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I have a MX5 SE, misfires and pops/splutters at around 4000rpm... then it hits boost and pulls smooth to redline...

I changed the sparkplugs gapped to .8 : Plugs are bcpr6e

the car is stock... i have a boost gauge... before I swapped out the sparkplugs it would misfire like that when cold... but when it was hot it went away.. after i changed it..its like all the time... i gapped it correctly too? I am thinking the coilpacks are broken? or could it be the bog solenoid ( boost)

That's common behaviour on modified SEs producing more than ~135 rwkw and is the fault of the OEM ECU because it's not providing enough fuel to the engine. 4500rpm is the usual number people quote for this (use this number if doing Google searches).
At ~5000rpm the ECU switches from Closed Loop mode to Open Loop mode which is why you see the improvement (ie. in Open Loop mode it's just doing a table look-up, supplying the fuel that corresponds to the air provided; in Closed-Loop mode the ECU is getting feedback from a bunch of sensors to calculate the amount of fuel required - unfortunately at around 4500rpm the OEM ECU thinks it's providing too much fuel and actually cuts it, which is why you feel the stumble).

The OEM ECU was designed to make the car feel like it had a small V6 and power delivery was smooth (and dull). It was never designed to operate much beyond this and you start seeing issues after mods creating over ~135rwkw. I quote this "over ~135rwkw" figure since I never had any issues with a FM downpipe, AEM CAI and boost increased to 10psi and that combination generates ~135rwkw. Once I added a 2.5" exhaust and bumped the boost to 12psi, I experienced boost cut problems and the car ran rougher (even after adding larger 305cc injectors).

There are band-aid solutions like FM's O2 signal modifier - this works well for some but not for others.

When I had a stock ECU, I had this and the "boost cut mod" which "fixed" this issue:
http://www.mazda-speed.com/forum2/index.php?topic=11873.0
http://www.mazda-speed.com/forum2/index.php?topic=19608.0

Even with these mods, including larger 305cc injectors, my car ran rough until warmed up.

The best solution for modded SEs is an aftermarket ECU.

FYI: The SE is referred to as a MazdaSpeed Miata in the US and there is a dedicated site for this, http://www.mazda-speed.com. Unfortunately it's now read-only, rather than being shut-down completely. You can still find valuable information there if you use Google to do searches using the site keyword:

eg. use "boost cut site:www.mazda-speed.com" as the search string.

Unfortunately some of the pictures no longer work but it's still the best site for SE-specific issues.

It shouldn't sputter and pop under the "hesitation" though, I would think. And if it's all stock, the hesitation should be a little less pronounced than with bolt-ons.

I would double check your plug gaps and make sure your coils are seated correctly. Are you getting any CEL's? Have you connected a OBD-II diagnostic to check for codes?

TONTON wrote:the car is stock

Given above, engine basic & other trouble shooting docs for the SE at following:
https://drive.google.com/drive/folders/ ... 0xSNGVXMzA

But the car is stock, it for sure doesnt have 135rwkw? There are no mods to it. Bone stock.

OBD2 shows No codes... plugs are gapped corrected, coils seated right.

Cam angle sensor or coils breaking down would be my guess.
Change the cam sensor first. They cost about $50.00
The coils will cost you about $300 if it's them.
Shop around for these prices vary by a lot.

Call Mania speak to Dave

Richard.