As Mark 2 owners I'm sure we've all been there. After a nice gentle drive and an invigorating coffee, you come back to your beloved Jag and suddenly that warm smile evaporates as the engine chunters without catching. The usual on spot troubleshooting is followed by the ignominy of calling the tow truck. This time though I felt a tipping point. All the garages I called would not touch it, no technicians qualified to work on "old cars", so I had no choice but to tow it back home. After considerable research on this forum cross referencing the service manual, I was finally able to determine the cause of non-start was a dislodged distributor vacuum suction pipe. I post this not seeking any kind of response other than to say I really appreciate the members of this forum being so passionate and knowledgeable about these beautiful machines. Going forward I think my experience will be repeated many times by other owners and if we are to keep these beauties on the road - its forums like this that will be keep the wheels turning. Thanks forum members.

 

I am curious how the cause of non-start was a dislodged distributor vacuum suction pipe.
Presumably the worst effect would be a small air leak in the vicinity of the front carburetor which would hardly prevent a start.
Can you elaborate on this?

 

Yes, I would kike to know too.
You would simply have no vacuum advanced, but that should not keep the car from starting.

Wait, maybe it would, with that disconnected, it may be possible as the rotor turned, the plate where the points are bolted down, was now allowed to move enough to throw it out of time to keep it from starting.
The return spring for the vacuum advance would have to be broken or very weak.

Still very odd though.

 

It has me scratching my head too. I think that JeffR1 is on the right track.
In a normal course of events the vacuum hose being dislodged would only create a minor vacuum leak which might affect the idle and contribute to a slightly lean mixture.
It should not affect the starting.
Try to find a distributor overhaul specialist and get it checked by them.

 

More likely a fuel vapourisation problem after heat soak than vacuum pipe. The conditions are not clear.

 

Interesting responses but I also think JeffR1 is closest to diagnosis. When it first occurred, I had excessive gas on the ground below the aux carb. The engine would crank but not catch. During my diagnostics at home, I established that the Coil had correct ohms, nothing was unusual inside the distributor cap, the ignitor (electronic ignition) was functioning, and I had a spark. When I switched my attention to the fuel delivery side I discovered the vacuum pipe dislodged on both ends. I reconnected both ends and bingo. All seems fine now, and she is purring again. I'll have a look at the vacuum return spring as noted.

 

So your Primary diagnosis is Chronic Flooding?

 

At first- yes....but quickly moved past that after letting it sit and doing all the normal stuff to get it going (patience and time etc) before calling tow truck . The gas dumping never happened with all my troubleshooting at home over a number of days.

 

Did you touch the wiring to the AED (hisser) or the enrichment device at all? Keep us informed then we all learn.

 

No- but that was one of my check points in troubleshooting and all was good. It's still running fine 3 days after resolving issue and no problems starting. I will look at that vacuum return spring at some point but as of now all good.

 

My 3.8 starts “on the button” when cold. But after a half hour highway run, and allowing it to sit while I shop (perhaps another half hour) it is very reluctant to fire (this is a very common use pattern for me).
It always does start (eventually) but it takes some time. This is not new behaviour - it has happened for the last 15 years!
I have always attributed this to fuel vaporisation, but now I am not certain.

Comments? This is the single complaint I have about a car that otherwise runs beautifully. Any help appreciated.

 

Prof Gregory ~ You know I'm going to say fuel vapourisation. Let's see what others think. (my oil industry influence & where you live ~ more light ends in the fuel to aid cold starting.)

 

I think I'm wrong.
Distributor timing is set up with vacuum advanced disconnected, (if memory serves) so unless something has broken, then I'm wrong.
Easy enough to check though, remove the distributor cap and see if he base plate moves, there may be some play, but not much _ that's normal.

I think it's just a coincidence, and heat sink very likely, as Glen said
Setting the float forks at 1/2 inch should help with this and be helpful from gas flowing out of the AED on a hot day. The present setting is 7/16.
I've also in creased the vent size in the line and opening in the gas tank, this assumes that your gas cap is not vented.
But vented gas caps seem to make for a smelly boot.
Keep in mind that I have not done this too my car, but on the Bentley, but it's been off the road for 1 season, but there is no reason why it would keep it from running with a somewhat lowered float level.

I've even contemplated putting a fuel shut off valve to the AED to totally stop the flow from the float bowel into the AED.
This however would require some butchering of the carb for this to work, so I've never done it.

Wouldn't be much, I would replace the banjo fitting with a solid bolt, and then drill and tap some small pipe fitting into the aluminum casting that runs from the float bowl to the AED _ the valve would go there to be controlled by another switch inside the car.
I suppose it could be wired in parallel with the switch I already have that controls the solenoid, it would disengage the solenoid and the fuel valve at the same time, thus shutting the flow of fuel off.
Would have to find the right fuel valve that when it's not energized, the flow of fuel is cut.

I did look around and I think Ford made one at one time.
Making a mechanical valve on the bolt may be another option, but that may present a source for a leak _ lots to think about.
Or drill out the bolt and make into a custom banjo fitting where the fuel only flows out the bottom and put the valve there.
The same on the AED banjo bolt.

 

thanks Glyn….but is there an antidote for this condition? Or a specific procedure to be followed under these conditions? The difficult hot starting is not random…it is entirely predictable. Cold/warm engine, No problem: instant starting. Hot engine left to sit for a while, very reluctant to start.

 

I will throw in my ten pennies worth and suggest it might be float height related.
The float height as mentioned has a specific height which controls not just when the fuel is allowed to flow into the float chamber but also the height at which the fuel sits in the chamber. The fuel height in the chamber is also reflected in the bottom of the needle valve seat below the butterfly. If the float chamber fuel level is high then fuel can over flow the needle valve seat causing the carbs to run rich but it can also cause over run which is what I was suffering from. Even though I turned the ignition off and therefore the fuel pumps where no longer pushing fuel into the float chamber because my float height was too high a certain amount of fuel was still rising out of the needle valve seat causing the run on. Once the fuel had burnt off and the level in the float chamber had dropped the engine would die.
Taking this scenario into your problem there is a likelihood that if your float height is such to allow more fuel to seep through to the carb body via the needle valve seat then cold starting would be great but hot starting would be like trying to start the engine with the choke out. If you look at the diagram below the fuel level in the float chambers is marked as a line number 21. This line runs across the float needle seat and also defines the height of the fuel in the AED.

 

So the condition is only prevalent after heat soak = vapourisation.

On carbs with throttle pumps its easy. Pump until you get liquid fuel. With SUs not so due to their operation over the jet that acts as a throttle pump & the hisser/AED set up.

For areas of the world that suffer this condition ~ One fix that I've contemplated is to make a ring main like a fuel injected car (might need an additional LP/higher pressure pump ~ might not) where you constantly circulate the fuel back to tank the minute you turn on the ignition with a pressure regulator before the carbs set to what SU's like at approx 3psi. Then vapourisation would be killed the minute you turned on the ignition. Fresh cold fuel would push the vapour to tank.

The oil companies are stuck between a rock & a hard place. Do you want easy cold starting? We do try to blend for easy cold starting but changes in ambient conditions are vast. Fuel injection rids one of the condition.