I have an old os 91 surpass. Great engine cant kill it. Suddenly I cant tune the engine. Needle valve is turned all the way in the engine still runs! But cant tune it, when I turn the needle out it richens up quick. O rings?? maybe the old girl is just worn out?

A little more info would help. How much run time, does throttle transition? Could be an air leak someplace. A standard fix for the hs needle O-ring is to cut about 1/4" of fuel line and slip it over the hs needle and screw it back in. Check places like the intake manifold and carb to intake gasket. Could be something stuck in there but it usually just dies if thats the case. Adjust the valves if they havent been done in awhile. I do it about every two years during maintenance.

I do high time maintenance about every 50 to 60 hours. If you're a gear head its not that difficult. Strip down, clean the inside, and decarbon the valve train and head.
Edwin

Forgot to mention, I have four of these engines, older FS. You're right, they're great engines.

Maybe a BIG chunk of crap in the needle valve keeping it from closing???? To big to pass on through, but won't restrict fuel flow either?????

I would take the carb apart and give it a good cleaning.

Ken

Not sure about run time I bought is used and have put 4-5 hours on it so its overdue for a good in and out cleaning, valve adjusting etc. the engine transitions fine I just cant adjust the high end. The needle is all the way in it runs and flys but not getting the rpms I usually get. (didnt put a meter on it). when the needles in it should stop running right?
Heres the kicker I took another complete carb off another os91 SAME THING, needle all the way in to run it now im really stumped???

I've seen the same thing with one of our club members' OS .91 4/S. As it turns out, not all OS needles have the same taper on them and the incorrect one can and will cause this.
Also, if the needle was ever overtightened or a piece of crud was caught in it, the needle seat could be deformed and not allowing full closure.
Just a thought or two.
Good luck!
Jim

This is one of those cases where, if it were me, I'd do a high time cleaning, bearing check, carbon clean up and reassembly. What Jim says would be a big concern. But based on the trouble shooting you've done. I'm stumped. If it was running good and suddenly doesnt run good, the crud would be most likely. All of my OS.91fs were bought used and first thing I did was rebuild. All got new bearing (cause the really did need it) but all other parts were stock and they ran good afterwards.
Edwin

Oh yeah, watch out for the valve stem keepers. They are the round wedges. Best to remove them with the head inside a big baggie. If the springs pop on you, your chance of loosing the keepers is lessened. Been there done that.

Originally Posted by edwin1

Oh yeah, watch out for the valve stem keepers. They are the round wedges. Best to remove them with the head inside a big baggie. If the springs pop on you, your chance of loosing the keepers is lessened. Been there done that.

Good advise. They are easy to loose even if they are off a Chevy small block..... Don't ask me how I know

Ken

I would suggest you take a close look at your idle needle setting. When the O-ring on the idle adjustment give up the ghost, the idle needle tends to move with vibration/impulse shock. And it will leak air and cause all kinds of problems. So, yes, I would indeed replace the O-rings, reset the idle needle and then see what you get.

If the Idle Needle is to far in, it will close off all the fuel, if it is leaking air, it will upset the fuel ratios even though it may be in the right place. The only thing holding it in place is the O-ring. You can slip a piece of fuel line over the High Speed Needle valve, compress it by screwing in the needle and it will seal that off pretty good. But you do not have that option with the Low Speed Needle valve, it is all internal to the carb so relies on the O-ring to hold it in place.

And yes, a piece of crud can be doing some mischieve too. But more likely the old O-rings need to be replaced. They do wear and age.

A lot of good advise. I'm thinking change all O rings high and low needles clean. Check gaskets, adjust valves. test. Do I need to do anything with valve seats, seals.

You might try lightly lapping the valves. A little tooth paste will do the job nicely. You might not even have to remove the valves to lap them. Just pop them up a bit, apply the paste and turn to lap. It he exhaust valve is really carboned up you may consider removing it though. I'm not real familiar with this engine, but I don't believe there are any seals on the valve stems.

Good luck!

Ken

Nope, no seals, just guide. I am a believer of the crockpot cleaning method. What carbon it doesnt remove, it at least softens up. Then Q-tips inside the ports. I use my dremel with the crown wire brush on the seats and the wheel wire brush on the valve stems to remove any stubborn carbon and usually the cam could use a good cleaning. Thats a pretty dirty area.
Edwin

I have 3 of the old 91 Surpass I 4 strokes. Picked up all of them used. I have lapped the valve seats on all of them. Made a significant increase in compression. That is an older engine and without knowing what the original owner has done to it, As long as your working on it I would lap the valve seats.

The valves are not that hard to disassemble. I have used gritty toothpaste or auto rubbing compound mixed with a bit of water to a toothpaste consistency. I like the rubbing compound better. After disassembly put the valve back into the guide, with some paste on it, then take a short piece of small silicon tubing and push it over the valve stem. While holding the valve in place with a fingernail spin the tubing between you fingers.

When done remove the valve and clean the valve and seat very well.

will lap the valves. What is the crock pot cleaning method?? any other advise as far as cleaning fluids to remove carbon and fuel deposits?

Crock pot with straight antifreeze, on low, 4 hours to over night. Remove the carb. Rubber gaskets dont do too good in it. I use some big hemostats to pull the parts out and rinse off. Cleans inside and out. I use a tooth brush, Q-tips, to reach hard to access places and stubborn gunk. For me, I completely disassemble except for the bearings and crank, unless it needs it. Rinse real good and re-oil before re-assembly. There are other cleaners but I like to get the insides good and clean too. For a quick cleaning, I'll remove the back plate, glow plug, carb, and any other rubber. Flush, oil, then re-assemble. Make sure all the valve train parts go back in the same places.

If you forget about it and leave it in the crock pot for days, the aluminum will turn a dark grey and stay that way. Doesnt hurt the engine, just a looks thing. I use an appliance timer set to turn it off after 24 hours. That amount of time wont turn it dark. Need a well ventilated space.

It should go without saying that you never use anything thats touched antifreeze with food again. And wash your hands real good or use rubber gloves.
Edwin

Originally Posted by Co.Rcflyer

I have an old os 91 surpass. Great engine cant kill it. Suddenly I cant tune the engine. Needle valve is turned all the way in the engine still runs! But cant tune it, when I turn the needle out it richens up quick. O rings?? maybe the old girl is just worn out?

I've been running Surpasses (I & II) for about 15 years in competition. Discovered some years ago that over time, the main needle to seat clearance gets larger and larger from vibration until finally one day it won't hold a setting or run properly. You can check by carefully pushing the needle valve side to side while the engine is running. The engine should NOT vary in speed when you do this. If it does then the set is worn enough to replace it. If it won't lean out then it's WAY worn. Replacing the O-rings is unlikely to help much, unfortunately (BTDT). The best and least troublesome solution is to spend $17 or so and replace the whole needle/seat unit. Here's a link to the Tower webpage:

http://www3.towerhobbies.com/cgi-bin...?&I=LXWY32&P=7

Since the Surpasses are no longer in production, there's no telling how long parts for 'em are going to be available.

Good luck with it. Don't forget to check valve clearance occasionally... :-)