Hi all!

I am new on this forum and just purchased first Jag XJ 4.2 -76.
Car is working otherwise great but now after cleaning the fuel tanks very strange problem occurred. Both tanks and fuel pump filters were full of dirt and thick "gunge". Now after everything is cleaned and this happened:

- Doesn't run idle
- Thick black smoke when you get it running
- Gasoline smell
- Spark plugs are black

This everything relates to problem that car is running too rich.

- I have checked that fuel level is correct
- Needle valve is cleaned and holds pressure. I tested sealing with air and both valves are holding it. However small leakage is coming from AED unit overflow pipe. I guess that is correct?

Yesterday I tested fuel pressure and it was only 1psi (both pumps) but fuel flow is very fast (faster than before cleaning the tanks). It's roughly 1litre/20seconds.
I also tested to run car with an external small tank without fuel pumps. And magic happened. Car is working great and same as normal but immediately when car's own fuel line is connected same symptoms occur.

Please help What could cause this problem? Fuel pressure is very low that it should not be the problem. Also needle valves should work normally but I haven't tested them with fuel.

 

Welcome to the forum hakunti. People will along with some answers for you.

When you get a moment please post an intro in the new member area.

New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum

 

Welcome also.

My first thoughts are that the pressure gauge is wrong, and you are in fact getting more pressure than it is showing.

Over 7PSI and those needles will not hold back the fuel.

Mostly about 4PSI is looked for.

The fact it works well on "gravity" feed indicates pump issues.

Being 1976, it has the pumps inside the fuel tank/s???

 

Thank you for the comments. Pressure gauge is new and made for fuel pressure measurement. Of course there is a possibility that it's broken. Pumps are inside the tanks. Is there any restrictor or pressure reducing valve in fuel lines? I mean that if pressure has been too low previously and something has been removed?

I have spare set of needles which I can test. Before that I will try to choke fuel hose and see if pressure increases and flow decrease. Could it be that flow is too strong and pressure too low?

 

Mmmmmmm.

No restrictor that I can remeber.

There is a one way valve just where the fuel hose exits each tank. This prevents the fuel from apposing tanks being pumped into each other. Maybe this is gummed up, BUT, I would expect very little floe in that situation.

When you have the flow happening, place your thumb over the hose end (just like the garden hose when ya want a jet of water), and see roughly how much pressure you actaully "feel". 1 PSI would hardly be felt at all, but 4+ PSI would enable you to "squirt" fuel over the fender.

The supply fuel line travels under th car on the RH side, and I have seen that pipe crushed many times, and that would restrict flow and pressure, but not only one.

I have never seen this on carby Jags, but some markets have some strange stuff. Where the supply fuel line enters the engine bay, there is a size reducing thingy, and that "sometimes" incorporates a one way valve. Carby engines do not need a one way valve, coz the carby bowls always have a reserve of fuel for starting purposes.

I still reckon you got too much pressure.

Carby pumps are always LO pressure, HI flow.

 

Now testing has continued and I have tried to find the root cause. I have removed and installed carburetors at least three times, tried to adjust fuel level etc. Today I changed float chamber needles but it didn't change anything (nneedles are with rubber tib from newer carburetors). Fuel pressure is quite low, but flow high but they should be enough. I don't believe that problem is in fuel pumps.

I tested also again with separate small fuel tank and engine is running great. Immediately when I switch back to car own fuel system it's running very tough with black dark smoke. It doesn't run idle either and feels that carburetor overflow from fuel.

It's quite strange problem.. Maybe needles are worn but somehow I don't believe that.
Could there be some problem in AED unit?

 

These AED devices can be troublesome, the trouble being them not turning off, but this is normally a duff temperature detector. This problem goes back to the 60s and lots of people fitted a separate switch to turn them off.

So, yes, you could have a problem with the AED. Try disconnecting it entirely.

 

Hi,

Finally the problem is found. AED is feeding huge amount of fuel directly to manifold and also fuel level in carburetors were too high. Fuel level was set to same as in SU instructions but it seems to be too high and both carburetors were flooding.

Now I need to somehow fix the AED (as I don't want to order manual choke yet). AED fuel level is correct and it's not flooding. What could cause the problem in AED that it's feeding huge amount of fuel to manifold? Does anyone know good instructions how to fix or overhaul the AED?

 

Fuel pump sock filters?
If you didn't replace or seal those tanks, you'll be fighting this relentless gremlin forever.