops suggests an issue with the fuel pump.

 

I agree having had a similar problem that it is fuel related.
Two things come to mind. The first is as stated above that you have an issue with the fuel pump. An intermittent fault it seems which could mean a new fuel pump, a clean of the points if you still have the old original SU pump or a fault in the wiring which is switching the pump off every now and then.
Secondly you might have a problem with your fuel tank or the lines coming from the fuel tank. These can get blocked or restricted. A small amount of dirt or debris in a fuel line or where the fuel line leaves the tank would certainly cause the symptoms you have got. Fast running and the fuel cannot keep up with the engines requirement. As soon as the engine slows or cuts out and the pump is able to fill the glass bowl and the float chambers.
I have two tanks on my S Type. The right tank and pump are working fine. The left tank has the same symptoms as you but I have not had the weather or time yet to tank the tank out of the car and to clean it. The fuel pump seems fine when disconnect from the line as it is pumping fuel but it is not getting enough through to run the engine.
I would pull your fuel tank, clean it thoroughly and blow the fuel lines out from the front to the back with an air line when disconnected from the fuel tank. Check you do not have an obstruction in the fuel line like a collapsed rubber hose (could be caused by E10 fuel) or a dent in the pipe cause by a hit with a stone or rock if it is a solid fuel line as on the S Type. Fitting a new solid copper pipe fuel line from front to rear would not cost much and easy to do.

 

 

A car underside that you could, almost literally, eat your dinner off! In case it's not obvious, the fuel pipe is running parallel to the brake pipe along the far 'chassis' rail.

 

New fuel tank, lines were blown out before fitting. New fuel pump. I'm going to try to figure it out again today after sleeping on it.

 

Proof that Jaguars do not have to suffer incontinence Peter.

 

Air leak at a joint or union?

 

Blocked tank vent?

 

Hard as I found it to believe or accept. This caused no fuel smell or leak. Filter bowl always about 3/4 full. But the minute you loaded the engine it produced no power & backfired through the carbs. Dual system S Type. Same with either pump switched in & both tanks full. Brought down the whole system..

 

One would think it has to be before the glass fuel filter if that is half empty when the engine stalls. I would say that it is because the engine is not getting enough fuel and although this is beyond the glass fuel filter have you double checked the float heights in the float bowls. If set too low they might be cutting off the fuel going into the float chambers too early resulting in not enough fuel to run an engine at higher revs or under load.
Other than this as Pete has suggested fuel vents in the tank. If the vent is blocked then no amount of sucking by the pump is going to draw fuel from the tank. Try running the engine with the fuel cap off to see if you get the same result.

 

In my case fully reconditioned carbs. Replaced pipe & all was back to normal & still is. We did the clutch no favours in the process. I now have another new spare. Just in case.

 

Today I think I solved my problem which has a lot of similarities with yours as stated. See my post "Seeking advice" https://www.jaguarforums.com/forum/m...advice-267522/
I have found that one of the rubber fuel pipes from the tank to the fuel pump had slightly collapsed on a bend restricting the amount of fuel being sent to the carbs. Might have been a combination of E10 fuel making the pipe more malleable and prone to collapsing on a bend and a fuel pip that was too short to allow a decent enough bend. I have replaced my rubber fuel hoses and just have to fill the tank up now to check it out but visually I can see the fuel flow better on the new hose.