As it happens, a long time ago I noticed that there was a rush of air (suction) when I open the tank to refill. Is this a bad sign, then? You’ve been so helpful I hesitate to ask where the end of the breather is and how it can be cleaned out? (may be the answer is in the Haynes manual that hopefully is on it’s way to me now!)
Absolutely. I forgot to mention this as a good sign of the problem. Cheers for highlighting it. 
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reckon you’re onto something here First_rider_off!
Is the fuel filler cap supposed to allow air in and the charcoal breather let air pressure out?
Not exactly sure about the setup on an MX5 which uses a charcoal breather. I just know that most car petrol tanks have a breather system to prevent pressure or vacuum building up. The cap is usually sealed to prevent leakages. I had a problem with this on a Triumph Spitfire many years ago. It had me completely baffled. As 999to5 said the car would start ok but would start to show fuel starvation problems sooner or later depending how full the tank was. The fuller the tank the quicker the problem manifested itself as the vacuum increases quicker with a full tank than an empty tank.
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Should be positive pressure when you release the fuel cap
I’ve never experienced the fuel tank vacuum theory myself, but run it with the cap off for 100 yards, it’ll prove or disprove if this is your issue
There are a multitude of things it can be, and without any diagnostic error codes, everything is going to be trial and error.
If you still have the standard airbox, then my first plan of action would be to reinstate this.
It takes more than a hundred yards before the effect takes place. There has to be someway of air being let in to replace the petrol that has been used. The breather valve may be in the cap itself which could have become blocked or stopped functioning correctly. I have heard of problems where people have lost their original petrol cap (which had a built in breather) and replaced them with a cheap temporary cap which doesn’t. If the engine starts first thing in the morning (or after the car has been sat for a while) and then shows symptoms after a few miles this could be the problem. It is an easy fault cause to check.
Hmm, check a few things along the tubing between the “Purge Solenoid Valve”, “Catch Tank”, in the engine compartment, back through to by the fuel tank “Charcoal Canister”, “Evap Leak Detection Pump”, “Air Filter”, “Evaporative Chamber.”
The Mellens site had excellent info on this system in the “Emission System.pdf” the full Mazda service info. But it seems to be off-line now and the other archive I’d found is incomplete and hasn’t got a copy.
If you pm me your email address, I could email you this pdf as it is only 1.243MB.
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I had it with a BMW R80/7 and I’ve seen it happen when someone failed to unscrew the breather valve on a little outboard motor.
People often fit aftermarket filler caps to these cars.
Could that be a possibility??
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many thanks - will pm you
reply to 999to5 - I’ve done that to an outboard!
It doesnt look like an aftermarket filler cap, and we’ve had the car from only a couple of years old with one lady owner before us. Not sure how I would go about testing the valve - I dont really want to put my lips to it!
Try running the engine with the filler cap open/off.
If it keeps running fine for say 10 minutes and you can rev it etc normally, that could be an indication it’s the breather.
If it falters and stops after a while, that would indicate your problem is elsewhere.
I wouldn’t worry too much about touching it with your lips, it’s not novichok…
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Check the filter on the bottom of the fuel pump. When was the fuel filter under the car last changed?
See if you can borrow a coil pack and HT leads.
I don’t suppose it would show anything but have you checked for fault codes?
Good luck.
D
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Is it safe to run the car with the filler cap off?
Regarding you comment on wrapping my laughing gear around it, I havent allways been complementary about certain dictators, so you never know…
I was previously very sure the air was sucking in when I opened the cap, but now I am looking to make certain, I cant be absolutely sure. Anyone know how to tell for certain without introducing dust or other contaminants into the tank? Perhaps if I put a plastic bag over it as I took off the cap, it might show a discernable decrease or increase in volume.
When was the fuel pump filter last changed? -possibly never for all I know.
Sadly I havent got spare HT leads or a fault code checking computer. Really impressed you guys are so into your MX5s that it’s expected to have this stuff around (I am not worthy!).
Perhaps I need to take it to an MX5 specialist like “Total MX5 Repair” in Barry near Cardiff (an hour and a bit drive from me, so dropping off the car & picking it up’s going to be a pain)
I wouldn’t want to drive anywhere with no cap on the tank but as long as you don’t smoke I think it should be fine to run the engine like that for a while.
I have had very similar problems, down to debris in the tank blocking the pump pick up filter, it’s a fairly straight forward process if a bit of a fiddly pain, you will get a rush from the tank when removing the filler cap as has been mentioned it’s a pressurised system but of course it still needs a vent (I yet to get my head around that one), I have done a couple of posts on here about this and if you want any help please ask. I have just finished doing it for a second time as more debris and fine rust powder gets disslodged I also stopped and removed the cap and there was convinced that was the problem as it drove fine for a while after, the key for me was it happening after turning left at a junction when low on fuel, it’s a simple free check you can do yourself, one tip is to run till empty and it’s surprising how far you can go with zero fuel showing, when it finally was empty I was still able to limp home and removed about 1 litre. Just realised I have alread posted to you DOH! A well it’s a 2 for 1 offer 
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well, thanks again Baabaa!!
So good you had to give the advice twice!