Are you noticing any liquid coming out of the stem end of your pipes (the lip)? I mean we all slobber to some degree and some of that gets caught up in the stem. Couple that with the fact that you're creating condensation each time you puff on it. I find that if I hold the pipe with the stem facing down a little bit the nasty "juice" will run back to the opening (lip) and as a result some will end up on my tongue. That stuff is bitter and nasty tasting.

An experiment to try if you suspect this...You'll need a napkin or paper towel for this
Puff like you normally do
Hold the pipe like you normally do
When you notice the bad taste beginning to appear cover the bowl with your hand, turn the pipe vertically so the stem is facing down at the towel, then gently tap the lip of the stem on the towel. If you notice brown liquid drips on the towel you've likely found your answer to the nastiness.

This used to get me early on in my smoking career because I, like many new puffers, tended to stoke that thing like a steam train. You create a ton of condensation in the stem and you also tend to hear gurgling in the bowl.

If this is what you're experiencing then the answer is to:
1. Slow down your puffing
2. Make sure your tobacco is dried out a little bit. It doesn't need to be crunchy but it shouldn't be too moist (think brand new pouch tobacco). I let mine sit out for about 30-60 minutes before I smoke and that helps a ton
3. Hold your pipe so the stem is facing more up towards the ceiling as opposed to the bowl being higher than the lip

If not, someone else with more experience than I will come along and offer up some more suggestions.There are more experienced smokers on here than myself. That's the beauty of this site.