Hello everyone so the past week I replaced my 408 with a 421. And when I went to start it it cranks and back fires our of the exhaust and once out of the intake. I'm not sure why and looking for ideas is it in the tune? The 408 had 1 3/4 headers and 11.5:1 compression nick Williams tb and the map sensor in the back. The 421 has the sensor in the front a fast big mouth tb and 1 7/8 headers and 10.5:1 compression. Would those few things make the tune bad causing it to back fire. I'm pretty sure the timing is right because I triple checked and the reluctor wheel was welded on by the machine shop so I'd assume that's correct as well. Any advice would be appreciated

 

Does it even start?
You might have a harness flipped.

 

^^^ double check the coil harness?

 

Backfiring out of the exhaust and intake is often times too long pushrods (or adjusters on rockers if applicable, are in too far). Try a compression test to see if the cylinders hold air.

 

Sounds like you either have you coil pack harness's flipped or your pushrods are too long and causing valves to hang open when they're not supposed to be.

 

It was the driver's side coil harness was flipped. When I put then on the brackets I didn't even think to make sure the purple wire went to cylinder 1. The car runs great now just hunts for idle which the tuner said is normal because of the different intake I put on it and the other things that aren't the same

 

Good to hear. Tuner says hunting for idle is normal…is he going to fix this for you?

 

it was hunting for idle because a vaccum cap blew off when it backfired. But it still hunted but stayed on this time. As for the tuner he has it now and will fix it. But my new dilemma is that the racetronix pump I want to put it before he tunes it won't fit in the fuel tank because someone put some cheap pump and bucket in their before I bought it. But I seen tick has a setup with the 340lph and a tank that goes right in the tank so I just bought that.