Tooka, The harp in the video is a Stage 3 Spiers. Out of box overblows/draws are luck of draw. If they happen to be well gapped, no problem.
Personally when I started being serious about overblows I could not afford (And still shouldn't afford) custom harps. I taught myself the basics of gapping. YEARS later I taught myself the basics of embossing. In my opinion, these two things are VERY easy to learn. I am very uncoordinated and a caveman at machinery yet I can do these two things and have a full overblow harp in around 15 minutes. It is nowhere near as nice as a custom harp, but it is pro level. I can gig without fear.
If you want a skype lesson on this procedure, contact me offlist. If you take one lesson and send me a harp, pay for shipping both ways and agree that if I break it, I am not responsible for it, I will set up one harp for overblows for free.
Thanks for the offer Michael but I had an old C harp that I've been using and regapped to tighter tolerances and mildly embossed the 7 draw. I can now get the Db overdraw 3 times out of ten so I'm a good deal further along than I was. I think with more practice and a little experimentation I should get there. The overblows havn't been a problem. Just that pesky overdraw but I think I'm on my way now. Thanks again for your offer.
