My four stroke dead stick today, first time I have had this happen to them in three years. I started it up for the first time since Oct. of last year and I thought like it felt a little soft for the compression when I turned it by hand.

I did prime it and it started right off. Did a short run up and off to the air we went. I was about 5 or 6 minutes in on air time and the engine just stopped like it ran out of fuel, which it still had over half atank left when I landed.

After landing I tried to restart the engine and I noticed I have no compression now, I can't even get it to suck up fuel to prime it now. What happened?????

I am running this engine on a 4* 60 so it is getting air and I run all my engines rich, well I guess I should say to rich. I think I would rather run rich and lose some power rather than burn it up. This will be my third summer with this engine so I am guessing approximately 5 gallons of omega 15% fuel has been thru it.

I have never done a thing to this engine so I am thinking maybe I am needing to adjust the valves and am hoping this will take care of my problem. Never have done that before, but it does not look hard to do. I am just asking what you all think or can recommed anything else to help trouble shoot this. Thanks.

OS 91 4 strokes are very good engines, have 4 of them and maybe have owned and traded off 4 more. First, try a new glow plug. If still won't run you need to remove the valve cover and see if they are functioning correctly. If so then check the valve clearances, get help with that if not experienced with setting close tolerances. Use fresh fuel and good luck. 5 gallons though not alot for one of these engines could be a little gummed up or a little wear on the valves. Find someone who knows these engines and should be working fine in about 30 minutes.

brooke,

Sounds like the valves but I'd check the fuel filter, all nuts and bolts, and pull the needle valve out and give it a good cleaning. You just never know, it could just be a spec of dirt on the needle valve. Doesn't sound like anything major.

I have an O.S. 91FS in my 4 star and it died like that. It turned out to be a broken valve spring which kept one valve from closing. Might be worth a close look.

Delman is giving good advice, you need to remove the valve cover and and check out the valve springs and rocker arms. If you have been running a castor based fuel you could easily just have a gummed up engine that requires a good cleaning. Look in the exhaust port at the valve stem, does it have a cruddy build up on it?
Fred

I had an OS 70 4 stroke that lost all compression after only 5 flights......turned out to be a broken valve spring.

Thanks for the input everyone. I will have the time to look into it latter today and i will let you know what I find.

Well, what do you know, I removed the valve cover and found I have a broken valve spring. Now my question is just how do you remove it? I removed the rockers and now it looks like it has a retainer on top of the spring, but how do you remove it. It didn't look like it had a "C" ring/clip on top of the retainer to hold it on. Did I just miss seeing it or what? Also when I replace the valve spring shoould I also order new retainers and/or "C" ring/clip (if any) at the same time. Thanks for all the help.

Page 24
http://manuals.hobbico.com/osm/fs91-ii-fi-manual.pdf
Looks like springs, retainer, split cotter.(split collet according to the part number)

Part Numbers
Stock Number / Mfr Number / Product Description
OSMG9242 45960200 O.S. Valve Spring FS-91Surpass
OSMG9231 45960300 O.S. Valve Spring Retainer FS-91Surpass
OSMG9227 45960400 O.S. Valve Collet FS-91 Surpass

It might be a good idea to work on the valve spring removal with the engine cylinder head in a large plastic bag to keep from losing the small parts never to be found again. Just a suggestion.

Thanks for the info., I see the parts that I need now. So I went back to the shop to take another look and I see the parts, but how do they come off. I do not see how to get the valve spring retainer and/or the split cotter out to be able to remove the spring. Do I need a special tool for this? Or do I need to remove the head and pull it all out from the bottom side? I'm sure it is not as hard as I am making it, but I have never done it before and i would rather do it right than just guess and cause further damage. Thanks.

Take off the head and find a round piece of broom handle to sit the head on. That will hold the valves closed. I used needle nose pliers to depress the springs and a pin and tweesers to remove and install the collets and keeper that retains the spring. I use a baking/cooking bag to keep the little collets from getting lost.

Just to let you guys know I got my new valve spring in from tower last night and just got done getting it put back together. That is one of those jobs that is easy, but a real pain.

It runs great and I am thinking you guys for your input. You nailed it on the head what the problem was.