Hi there. I picked up a ys 91AC in good shape off of the net that l had to change the cylinder head on. Well l picked up a head as well off of the net. Now a minor problem. I hope. When l took the engine apart there is a clear plastic gasket between the head and the case. It goes under the cylinder sleeve. I cannot get the sleeve out of the head l picked up. I looked up the engine at the central hobbies site and it does not show a gasket located there. So l am wondering. Do l really need it?

Thanks

Tim


You have one the two types of .YS 91AC engine's offered. Some use the clear plastic gasket and others use an o-ring in lieu of the gasket. I believe the 91 realized some evolution from Japan.

You'll need the other type of cylinder assy. Let me see if I have one.

How much do you have invested in the engine you purchased?

As has alreadt been said, The early model AC had the clear plastic gasket under the sleeve/cylinder assembly. The last model of AC had a o-ring under there only.

If you order as gasket set from central hobbies for an AC, you will get the clear plastic gasket and no o-ring to go under the cylinder.

If you order a gasket set for the 'FZ', then you'll have the o-ring for under the cylinder but the carb gasket may be wrong?

The gasket type crankcase is no longer available.

The regulator diaphragm is not included in the gasket set.

The difference between a late model AC and the FZ are as follows.
The crankshaft/drive washer/key has been changed to no-key. The FZ has a taper lock type front drive washer. Also the FZ has a bigger carb. If you buy a new FZ, it will still say AC on the side of the crankcase.

You need to inquire about your requirements when you call.

Typically, the muffler is what tears up the cylinder head on the AC's. If you purchase a FZ muffler assembly, then the adaptor pipe will break away in a crash rather than tear out the cylinder assembly.

If you heat up the head, you should be able to get the sleeve out. Those sleeves are generally pretty hard, the ones I've seen looked new inside. Heat the head up in the oven on the low setting or warm. Somewhere around 250 degrees F.

When you go to put it back together, generally the airbox gasket presents a problem and if it doesn't then the carb gasket to rotorbox gasket will give trouble. The proceedure is to put the carb/rear rotorbox together and on the motor without the cylinder. Put the box on the cylinder before you put it on the crankcase. Then you set the cylinder assembly down on the crankcase using the o-ring between the airbox and the carb.

These little thin gaskets are delicate and will "suck-in". Then you will have an air leak and you will be chasing your tail for a few days until you figure out the problem. A common problem is that early on a person will realize this and put silicone sealer in the gasket areas. Then first thing you know, there is silicone in the engine or the regulator.

If you take the regulator apart, the *** goes to towards the front. The diaphram has a top and bottom as well as a front.

The idel screw (if you have one, it will be in the center of the throttle shaft on the fuel inlet side) is an airbleed, so it works backwards. Out is leaner and in is richer.

Look up YS engines on Google. There are several sites there devoted to setting up the YS engine. Also central hobbies has a parts exploded view and price breakdown on there.

Good luck,
Jim

Thanks for the reply guys. I am positive the gasket came off of that head when l took the broken one off. Maybe not. I had one of the old style heads that l had picked up as well. But now l have two clear gaskets. The engine l have is the one with the O ring. So l put it back together last night and will run it today. Hey Michael. no need to look for one. I got this one from you. It came in yesterday. Fits lke a glove.

Thanks

Tim

There is room for the clear gasket under the cylinder as well as the o-ring. But with the o-ring, the clear gasket doesn't seem necessary. It is up to you. Now that you have the same ventage AC crankcase and Cylinder, it sounds like you are well on your way to enjoying a Y-S?

Jim

Hello Tim ; I have a YS AC 91 that I got from one of the guys at the field that had been in a crash & broke the cylinder @ the exhaust. I had the machine shop at the factory where I work drill deeper into the head & re-thread with a bigger tap. seems to work fine but I can't get a muffler to fit it now. My question is do you have an extra cylinder? It sounded like in your post that you had both old & new style. Mine is the old one with the clear gasket. If you have one & want to part with it please let me know. Also if you would like I could have our machine shop do the same thing as mine so it wouldn't be a total loss. Thanks Rob

Sorry Rob. Someone else already snatched it up.

Thanks Tim ; just my luck, day late & a dollar short. My offer still stands to have your old one fixed if you want. Thanks anyway, Rob

Thanks for the offer. But l went to my LHS today and he happened to have the right size and length l needed. Reassembled the engine and ran it tonight. Couple of flips and away it went. What a smooth running engine. Now to finish those engine mounts and put it in my chipmunk for tomorrow.

Cheers