Belt walking/jumping grooves

Ive recently got my engine running but have a couple of issues.

One is that I have 12 rib pulleys and currently only using a 6 rib belt. But when I fit the belt on the inner grooves, once driving ( I assume at high rpm's ) the belt travels to the outside of the pulleys.

There is good belt wrap, and its a manual tensioner, which is secured pretty tight. There wouldnt be any real slippage issues yet for all the power/boost Im running.
The grooves appear to be inline with each other.

I assume this means the blower pulley and crank pulley arent running along the same plane ? even if the grooves appear to be in-line ?

Ive already tore one belt due to this, so something is up. Bracket flex wouldnt be a major issue, although it does move a little bit when tensioning.

I can remove or fit thinner washers in the area circled which would move the head unit in the direction indicated.
The belt is travelling in the opposite direction, which makes me thing it is tilted forward a bit ??
http://img151.echo.cx/img151/8190/img1209sc5hr.jpg

The pullies should line up as is unless the pullies were made incorrectly.

When you say to the "Outside" of the pulley, I assume you mean to the part closest the radiator. If so I would assume it is because the braket is flexing and pulling the "Outside" edge of the pullies in thus making the belt walk to the outside.

That is my guess. If it is not coming off now, then once you put a 12 rib on there you should be fine.

I know you said it was hard to find 12 rib belts, so you could try a 10 rib an mount it on the outside 10 ribs and see if it stays on. My guess is it would and you'd be fine.

I do have a couple of 10-ribs ordered, but still waiting on them. Though I'd like everything to be fine with the 6 first. I'll try re-shimming the bracket and see how that goes.

Try to mount it to the outside and see how that goes.

Correct me if I'm wrong, but would two 6 rib belts work?

I tried removing a washer from behind the bracket. Ive only gave it a couple of blasts, but belt is still sitting on inside grooves where I fitted it.

I did try 2x6 ribs initially, but they walked, and one got tore up. Perhaps if the alignment was good, 2 might work.
But when sourcing belts, I also found that getting 2 the exact same length was difficult, despite their part numbers.