you should ask in my link I posted here, I would bet some one will know,,
everything I have read tells me the 91s are strong,, but their Needles are loose, and the case on my 180 4St is rather soft, and you may need to use a regulator as I did need one,, other than that they are usually strong engine
http://www.rcuniverse.com/forum/glow...-overhaul.html
Jim