Hi all,

I have a moderately tuned jdm s5 with an odd misfire - my tuner is busy / difficult to get hold of at the moment so I was wondering if there is anything i can check on my own in the meantime. Here's a rundown of the spec:

Freshly rebuilt 13BT major extend street port
uprated custom apex seals
HKS T04E
Mapped for 99 octane (UK) on a PowerFC @ 0.8 bar
FMIC (I have to admit the hotside pipework isnt great, and needs re-doing)
1680cc secondary injectors, stock primaries
FD MAP sensor (no AFM) and IAT
FD intake (inlet elbow, plenum etc)
FD coils & leads
FD loom
Koyo rad
Relocated battery
Re-wired stock fuel pump
HKS Twin power (see below)
BUR9EQP all round

2nd gear WOT - fine.
3rd gear WOT - somewhere around 5000 - 6000 rpm bad ignition breakup. Backfiring a bit and loss of acceleration. The engine just feels like its struggling to pull. Once i get past 6k i get a big backfire or two and seems ok after that. If i have my lights on this is sometimes accompanied my a blown lights fuse
4th gear - initially a backfire or two, then fine.

First, I changed my plugs (no difference). Then... I tried unplugging the twin power - this makes a big difference. Misfire is still there (sometimes), but I dont get the noticable loss of power. Maybe 2 or 3 big misses and backfires, but still pulls well. I can also feel the misfire off boost occasionally, but with the twinpower unplugged it isnt regular or reproducable.

So.. it seems like the twinpower was somehow making an underlying misfire worse??

Has anyone else had anything similar or can suggest anything to check next?

Thanks in advance...

 

I should say that all of the FD conversion parts were done as part of the rebuild, so i believe they would have been sourced in good working order.