Sailcloth too THIN

I have a split seam in my largest genoa. It is very light dacron, I don't know the weight but it feels nearly like spinnaker material. I can not sew it at all. It seems as if the timing is all off, there is only very occasionally a 'knot' made  below the fabric. I am using V-69 with a #18 needle and I have increased the pressure quite a bit from what I normally use. I also switched to a #16 needle, same result. This happens both with straight stitch and with zig zag.

With the exact same machine setting and thread, I can sew other fabric (sunbrella, denim, ...) without any problem. Just for kicks, I just put some paper tape (the left-over 'cover' from the basting tape) on top of the seam and it worked perfectly! This tape itself if quite slick (very little friction) so I think the problem is just that the sailcloth is very thin.

Any ideas how I can sew this?

Thank you!

I watched the video, very instructive! I don't get any of the pucker shown in it, my problem is that there are simply no knots formed. But I suppose your diagnosis is correct, I have too large a needle and thread. 

I will let you know how that goes. And thanks again!

As promised, here is my update.

Unfortunately, I could not get it to sew consistently. I used the V-30 with #12 needles and sometimes the seam turned out very nicely, at least for a straight stitch. But it was just very inconsistent: most of the time there were problems, skipped stitches (many!), broken thread, even a broken needle. In the end I was worried that I was not only getting nowhere fast but I was also punching so many holes in the sail with all my practice seaming that it might weaken the sail. 

I have no explanation what went wrong. I have sewed sailcloth with great success previously but it was all much heavier fabric (mainsail, working jib etc). 

So, after some playing around I broke out one of my other sewing machines, a Singer 99K (made in 1955 in Scotland!), put in the V-46 thread I had bought (the V-30 still being in the 'big' machine) and it worked beautifully! The machine sewed it like a charm, with absolutely zero problems and zero experimentation (I did not even change the tension, and I used whatever needle happened to be in the machine). Of course it has no zig-zag so I seamed it twice (it was just repair work so not too much length). It was such a pleasure to use this machine!

I am sure an LSZ can be made to sew this material (I have seen it on the Sailrite video) but I just don't have the experience and expertise. So I will just use the Singer for the really light fabrics, like spinnaker, drifter etc and reserve the 'big' machine for heavier sails, and of course canvas. 

Again, thanks for your help!

Can I send you email about this? 

Thanks