I have over a dozen Jaguars from the 50s and 60s. All of the start easily within a week after the last run, but most of them are somewhat difficult to start after 3-4 weeks. One or two start at the first push of ghe button even after 6-7 weeks.
What causes difficult starts?, i.e. It takes many pushes on the button for the engine to start.
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It’s my guess that the oil has drained from the cylinder walls thus causing lower compression until continued cranking coats the cylinders. I had two Nikosil V8 engines that I had to crank a long time before they would start. If I started them every day - no problem.
Difficulty in starting an engine with carburetors after a period of setting really has nothing to do with oil draining from the cylinders: it most likely has everything to do with the carburetors evaporating their fuel load.
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Ok, that makes sense, but why do some carbs let the fuel evaporate, and others not?
What is the cure?
Drive’em more often…
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With carburetors and points-controlled ignition, there are several factors in which the exact settings are manually achieved.
Main jet height, starting carb needle stop nut, and carb float height determine the cold cranking fuel mixture. Unlike modern cars with their engine management computer controlling the fuel injection.
Points gap determines dwell and charge buildup in the coil.
Ignition timing and vacuum advance also have their effects.
Finally valve lift and cam timing are variables that affect starting.
These are all factors that are manually set by the mechanic, according to his skill and ability to adjust them.
I know what you mean. I have two early Jaguars that I start regularly. The '38 SS starts surprisingly quickly like in 2-3 seconds even when not started for weeks, but the XK120 has to be cranked 5-10 seconds and sometimes will cough a couple of times before starting when it has been a few weeks since the last start.
Both start within 1 second when warm, or even after sitting at a car show all day.
Thank you Rob.
Do you crank the 120 for 5-10 secs in one long push?
I tend to do 5-10 short cranks (3-4 secs).
Perhaps I should try your way.
Agreed on immediate starts, later the same or following day.
I never touch the gas pedal during, or before starting.
I drove 5-6 cars yesterday as the weather was very nice. The pastel green 120 OTS started on first push, as it always does, but Jack Stamp’s blue 140 DHC took 5-10 pushes, as it always does. Both cars had stood for a month.
Taking a 420G to a nearby town this morning. A good 40 mile drive there and back.
I drove the Willow green S1 E-type from Stockholm to my summer house 2 days ago. 350 easy miles with the 5-speed box from Paul Cangialosi.
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If the Mk VII has stood for a long I’ve found that starting with my foot hard down for 3 or 4 cranks then releasing it suddenly induces fuel to be drawn through the hisser and the main jet to get things going.
Cranking for 10 seconds is not a problem and saves you some wear on the flywheel and pinion.
It may depend on the hisser. They don’t seem to start immediately and don’t all give the same amount of fuel.
What about choked cars? Sometimes there’s cars that start right up after sitting and others take a bit. Likely it’s a mixture difference.
I crank continuously, not short shots. Be aware that the starting carb has an adjustable mixture, the nut on the top, which raises and lowers the needle. Depending on the ambient temperature, it may want to be a bit richer or leaner.
Jaguar added a built-in timer, if an Engine has sat for more than 30 days, it will not start until it has cranked over for over 120 seconds, this is to allow the Engine oil to circulate and lubricate all moving parts!
Ok it’s ball shit 
Imagine the wear if an engine had sat for months, then started instantaneous, especially if it had one of them new high toque starter motors, only good for door stops in my opinion 
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And anti clockwise on the nut makes it richer if I remember correctly. Half a turn perhaps!?!?
I have a question good sir,
I let the fuel pumps fill up the bowls, which presumably would also let fuel into the jets, before cranking.
However if I leave the vehicle unstarted for more than a couple of weeks, it always seems to take longer to start than normal cold cranking, maybe 10 secs as compared to much less if the engine is only one day unstarted
I cant understand this, (if fuel is filled in the jets), are you or anyone else able cast further light ?
If the fuel didnt fill the jets up to the normal level, ( like I think it would) that would explain things
This purely out of interest, it doesnt matter in a practical way
Hi Tony…as your aware you only need fuel, compression, ignition to start the car…so all things being equal if the car is left for a couple of weeks you will still have the same ignition and compression…but probably not the same fuel due to evaporation…yes you let the pump fill the carb bowls but what about your choke/enrichment carb…this probably needs adjusting as mentioned above…cold and damp can also have an affect…Steve…ps the small channel to the enrichment carb can get block/partially with sediment that wont help
Hi Peder,
Yes, turn the hollow nut clockwise to lean, anticlockwise to enrich. It stops the needle travel. The needle is a pointy tapered thing that sticks into the jet, and is sucked down when running. The jet is where the fuel flows through, and the needle partly blocks it depending on how deep it is inside. Notice there is a tab that rests on a flat and prevents the nut moving due to vibration.
Hi Tony,
Yes, allow the pump to fill the bowls and the jets will be full too. Quick or slow starting depends on the temperature of the intake manifold and head. You want vapor, not liquid in the intake. Cold means you don’t get as good vaporization of the fuel mixture as it is sucked in by the pistons.
Thanks Rob,
I turned the nut 4 slots ccw.
Will know if it helps on the 29th of Sept when I return to my summer house.
Maybe some lighter parts of the fuel evaporate over time making the mixture harder to ignite?
Thanks for the replies Gents,
Kind of like the No and Yes I got from two expert accountants when I had a complex tax issue 
So I understand the fuel evaporates, and we are talking an engine that is fully cold, whether 2 days or 2 weeks apart
My assumption is that once the fuel bowls are completely full, the passages to the carbs and ASC will be also full, and the level of fuel in the jet well, ready to be “sucked up” for atomisation will be at the level determined by the float setting
So the same on both occasions (unless for some reason its not, that I dont know about)
So this has been the same on all my ASC Jags, but I cheated a bit and omitted to say that the current engine has a full manual choke setup, no ASC or AED. I lock the choke out to the same number of clicks each time for starting
I am aware of the drilling that leads from the bowl to the ASC, and have reamed them out with a ~.030" wound steel guitar D string, which makes the perfect tool
I can only think of evaporation as being the answer, but it doesnt make logical sense if the system works as I thought