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O.S. .91 surpass II not reaching top RPM

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Old 06-08-2011, 04:15 PM
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dingo9882
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Default O.S. .91 surpass II not reaching top RPM

I have been having some problems with my .91 that I just cannot figuer out and maybe you can think of something that I have missed. My engine is not coming even close to it top RPM. It is only reaching about 7200 with an APC 14x6. Now, I have replaced the bearings recently so I thought that might have been an issue. But, I ran a tank or so through it and nothing has changed. I have adjusted the valves from the .004 to 100. (I used the feeler gauges that you can buy from O.S). When that didn't work I thought maybe I had a bad spring, so I replace both the springs. Still, I have gotten absolutly so results. I tried sealing up supposable air leaks with permatex and I still have had no results. I am running Cool power 15% nitro and 17%oil. So, I tried different fuel and still no results. Could I be adjusting the valves wrong maybe? I have checked and rechecked over and over. I even thought maybe I put the piston in wrong and that could do something. Any ideas would be better then what I am going through now. What scares me is that before I changed the bearings I was getting around 9400 or 9800 rpm so that could be a problem to maybe.

Chris
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Old 06-08-2011, 05:12 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

Recheck the position of the timimg mark for the cam shaft to crank position. It's very easy to get it one tooth off and cause poor performance and it is something you moved while changing the bearings.
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Old 06-09-2011, 05:42 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

You'll need to check the camshaft installation. If you're not familiar with the engine, it's easy to get it installed incorrectly. With the piston at Top Dead Center, the dot on the cam gear is in alignment with the pushrod tubes. It is not at 90-degrees to the crankshaft. PLease see the diagram below:
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Old 06-09-2011, 08:28 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

I was actually just about to ask you to see if you could help me with adjusting and checking that. Thank you for your reply, that idea popped in my head but it never occured to me that could make so much of difference.
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Old 06-09-2011, 04:21 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

I just checked the camshaft and I must admit that when I put the new bearings in I must have took the camshaft out of alignment. I will run the engine up and get back to you.
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Old 06-10-2011, 04:53 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

Bax, thank you for your help. You solved the problem. I ran the engine up and it is turning a 14x6 APC at around 9500-9800 rpm. Thank you again.
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Old 06-24-2011, 02:17 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

Bax, does this apply exactly the same to the OS 120 Surpass III also? For some reason, I thought the dot goes opposite where you show it, so that is where mine is now, and it runs, but maxes at 9k rpm with a 16x6APC. I recall reading, probably in error, that the cam dot goes at about 5 o'clock, but you are showing it basically at 11. Just want to confirm before I tear the engine out of the plane again, lol. Thanks for your assistance. Jon

BTW, I bought this engine used a couple years ago, and just took it apart to do the bearing replacement, and that is when I found the cam dot at 5PM on this engine.
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Old 06-26-2011, 06:49 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

Well after reading this far I probably know the answer BUT!

I have an OS FS91 SII-P engine in a scale model.  It has enough power to pull the model but it idles rough at 2100 - 2200 RPM and only hits 6500 RPM on top end swinging a 15 x 6-10 Zinger wood prop.  I have replaced the glow plug with a new OS Type F plug and have new 15 % Nitro fuel with 18 % oil. (Klots and Castor blended)

Should I expect more top end RPM?
Will the cam being out of time cause the engine to vibrate a lot at idle?

Thanks in advance,
Clancy 
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Old 06-27-2011, 06:41 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

All of the O.S. 4-stroke, single-cylinder engines have the camshaft set the same way.

Clancy, you just have too much propeller on your engine. Go with a smaller prop to get it turning around 9,000 RPM or better. The engine will run better and set up better all the way around. We'd suggest a 12 x 10, 13 x 8 or so. Don't worry about scale looks. You want the engine to turn around 9,000-10,000 RPM at full throttle when leaned to just rich of peak RPM. Then, any prop that gives you that RPM and will fly the airplane is the "correct" propeller. Go for a scale-like prop when you're setting the model up for display. Fly what makes the combination perform best.
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Old 06-28-2011, 11:25 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

Bill
I have commented on three of the RC Fourms about how my OS FS91 SII-P engine has been acting. I got everything from add more nitro to get a smaller prop to check that the cam is properly timed to the crank.
Yesterday I spent two hours taking the engine out removing the cam cover and bearing and checking that the Cam was timed correctly. It was OK. Today I called three hobby shops about 15 x 5 and 15 x 6 molded props. One Chain HS said that they had the props in stock.The wife and I drove 20 miles only to learn they had 1 each Evolution 15 x 6 molded prop in stock and that they do not stock 15 x 5 molded props.
The test comparison between the wood 15 x 6-10 and the molded 15 x 6:
J. Zinger 15 x 6 - 10 wood weight 1.395 Oz. - Idle acceptable at 2100 - 2200 RPM, Top speed 6500 RPM.
Evolution 15 x 6 moldedweight 2.880 Oz.- idle at 1700 RPM (idle stop set at 2000 RPM), Top Speed 7500 RPM. I will never need that much power on the Taube.

The build of my Taube is under"RCScaleaircraft." It had the highest Static score at it's first contest but finished in Third palce due to the engine not set correctly.
NATSstart in 3 days.
The plane is in the van!!
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Old 06-29-2011, 05:55 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

OK. If you insist on running the engine with such a propeller that it will only turn the RPM levels you mention, then you're going to have problems. You need to get the engine running up in its preferred power band to get smooth operation. If you have the full-throttle RPM held down by a too-large propeller, then the carb won't be properly-balanced between high-speed and idle mixture settings. Get the full-throttle RPM up to the high-8,000's, at least. Don't worry about a "scale" propeller. Props can't be seen in flight, and a reliable engine should be the priority for flying.
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Old 06-29-2011, 10:07 AM
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Default RE: O.S. .91 surpass II not reaching top RPM

A Friend is bringing a 14 x 6 molded prop to the NATS for me to use.

Will report on the performanceafter I haveflown the Taube.

Here is a picture of my show prop. It is 17 1/2 Inches long.
Thanks,
Clancy
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Old 07-03-2011, 04:56 PM
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Default RE: O.S. .91 surpass II not reaching top RPM



Saturday came up windy and gusty.
The perfect weather to keep the Taube in the car but that was not what I did. I had full Wing Warp and Rudder offset cranked it but the wind still got under it and turned it in some. The model tried to save it's self but the three point landing was two mains and the prop.



Both SIG Glass Filled motor mounts broke at the rear motor mounting bolts which prevented damage to the engine and airframe. If I had had a spare set of mounts I could have flown today.



The big problem is that the engine is still down 1000 to 1500 RPM from what you tell me it should be turning. Saturday morning I mounted one of the 14 x 6 APC props and made my first attempt. The plane had NO pulling power and it could barely make headway into the wind and as soon as it got up wind even with full Wing Warp and Rudder offset it turned in some but it tried to save it's self. Three point landing, two mains and the prop.
Itore the engine down this evening and the only thing I found wrong was the nipple beside the Cam that connects the crankcase to the intake manifold was loose. I tightened it and as soon as I get some new motor mounts I will test run the engine. It seems that the smaller the prop I put on the engine the lower the pulling power at full throttle.
Clancy



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Old 07-06-2011, 02:48 PM
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Default RE: O.S. .91 surpass II not reaching top RPM

Bax and all who commented,

Today I verified the loose nipple was the problem. A month ago my oldest son's table saw died. I asked for the steel stand it was on. Today I was up in the attic and found the Easy Just motor test stand I bought while a teenager. Price on the bottom was $1.30. It is made ofhard maple wood. I have never ran any thing bigger than a 35 in it. The FS91 fit and by starting it at lowthrottle, and immediately getting behind the prop,I thought it would be worth a try. I bolted it to the table saw stand and lashed the standto a 17 inch Green Ash tree.
I had tightened the loose nipple about a full turn.
First run - Evolution 15 x 6 prop - 7500 RPM. Last time it peaked at 7000 RPM. Did not touch the needle valve. Loosened the nipple 1/2 turn.
Second run - 7000 RPM. Re tightened the vent nipple.
Thirdrun 14 x 6 APC prop - 9,100 RPM. Saturday on a 14 x 6 APC prop It could not power through the wind like I needed it to. There was no 9000+ RPM available!! This is the highest RPM I have seen on the Tach on this engine. Started to adjust the needle to see how much more was available when it started to die, the two ounces of fuel in the test stand tank were gone. End of testing.

It is important to understand the key definition of how this engine operates to visualize the problem I was having.Note: there are TWOnipples connected directly to the crankcase. The first is the common breather vent which on this engine isplumbed to the intake manifold. The second one ismounted in the center of the backplate and is connected to the Fuel Pump.

"The varying pressure in thelower crankcaseactuates the diaphragmin the Fuel Pump allowing it toprovide sufficient fuel, under pressure, to the Pressure Regulator to allow it to provide the required amount of fuel, at a constant pressure, to the Carb."

The loose crankcase vent nipple was causing a loss of varying crankcase pressure preventing theFuel Pumpfromproviding sufficient fuel to the Pressure Regulator to feed the engine at high power settings. It was acting like a Governor!!.
Clancy

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