How many shims and gear ratio for OS 91SX-Cspec-PS

[Dave]

I am wondering how many head shims I should be using.

I have the OS 91SX-C Spec PS with a Hatori SB18 muffler in an Raptor 90. Stock head but I have added the third needle valve.

I am using Wild Cat 30% fuel.

I am also wondering what the ideal gear ratio would be as well.

I understand I may want to try a few combinations but I would like a starting point.

[marked23]

I was using that engine with the third needle and the MP2. And my fuel was coolpower 30.

On one shim I got a lot of vibration. I added a second shim and it smoothed out a lot. Not sure what to say with your fuel, but for me and my fuel I would start at .016 (two shims).

-Mark

[DavidH]

Stock head ( blue). I use the stock shim (.008). I use CP 30% and SB 18 Hatori.
Motor runs smooth in all speed ranges. I am using 7.91 gear ratio. 7.91-8.2 will be the gear ratio range to be in.

David