Hi,
I have little experience with 4-stroke/cycle engines but have recently acquired a Ys91ac secondhand. It has less than a gallon thru it but had been laid up for over a year. I ran it as directed in manufacturers sheet and it ran sweetly (with a slight pressure leak from air-chamber).

Problem is the plug had previously been cross-threaded and I did not realise this and made the problem worse, panicing and trying to fix the problem only made it worse.

Anyway, I've ordered a new head (rather than try to mess with a helicoil) and should have it in around 10days - WHAT IS THE PROCEDURE FOR REPLACING THE HEAD ? I've already popped off the head to check for any shards of metal from the stripping plug inside the chamber - this seems quite straight-forward, but I dare not touch the valves or tappets until I get some advice.

How do I remove the valves, and refit them, do they require any action to re-seat in the new head ? Whats the exact procedure for setting the gaps on the rocker arms ? Is there any special advice on refitting the head to the lower casing ? does the piston ring have to be rotated to a certain position ?

I've also ordered a set or replacement gaskets.

Thanks for your help, hope you have the patience to help me !

A little off topic ( I know you'll get help from someone here), but I have to wonder about the "good" people in our hobby who sell engines with a gallon run through it, but cross-threaded glow plugs., stripped exhaust screws not mentioned. I guess there are all kinds everywhere.
Good luck with the replacement. I'm looking forward to learning from your question.

I thought the head came complete with valves and springs. It is not easy to remove valve and springs; well it is easier to remove but I had a heck of a time trying to install them back in without a valve spring tool (if one exists, someone can enlighten me on that). I had to remove the valves when I wanted to clean the valve seats.

Vim

Well, I replaced my YS53 head once.
It was a little hard, but doable.
Assuming that you alredy removed the old head by removing the four socket head screws that hold your head to the crankcase body, already removed the cylinder liner and piston.

http://www.centralhobbies.com/Engine.../YS91ACEX.html
above is a exploded view of 91AC from Central hobbies. All parts has number and the part chart below the diagram.

You will need a small towel or rag and a wood stick that is preferrably round with smaller diameter that the inside of head and flat on top. I used rubber hammer that has wooden handle.
wrap the rag on the wood. Place the head on the stick. The stick should be mounted on vise or something to prevent moving.

place white papers or towel under the vise and around the work area to cover any dark space. This is to prevent losing small parts such as #5 valve springs, #6 spring retainers, #7 valve spring retainer clips.

I had a real hard time looking for the retainer clip when it bounced because of the spring.

1. Remove the Rocker arms from old head. set them aside. There has to be a screw #35 right in the center of the rocker arm shaft. #34.


2. I do this one by one. removing one valve and install on new head first before removing the other valve.
Try push the valves to make sure they don't go all the way in. (the wooden stick should prevent them moving in.
Use a needle nose plier to push the spring retainer, then remove the clips. set them aside. Pull the spring with spring retainer out.

Remove the head from the stick and push the valve out through inside the head.

Insert the valve to the new head and put it on the stick making sure the valve does not go in. Insert the spring and spring retainer and push the retainer with needle plier carefully and place the clips on. When you release the spring be sure to cover the top of the spring to prevent sudden expansion of spring that may cause the clips to fly away.

Repeat for the other valve.

Install the rocker arm assemply to new head.
install the cylinder liner, piston, air box etc.

Be sure to screw in the tappet adjusting screw #32 all the way in to the rock arm, so that when you install the head back to the crankcase, the tappet adjusting screw can be unscrewed to meet the pushrods. If you don't do this, it may be difficult to seat the head correctly or bend the pushrods.

Once head assembly is back to crank case and the airbox is tightly fit to the carb body.

Adjust the tappets for correct range of valve clearances.

For Valve adjustment. I always try to set them on .07mm.
Use the valve gauge from automotive part store.

Well, I am not good at explaining the adjustment procedure.
You will be better off by downloading the OS 4-stroke engine manual for OS 70/91 4-stroke. It is a PDF document so you will need Adobe Acrobat Reader.

Following link is for manual downloading. Procedures are almost identical except YS use flat screw driver to adjust the tappet adjusting screws while the OS use socket head wrench.
http://www.osengines.com/manuals/

Hope this helps,

Thanks PatternFlyer, that sounds like good advice, I'll be printing it off and once I've got the new head, got the worspace cleared, and a cool Guinness poured, the operationd will commence !

It'll probably be a couple of weeks before I have the head because the UK were out of stock and Central Hobbies are sending one on - but I'll put up a post to let you know how it works out.

Vim, I'll check with Central as to whether the head comes with valves - at $115.50 you'd like to think it does !

Unfortunately,
a new head doesn't come with valves.

I have removed head, muffler, valve cover and airbox in preparation for the new head - just to get familiar with the parts you referred to in your instructions, however you began your post with ... assuming you've removed .. cylinder liner .. is there a special tool to engage the liner, is it threaded at the top ? I have removed liners from 2-strokes in helis before, by jamming a piece of wooden tooth-pick between top of piston and edge of exhaust port on liner and heating outer cylinder with heat gun and then turning crank shaft to push liner out the top but the procedure here is obviously different !
Also, are you aware of a torque setting for the cap-screws which bolt the head to the crankcase - or is it just a case of 'as tight as you can'
Thanks again, I want to do this right !!

It will be a lot easier and cheaper if you send this to YS Service and let tham do it for you. There are 2 completely different cylinder/crankcases for the 91 and they will not interchange.

YS Service
1295 Industrial Court
Gardnerville, NV 89410

Phone 775 782 4562

next time work while the head is inside a large ziplock bag.
worked for me, someone else suggusted it for me onetime.

Max,
I don't think there is a special tool for that.
By the way, 53 and probably 63FZ don't have separate liners, the head and liner are in one piece. (Costs a lot!).

4-stroke liner should not be hard to replace since there is no hole on the wall of the liner. Maybe there is a notch just for direction. No idea on that.

Try to pull the outside skirt of the liner from the head. if it doesn't come out I would use a thin blade between the head and liner skirt very carefully.

I have no idea of torque settings on cap-screws. I always tighten them until it is just tight enough. Well, that's kind hard to explain. I would go no more than 1/2 turn after hand tightening and retighten after a shor run.

Sorry, I can't be any more help on this.

Dave,
I don't know why there are two different crankcase and cylinder heads. Central hobbies still have same part numbers for both 91 AC & FZ.

If they are different aren't they supporsed to have different part numbers?