I'm installing a HMF utility muffler today. will it hurt anthing if I don't rejet just add the muffler

 

It might. I would definately rejet in order to get the maximum gains from that pipe.

 

You need to put a new plug in and do a plug reading to see where you stand. If you're lean you need to reject right a way. If you're rich, you're wasting potential performance gains from the pipe. If you have a light grey or tan color your okay. I expect you'll be on the lean side.

 

You're still getting the same amount of air into your motor, so with the new pipe you may be a little lean, but they come on the rich side from the factory. To get the most out of the pipe, you need to open up the airbox. Then you will definately have to rejet.
Which pipe did you get. The regular utility pipe or the QS1.

 

yes you should rejet