dingo9882

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

skoda450

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.

Bax

My Feedback: (11)

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:

dingo9882

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.

dingo9882

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.

dingo9882

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.

maukaonyx

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.

Clancy Arnold

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 

Bax

My Feedback: (11)

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.

Clancy Arnold

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.

Bax

My Feedback: (11)

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.

Clancy Arnold

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

Clancy Arnold

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

Clancy Arnold