HEAT RISER VALVE = TOTAL CRAP!!! These damn things always wind up corroding and getting stuck, usually in the closed position causing burnt valves or worse! They also restrict exhaust flow (even when open) which reduces horsepower. And I'm assuming that since you are a Mopar fan you like horsepower ;))
This is the FIRST thing I remove from any car that has one. It's not too hard to remove it, just drop your head pipe and remove the exhaust manifold for access. I removed it from each of the 4 road Runners I have owned, even back in 1970 on my brand new RR. Even back then everybody knew to get rid of those damn things before they damage your engine.
For maximum exhaust flow efficiency out of a stock manifold, it's best to remove the whole thing including the shaft the flapper valve rides on. Then you can plug the holes (from the outside) with a short oversized bolt or have a welder fill it and and grind flat. Lots of options how to do this just make sure you get rid of that damn valve.
Note: When you remove the exhaust manifold take a look inside it. I'll bet it has carbon build up from restricted flow.
Good time to clean it out.