More from Chris, the Miata Whisperer...

Scan it for codes and check for vacuum leaks.

Could be the idle bypass screw and the idle is drooping too fast for the idle air control valve to catch it. Has the throttle screw moved any? Does the idle need to be barely cracked for it to run or is it a case of needing 1500 rpm to keep running?

No actual idea other than agreeing with Zeke.

My experience: look at the last thing you messed with. This is so often the issue.

vacuum leaks and check whatever you did last to it.

Right, that's backwards. Normally a clogged filter will allow enough fuel to idle, but then restrict the fuel required to perform under load and/or higher RPMs.

We checked for leaks, nothing showed up. I mentioned the throttles needing adjustment but what do I know, I just watch Vice Grip Garage a lot. It’’s also a Bit more challenging for a non stock car imo. I have another question for later on, I’m burning out my asks.

Did you get your son to clean the plugs to see if it would idle okay with clean plugs? (of course they'd foul again, but you'd know it was probably a tuning problem then.)

No expert on Miatas, but if it were me, I would try disconnecting the Idle Speed Control Valve ISCV. When disconnected, it should produce a somewhat normal idle. If so, Google Miata ISCV troubleshooting and you should be able to figure out what needs done.

if its running ITBs it no longer has an idle control valve.

and 99% chance its a leak or other issue in the ITBs. ITB converted miatas tend to be a little fickle.

We cleaned the plugs, same issue. On hold until he gets back next weekend. Also, would a light come on if throwing code? I unfortunately left reader in far away car. He was going to call the ITB manufacturer.

depends on the programing for the ECU. if its aftermarket, all bets are off, you are probably on your own.

Did you get it sorted?

We just finished up an 8 hour endurance race with the 89 NA miata… my engine now has 80 race hours on it. We got bumped out of E3 into E2 because of 4 laps that were a second too fast for E3… still got 2nd in E2 and 6th overall. Not bad for the car with the least displacement and lowest horsepower in the race.

Making Miata guys proud. He’s tinkering with a motor he has on a stand so he hasn’t messed with it.