Hello.
Faced with such a problem.
XJ-C 4.2 su carbs.
When cold, the engine starts and runs normally.
But when hot it turns off very badly.
I turn it off and the engine snorts and continues to turn for a while and turns in the opposite direction.
When stopped, oily smoke comes out of the intake pipe.
I'm not trying to figure out what to do about it.
Sparkplug are clean without re-enriched plaque.
CO and CH are almost normal at about 2% CO and 50 CH.
Rides well without detonation knocking.

I tried to attach a video but failed.

 

I have had that.

Jaguar installed an Anti Run On Gulp Valve in the system, they worked when they fekt like it, and that is probably missing for that reason.

All mine that did this were very touchy with idle speed, and timing.

Too fast and they ran on. That too fast could only be 100RPM, they were that fussy.
Too far advanced, ran on seriously.

I idled them at 650rpm, and retarded the timing a SMALL amount.

One was a PITA, and always got shutdown in Drive, and the A/C ON, that stopped it dead. Became habit after a while.

 

Hi Grant
Anti Run On Gulp Valve? I've never seen such a thing for carbs.

I will try low idle. But I'm worried about low generator charging.
I made a conversion to Bosch 120A. But this is not a panacea in deep traffic.
Our laws require lights to be on. Plus, air conditioning and charging drops.

 

I adjusted the ignition timing as early as possible before the short detonation. As Grant mentioned.
Idling increased. I reduced it as much as possible. In D drive is 450-500rpm.

And this back(front)fire is gone!

Thank you Grant

 

Here is the Jag/Triumph/MG/Rover used Gulp Valve.

The circuitry to make is Gulp was about as fickle as Lucas could dream up.

This the one off our 1976 Daimler 4.2 from 30 years ago, maybe more. It works fine, just the rest of the "required" was crappy, so here it is, in storage for that rainy day. Car is LONG gone.


Gulp Valve AKA Anti Run On Valve.

 

XJSV12
"Running on" the term for when a petrol engine effectively diesels on when the spark has stopped to the plugs, was quite common before fuel injection. What a gulp valve (anti run-on valve) does is to allow air into the manifold by bypassing the carburettor butterfly, thus equalising the pressure each side of the carb. By allowing air directly into the manifold, it immediately removes any "suction" through the carbs, and thus any drawing-in of fuel droplets. So the engine stops firing, as there is no fuel. ie stops running on.
You could have the same effect by fitting a solenoid valve into the manifold activated by a switch in the cabin.

 

Greg,

Thanks for that.
I've had lots of older Jags with the Gulp, but truly didn't understand them.
Rob

 

dieseling is caused by a vacuum / air leak.

 

Before it started, I adjusted the carburetor with slightly increased idling . The engine runs more steadily.
But this has led to the fact that when you turn off there is a dieseling effect. Closing the flaps-reducing the idle speed, you turn off the engine does not allow the air to suck in with fuel and the engine normally stalls.

Thank you!
Without you, I would have thought about it for a long time.

 

Good to hear.

SIMPLE cars these Jags.

Beer O'clock is declared, enjoy.

 

the other main cause of running on was the petrol carbon deposited in the cylinder combustion chamber. This would get hot enough to ignite the fuel and air mix.
A good old decoke would clear it up. It shouldn't be such an issue with modern fuels and oils.

 

That is true, but the point of the gulp valve is to stop any fuel getting onto the cylinder, regardless of glowing carbon being in there. Consequently running on stops because no depression over the carburetor means no fuel vapour getting to the cylinder, means nothing to ignite.

 

Yes I remember "gulp" or anti-run on valves back in the 70s and 80s on mgs. minis etc which were also available as an after market gadget. Fitted to a few escorts and cortinas etc when they got all coked up.