I have just purchased a used, but apparently low time, .91AC which has some minor rust marks on the valve rockers and a small spot on the crank counter weight / web area. What is the best and safest method to clean this up prior to running? Because this engine quite possibly has been sitting around for some time what items should be replaced, eg gaskets, seals, as a precautionary measure?
Unfortunately no manual was supplied, and as I have no experience with YS, or any four stroke's for that matter, it would be nice to know how to make the necessary adjustments. Is a manual available for download anywhere?
Are any mufflers available? It only has a header pipe and I would like to quieten it down a little.

You can use the setup instructions at www.ysperformance.com They work for most any YS engine.

If the engine is rusty inside, it will find it's way into to the regulator and cause you all kinds of grief. I would strip the engine completely and replace all the gaskets and o-rings, along with the diaphragm, and regulator plunger. The rust can be wire brushed off the steel parts.

For mufflers try www.pspec.com

I have just purchased a new .91 AC and would like to know what is its output power and the best prop to use in a 3D plane, say FuntanaS 90 by Hangar 9.

Thank you in advance.

IGOS R/C

Use a 16x4 APC prop, 30% Cool Power Heli fuel.

This engine is NEW? We haven't made a 91AC in about 5 years. Where did you get it?

Thanks Dave. I am also wondering whether it would be an idea to install a new piston ring while I have it disassembled. If I did this should I install it in the existing bore as is, or should I lightly hone the bore with fine wet-and-dry emery paper (on the end of my finger?).

If the hot compression is OK, no need for a new ring.

Hi Dave !

Thank you very much for your prompt reply.
Yes, my YS .91AC is new, brand new as all the stuff inside the box (4 allen, 1 hex, 1 check valve, manual, Futaba garantee, silencer and the box itself). I get it from a friend. I know it is discontinued for about 5 years. I also get a YS F-120 but this one is used and so I adjust the valves clearance (was close to 0.55 mm), change the gaskets and clean inside the regulator. Could you tell me the output power for both engines ?
Thank you in advance.

IGOS R/C

91 is about 1.75 hp
120 about 2.1 hp.

Hi Dave !

Thank you very much !

IGOS R/C

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