Hysteria

Bought (a bit blind..) a '99 NB 1.8 Flying Miata Turbo conversion car yesterday.
Car did not drive since October 2021. Test drive and first part driving home (30 km) was sort of ok. Coming of the motorway it choked on the gas, lacked power and started coughing through the air intake. Petrol tank was already low on departure (at least 1.5 year old fuel) so filled it up with fresh RON 89 with as minimum ethanol as possible...

Drove still very problematic for the 15 KM's afterwards. Car will only pick up a little at 3000 rpm. Backfiring from intake predominantly, but also exhaust. Limping home…

Supposedly put in storage in good driving condition. Where to start. Do the Flying Miata programable unit and after market programmable ECU keep their settings after 1.5 years of battery disconnection? What to clean to make sure potential “bad petrol” is out of the system.

SimBa

If that gas is 1.5 years old that's a good place to start looking for issues. Is 89 Octane as high as you can find? I assume you're in Canada?

If you really wanted to clean the system you'd be wanting to drop the gas tank to clean it. Some octane booster might do something for you, although I'm not sure how well it would mix in if the tanks already full.
Coughing through the intake makes me think it's running lean. Do you have an AFR gauge?

Watterson02

First off, I doubt it'll help your situation, but it is very likely your car was tuned for something more like RON 98 rather than 89. Not sure if it is a typo, but I never run anything less than (R+M)/2 93, which is roughly the same as RON 98. I understand avoiding ethanol if you're concerned with gunk in the tank, but that fuel can potentially cause catastrophic engine failure if the car wasn't tuned for it (due to its substantially lower octane rating).

I let my car sit for over a year and did the first drive with the new engine on the same gas, so personally, I wouldn't assume the issues are directly caused by the old gas. Like the previous comment suggested, I'd look at the AFR gauge. If it doesn't have one get one and learn about what target AFRs you should want to see. A good rule of thumb is 14.7-15.5 while cruising and 11.5-11.8 in full boost for safety. I don't think this would cause backfiring out of the intake though... It is possible that there is some sort of issue within the ECU, but my megasquirt retained the tune file after the longterm downtime, but your mileage may vary.

If you're considering cleaning out the gas tank, I'd just look inside to see if it appears to be an issue. It is possible that other things in the system are clogged too if it looks rough. It wouldn't be a bad idea to have your injectors flow tested and cleaned, as well as replace the fuel filter if you pursue this course of action.

Lastly, we've not been given a lot of information to help diagnose the situation. Just that the car runs poorly and backfires after getting fresh gas. Information like the AFRs, how consistent the backfiring is, does it idle well, have you verified the ignition timing is correct, and potentially a log/tune file from the ECU would help us greatly. We can't help much with as little information as you've given. Also, just go ahead perform the normal maintenance that the car probably needs, like new sparkplugs, oil, etc. I'd recommend the NGK BKR7E spark plugs if you can get them. They're two steps colder and pre-gapped to .036" from my experience, which is where you will want to be with a turbocharged Miata.

Oh, and welcome to miataturbo.net