Just built new engine and been mapping it. All is well untill about 6000rpm. It has a backfire for a few revs then clears up on load and in neutral. Number one plug is always coaked up black. We've checked /swapped injectors. Plugs, coil, leads,the map looks spot on. Any ideas please I'm lost now.

 

When dose it do it when you floor it or when you drive it up the revs slow . I ask this as my 3dr had this for a long time it would drive spot on all the way up the revs but when i booted it it would back fire hi up in the revs . It turnd out to be the leads were bracking down and they were to close togever when i moved them apart it sorted it i did put a new set on when i found this out . im all so running wasted spark

 

I'm also on wasted spark mate. I've swapped the leads around and it stays on cylinder one. Cheers

 

It does it on full or part throttle load or no load

 

Its the coil pack mate

im bilding a focus cosworth and we started it up for the first time about 4 weeks back and number 4 was not running right it was sparking but not firing the plug when it was in the head so i put the coil pack on the wifes KA and that run the same . i put a brand new one on and it ran spot on

 

No I've swapped it.

 

Try a new front pully . Iv all so had a lot of problems with after market pullys making the car back fire on start up

 

What management are you running? Surely you can see the output from the crank position sensor to check that? Have you done a compression/ leak down test?

 

I'm running dta p8pro all 4 cylinders identical. If it was the pulley wouldn't it do it all the time not just at specific revs.