Changed old plugs (screaming eagle ) with new plugs (screaming eagle) , bike runs like crap. Stumbling and throttle response is worst. Put old plugs back in and runs great. Repeat and same crappy results. Bought new plugs and same thing. Any ideas? Possible tune (fp3) is for old plugs lol? Gap for plugs looks to be same. Im baffled

 

By some 6R12 plugs from HD and try that. Gap them around 35 to 37 thous.

 

Use a plug gapper? Don't trust them to be gapped out of the box.

 

Try changing one plug at a time to id the faulty plug.

 

Sounds like you didn't set the gap.

 

It's something you did..Probably way before the plugs..like your tuner engineering

 

You could try buying something other than that grossly overpriced MoCo crap. That generally is the best way to get quality parts that just work as advertised.

 

Put two stock HD plugs in it.

 

Who would ever trust the gap out of the box, I am not saying the OP did, but you stated looks to be the same, gap them with a feely gauge.

If you hadn't tried a second pair of new plugs i'd thought it could have been cracked insulator or other plug issue.

First double check the gap.

 

You could ohm out the plugs, tip to tip. compare to the old ones, maybe the parts store got a bad batch. Where did you get the plugs from? If you got them from the internet, they could be knockoffs made on the Ho Chi Mihn trail .