something must be restricting your exhaust,
![Head Scratch :headscratch:](https://board.marlincrawler.com/Smileys/marlin/headscratch.gif)
ok i went back and read the first post,do you have a timing light??
if so, set the timing to 5* at low idle, then hold the light on the timing mark and slowly raise the idle up and see if the timing advances itself