Backfire on transition from full throttle

[manga_blue]

Can anyone explain for me what's happening here?

I get a backfire down the exhaust immediately after coming off full throttle at high RPM. This might occur either during an upshift or else after just plain backing off before braking.

AFR readings at WOT seem OK right through the range at 12.5-13.5 then go rich to around 10 for just an instant when I do back off. The rear bar is always filthy brown and splattered with spots of tarry black crud after a track day.

[Steampunk]

what's the condition if your catalytic converter? My guess is you've burned it out and there's a big hole in it.

[Hammer]

Backfire is usually a symptom of running a little on the rich side [where the unburnt fuel leaving the combustion chamber is ignited by the hot exhaust - in case you didn't know]. The other tell-tell signs you pointed out also points to running rich.

Are you sure your AFM is accurate? The sensor might need a clean - I had that issue on my SP few years ago which Mazda immediately replaced under warranty.

[Benny]

Easy solution, don't back off the throttle!

[project.r.racing]

backfire = sexy noise.

[StanTheMan]

I have the same issue .

I'm currently working to improve the predicted map table on the adaptronic.

are you running an aftermarket ECU?

[manga_blue]

It's an original Mazda ECU, STM, although the fuel map is slightly 'improved' - nothing dramatic, just a few tenths of AFR changed here and there from the original map. Also fairly standard NA8 motor, including standard cams.
It looks like a whole load of fuel gets dumped in the motor just as/after I close the throttle and that this then ignites in the pipes.

You've got me worried about the cat, Andrew. It's big, newish and fairly exxy. No holes in the side although the car's been developing some unpleasant droning and raspy noises lately.

[StanTheMan]

right

NFI.

mmmmm.

[Steampunk]

It's simple enough to remove, so I would do a visual double-check and rule the cat out of the equation.

When you say the ECU is "slightly improved" do you mean via that crystal mod thing? (I'm not sure what that is exactly)
Perhaps it screwed up the overrun setting? ie. instead of the injectors shutting off when the throttle is closed, it still pumps fuel, thus creating an overly rich mixture for a split second.

Bad batch of fuel maybe?
Fouled plugs not allowing complete combustion?

[StanTheMan]

is the ecu re-flashed or something?

1red5
The original idea of the crystal mod was to be able to increase the rpm on a standard ecu. Not because its required but because some one wanted to do it for the sake of it. However the side effect was that it run a slighly leaner fuel at high rpm which gives the mx5 an ever so slight increase in power. With the NA6 afm you could then take it a fraction further & adjust the air/fuel mixture.
But I can assure you gains were minimal....barely noticable. Main difference were slightly smoother idle, Increase in RPM for that very occasional stretch of racetrack where you don't need to change gear because you haven't hit the rev limiter.

in my case....I never had any backfire issues. Probably because it was running leaner anyway. I only ever got to the rev limiter twice after doing this mod. Once to test it. & 2nd time just because I wanted to. Never got near it after that. I still change gear at about 6.5-7.0....
My current max rpm on the adaptronic is set at 7700. I've never got there.

[Steampunk]

ah okey dokes, I told you I didnt know much about it.
Phil, I think Benny hit the nail on the head, don't back off and just do flat-shifts and you're shweeeeet

[manga_blue]

ECU is socketted with revised fuel and timing maps on the new ROM. It's a long, ongoing story on miata.net. So far only the 28 and 32 pin NA8 ECUs have been cracked but they're circling on NA6 and NB units this year. It's the best way I can see to get an ECU that allows rev limit, timing and fuel changes but retains stock driveability and features.

23 page read here http://forum.miata.net/vb/showthread.php?t=226162
As I said the changes to fuel on mine are very minor.

[93_Clubman]

MB, my first thought when I read this post the other day was what's been done to your car recently, as my understanding was that the socketing & remapping performed successfully for at least 6 months?

The following provides a summary of socketing & remapping:
viewtopic.php?f=55&t=38209

[bretauto]

Air leaks(exhaust leak) in to the exhaust will cause a backfire especially with richer fuel mixtures.

[StanTheMan]

Air leaks(exhaust leak) in to the exhaust will cause a backfire especially with richer fuel mixtures.

you're sooo right......will check that myself too.