Extra headshim needed on OS91HZ with 20% Rapicon fuel?

[cmouton]

Hi, I have been using 15% nitro with my OS91HZ. I want to convert over to 20% Rapicon and want to know if I must add a extra headshim? Thanks in advance.

[cmouton]

Are there anybody with experience for my situation? Thank you!

[marc8090]

Try it first just the way it is. I have run my 91HZ's on 30% Cool Power shimmed at .004, .008, and .012. It runs a tiny bit smoother with more shim, but is down on power. I was running the stock .004 shim for a long time with no issue and only tried thicker after realizing I had never put a thicker shim in during a bearing replacement. That is when I tried both thicker options. Pretty quickly went back to the stock thin shim. There is no tuning issue, or constant lean run symptoms. Nothing to indicate that .004 is too little shim. The thicker shims do give a slightly less sensitive needle valve, but it is not necessary. If you want the power, leave it alone. If you want it a tiny bit smoother and less sensitive to needle valve settings, then add more shim.

[cmouton]

Thanks Marc8090, I also think I must try it first as is and see what happens. I will richen up the needles before the first start up with the 20% and then retune the engine. I forgot to add, the areas where I fly 95% of the time are 2780ft above sea level and the other flying site are 1500ft above sea level.

[marc8090]

I am less than 100ft above sea level. Now that I know you are higher in elevation I would definitely try it first on the stock shim.

[cmouton]

Ok thanks! I'm going to give it a try and will let you know. I will get the fuel somewhere in the next 2 weeks.

[RontaryUS]

Never added shims on my 91hz running on 30% (kept it stock) and it flies great.

[cmouton]

Just feedback, I tried the 20% Rapicon and no extra shim were needed! I love this fuel!