Bigshot: I'm using Fiocchi (#616) 209 primers in my CVA Accura. I typically have to clean the carbon out of my breech plug every 20 to 30 shots. I am going to experiment with more frequent cleaning to fine tune accuracy.
Here is a very good blog on some issues to consider when selecting a 209 primer.
http://blog.cva.com/muzzleloader-cva-accura/
Here are two good posts on the Blackhorn 209 website on recommended primers:
http://www.blackhorn209.com/specs/primers/
http://www.blackhorn209.com/2011/10/which-primer-is-best-for-igniting-blackhorn-209/
Some common issues in using Blackhorn 209:
1) One problem when shooting Blackhorn 209, is having a primer that is not hot enough. Avoid special muzzleloading 209 primers.
2) Another problem is having a primer that is too "hot" and the primer itself exerts too much pressure on the load and pushes the bullet up the barrel before the powder ignites. This has its own set of ignition and accuracy problems.
3) Also as Old Hunter mentioned, the breech plug needs to be Blackhorn 209 compatible. Many breech plugs can be modified. In the past, the guys at Western Powders (Blackhorn 209 manufacturer) have been helpful with this.
4) Make sure you keep the breech plug clean. Occasionally use a close fitting drill to chase the carbon build up, and a torch tip cleaner to keep the fire channel open. Some guys are going to rifle primers in a 209 adapter to avoid this carbon build up. I have not tried this yet but it looks promising. Here is a link on that solution:
http://www.prbullet.com/lrvf.htm
TROUBLESHOOTING: There should really be no noticeable delay between the "hammer falling" and the load going off. If there is, you need to chase the problem down, and it is typically one of the four things above.
It is definitely worth working through the issues. My Accura worked right off the bat with the Fiocchi primers. Blackhorn 209 is awesome powder and definitely worth the effort to get dialed in.