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Hi,

I've been search for a while now and i haven't been able to find a solid answer other then it being caused by lean mixture.
When decelerating then tapping on the throttle it would backfire, this doesnt happen all the time though.

So what would cause this? Tune at fault here? Or is this a side effect of itb?

I'm no Efi expert but I would have thought when you tap the throttle the mix gets rich as part of the acceleration enrichment. So do this on the overrun and you've got excessive fuel with low air flow hence loads if unburnt fuel. Are you running a hot cam?

you could be right? I'm no expert either.
Cams yes 256IN/EX, but i doubt this would cause the issue.

Sounds like ignition and cam timing to me. IF your sure it's coming out the intake, then it has to be sparking the mixture while the intake valve is still open.

Lean mixture is the cause of backfiring out the inlet about 95% of the time. Blocked exhaust can do it too. Really bad cam timing is another cause. I've also had it from a sheared exhaust cam lobe. With the exhaust, cam timing or cam lobe it would be constant, not specific and momentary like yours.

I'm not sure how EFI systems work in terms of an initial charge when you blip the throttle. If you had a set of Weber DCOE sidedrafts there instead of the ITBs then I'd say you definitely had a problem with your accelerator pump or accelerator jet. In a carby a quick opening of the throttle results in a small pulse of extra fuel through the accelerator jet, momentarily enriching the mixture for the transition from overrun to power specifically to prevent that little backfire. I suspect that if you were tuning with steady throttle on a dyno then you'd have OK mixture for that steady state but still miss that little extra for transition. I assume there's a little piece of code somewhere that says add a little extra for that purpose. What ECU are you running?

I am not the one tuning my car, i have just got it back from the tuner and have noticed this on the drive back home and around.

Manga i think you are describing Transient Throttle same thing Zcootz wrote, and i think you guys might be right when coming off throttle is when lean conditions can occur (right?) and putting the throttle back the ecu is not putting enough fuel to balance out the mixture?

Sail, i some how feel this isn't it seeing as the rest of the car is working perfect.


Ecu is a haltech sprint 500.

Yes well, 14,700 hits on "Haltech Transient Throttle" with all of the first few describing problems like yours. Just take it back to your tuner.

Hi Koop, it could be your tuner or your ecu. I have itbs with a set of tighe 805c cams (274 and 264 deg I think) and whole bunch of other N/A mods and use to had horrible trouble with backfire thru both intake and exhaust and on start up as well
I changed my tuner as I wasn't getting any satisfaction. Things got a lot better with my new tuner but still weren't perfect. Then we discovered my piggy back ecu was also part of the problem. Pretty sure your haltec 500 is a piggy back. He said because it was a piggy back their was a lack of injector timing adjustment among other things (that I have no idea about) and this was not helping my problems. Then my piggy back started to die so a haltec 1000 stand alone went in and I haven't looked back since. He did wonders for my car.

Just my 2c,
can point you in the direction of my current tuner who is also a haltec dealer and knows your ecu well.

Hope this helps :-)

hi mmx005,

I've actually got this sorted, nothing wrong with my 500 (its setup as standalone) or my tuner.
Issue was to do with Fuel Decel Cut and Transiet Throttle, just needed to richen these settings because the itbs have more air flow any subtle changes have a big effect.

Ahh, fair enough. Good to hear :-)