OK, got my amp installed with the Polk 6.5 speakers. Everything sounds good, with bike off plays normal. I start the bike and increase the volume and the right speaker peters out....still plays very quietly but no volume at all..?? Anyone have a clue? It is a cheaper amp from ebay, but it seems to work while bike is not running. What u guys think? Thanks

 

Check all your connections. Something is shorting out.

 

What make and model Is the amp?

Amp possibly going into protection mode with the bike running and supplying more voltage which will increase the output
Did you set the gains of the amp with the bike running or off.

 

If you have a spare speaker lying around switch it out with the one that's playing at low volume and see if it does the same thing. I've ridden this horse before and I'm thinking it's the speaker. Polks are notoriousl for the scenario you described. Especially if you have an older set.

 

I have swapped the speaker out with another, and it does the same thing.. I found that if you really put the volume to it it will start to crack and pop and come on for a few seconds then cut out again??
I set gains both ways on the amp with same outcome.

 

Okay now were getting somewhere. What amp? Is the issue isolated to one channel or both? Check your RCA's and ground. If the RCA's and ground is tight. Sounds like your amp may have an issue.