That #4 module had a single "step" in the timing curve - and it was way to low in the rpm range to work right. Or maybe I should say work without issue. Some of us have the engine running pretty good with it. The step is at something like 2200rpm if I remember right. TR is correct stating many baled on that module in favor of one rated for 8.4v. If you see yours has that 8.4v rating, this step thing is not your issue. If you do have the older one - try going rich as stated earlier......
It's a cheap something to try that will only take you a minute.
If you have a 20 that's never burbling, I doubt it's tuned right. Great engine, but they run rich when they're right. ;