I bought a used model and it came with a YS FS91AC engine. Does anyone know what the large chamber on top of the cylinder does? Also, why is the intake routed down through the rear crankcase before going back up to the intake chamber?

Thanks,


Marco[8D]

The box is an air chamber. The YS is typically described as super charged and pumped. The 91fz is the later version, but very similar to the 91ac.

Go here to download the manual on the 91fz
http://www.yspower.co.jp/pdf/fz91-e.pdf

It is safe to use the fz manual to operate the ac

Edit. The case on both ac and fz has ac cast into the case. There is a minor difference within the airbox that sets them apart. Your engine could well be the fz

Argueably, the YS 91 puts out about the same power as many other 1.20 size engines. Typically it is run on YS 20-20 fuel, but will handle 30-20 heli fuel without incident.

Good job J-R.

There are two main differences between the FZ91 and the AC91. The FZ has a tapered collet retainer for the prop drive washer whereas the AC has a woodruff key type. Also on the FZ the carb body on the back of the engine is slightly larger but not enough to be apparently to the nacked eye.

In the beginning, the only way to tell a FZ from a AC was by looking for the collet driven prop drive washer because they both said AC on the case. Right before they quit making the FZ they cut off the AC part on the case and then, eventually, they started stamping FZ on the lug. But the majority of FZ's say AC on the case.

The engines run pretty much the same and it doesn't make much difference as to which one you have unless you are going to buy a crankshaft, prop drive washer, carb body, or barrel.

Enjoy, your engine

Jim

on the low speed needle do you turn it the oppostie of the high speed for rich lean? one other ys said it and I didn't know if it was this one

The LS needle is an air bleed device. Therefore, it turns opposite the HS needle

Jim