you just changing the pipe or re-jetting the carb as well?

rfvallejo wrote on 12/31/69 at 16:00:05:

Well then you only made the stock lean condition even worse.  I'm surprised it didn't run like crap.  It almost certainly ran hotter.  When you open up the exhaust, you increase the airflow (at least until you reach another bottleneck, like the air filter), which makes it horribly lean.  The stock mixture actually has too much air (or too little fuel) in its fuel/air mixture.  When you add more air by changing the exhaust, you MUST add more fuel.  That is accomplished in various places: pilot circuit (air mixture screw and pilot jet), main jet, and the needle position (white spacer).  There are a lot of posts on this topic and you really should read quite a few before just swapping mufflers.

Rejetting will be necessary, at least the main jet.  The pilot jet may not require changing if you can adjust the pilot screw sufficiently.

Please take a look at the Technical Documents/Reference forum and read some of the carb and tuning posts like:

Mid-Range Power & the White Spacer (Backfire)

Lancer's Straight & Simple Carb Tuning

A Carburetor Primer

Carb Specs.

Carburetors 101

Idle Mixture Screw

Also, please be sure to read the sticky post near the top of that forum entitled Posting Guidelines: DO NOT POST QUESTIONS HERE!.