I have five flights on the 120 now. Here is my set-up: GP Venus II with O.S. 120. APC 16x8. Green Omega Cool Power 15%. Factory fuel tank in factory position. Engine is mounted 90 degrees with a Bisson Pitts style muffler. Here is the issue: The engine seems to run great on the ground and initially during flight. It will pull straight up after take-off. After several minutes (less than ten) the engine seems to lose power, because it doesn't seem to have as much vertical as it does on take-off. The difference between 50% throttle and 100% throttle is less, even though I don't fly at 100%. Is the Pitts muffler a problem? Is it not keeping adequate pressure in the tank?

It is cooling off in the evening here in Georgia, so I went two clicks rich on the high speed needle. That still didn't help.[:@]

If I would have known during construction what I know now, I would have mounted the engine so that the stock muffler exits out the area factory-cut in the fuse. It took alot of cutting to make the cowl fit over and around the Pitts muffler.

Probably is overheating. Make sure that you have enough exhaust of hot air in the lower part of the cowl. The rule of thumb is 3 times of the intake. Also you could be a little lean.

VB

It has plenty of cooling due to the cutting of the cowl I had to do because of the muffler. And, over 1/2 inch of the head is protruding out of the cowl.

OK. Still need to make sure that you have 3 times exhaust area compared with the intake in the back of the cowl. This will avoid the hot air to remain inside the cowl and overheat the engine. Check or verify also the following.

1. Make sure that is not running lean when uses some fuel. How you adjust the engine?
2. Check tank position. I assume that you don't have a pump.
2. Is the engine new? If not it could be bad bearings.

Good luck,

VB

Try a couple of flights on 5% nitro. The OS 1.20 runs fine on lower nitro fuel, and it may run cooler.
JC

Check this link in regard nitro and cooling. This is very interesting analysis.

http://www.rcuniverse.com/forum/m_75...tm.htm#8069361

VB

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Yes I read that entire discussion and added my 2 cents several weeks ago.
Here's the bottom line:
A model IC engine burns fuel (glow or gas) to produce heat energy and converts heat energy into mechanical energy which turns the prop. If you have more nitro to get more power, this must create more heat for the engine to produce more mechanical energy. (Simple) The OS 1.20 will run well on 5% nitro. It will lose some RPMs and it will run at a lower cylinderhead temp (all other things being equal).
Regards
JC

In the OS web site they recomend smaller prop (Max. 15x6). I am wondering if the 16x8 is too much load for the engine using standard muffler. Probably good with tuned pipe. I don't have this particular engine so I don't know. Check here:

http://www.osengines.com/engines/osmg0650.html

In regard the fuel. I am almost sure that the lost of power with 5% will be too much for this application.

VB

VB Quote: "I am almost sure that the lost of power with 5% will be too much for this application."
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You probably won't even notice the difference in performance in the air.
If there is any drop in power, it will be slight.
If you have a good tach, you will see some difference, but in the air you won't see much difference.
The OS 1.20 is flown in many countries where nitro is not available (banned for illogical reasons).
So the engine will run OK on FAI fuel which is zero percent nitro.
But the engine will start and idle better with 5%.

The OS 1.20 on 5% will provide all the power needed for a Venus II.
You probably won't fly at full-throttle most of the time.
Regards
JC

I normally avoid posting on these threads, but I think I can help a little here. I've been flying a Venus 2 with a 120 Surpass for a few years. I've been using the 120 for about 10 years. I agree with vbortone that it's overheating and what he says about cowl opening. It will overheat if there is too much air getting in the cowl and not enough getting out. My engine is mounted at 45% with the stock muffler and I have never had a cooling problem. If your head is only sticking out 1/2 inch, that is really only the rocker arms in the wind. You need the cooling on the cylinder. Also the 16-8 is too big. Try a 16-6 APC. As for nitro, I use 20%. I've tried as low as 10% and as high as 50%. It ran fine on the 10%, but doesn't make much power. Anything over 25% and the needle becomes very erratic and you can never finish a flight without landing and adjusting it. I'll stay out of the nitro/heat conversation because I don't know who is right.

Fred

BlackS10 will have the opportunity to check and let us know the results.

Thanks,

VB

BlackS10

I imagine you're probably running a OS #8 plug, but in case you're not, that is the plug to use. I've seen several try to use other type plugs in smaller 2 stoke engines and get the same results you are stating. Also, a OS F will work good too.

Todd

Fred and VB,
The original post didn't specify 2C or 4C. "I have five flights on the 120 now. Here is my set-up: GP Venus II with O.S. 120."
Being a 2C flier, I assumed 2C and my comments were for the OS 120 AX.
If this is a 4C we're talking about, then my comments about nitro content don't apply.
I agree that 15% is a good choice for the 4C OS.
Regards
JC

I've got the 120AX and ran both a 15x8 and 16x8, no problems. I would check the fuel line - are you getting sufficient fuel when pulling to vertical? Maybe not enough fuel and therefore not enough oil/cooling? I use 10% nitro, and after a flight of tons of inverted, I can grasp my engine without any discomfort - runs very cool.

There is also a thread on using this engine with a Pitts muffler, and the poster talks about crimping one of the exhaust vents closed to gain more pressure in the fuel tank. Personally I use stock muffler with a perry pump - try a pump, may solve all your problems.

Hey guys, thanks for all the help. I thought I gave all the details, but I failed to mention that this is a brand new 120 2-stroke. Trust me, due to the close quarters of the cowl and a big engine, there is PLENTY of air getting out. I had to do alot of cutting. I have had problems in the past with Pitts style mufflers due to less back-pressure. I mounted the stock muffler in a Pitts style and I'm headed out to see the results. The 120 2-stroke comes with an adapter to do so. The reason I am running a APC 16x8 is because in the review by Jon Lowe on Great Planes' website, he used both a 16x8 and a 15x10 with pleasing results. I have always had satisfactory results with no greater than 15% nitro. I'll check back this evening with the results. Thanks for all your help.

OK. Let us know the results. Do you know if Jon Lowe uses tuned pipe and fuel pump? That will be a complete diferent situation. I agree that the back pressure could be the problem. Good luck and let us know if you solve the problem.

VB

Jon Lowe used a stock pipe and no pump.

Well guys, I solved the leaning problem...however, I am ordering another Venus II tomorrow. Always check your glow plugs for tightness! I was doing my second favorite move (favorite is knife), stall turns, pretty low. The muffler made the engine very quiet, something haven't heard in about nine months. Remember, I sold all my gas stuff and returned to glow. Anyway, the glow plug must have wiggled its way out during the very top of the stall turn. As I came back level, the prop was windmilling, unbeknownst to me. Well, I gave her the throttle and nothing happened. I tried to turn back, but that made the inside wing stall, and you know the rest...[:@]

I was overdue for planting one anyway, its been over a year. I'll be flying another Venus II next weekend. This time, the engine will be mounted at the angle so the stock muffler can exit out the bottom of the fuse. No more Pitts on glow engines for me. Thanks for all your help guys!

Sorry to hear that. Well, good that you are getting another one. I am sure that everything is going to be fine.

VB

I'm sorry for your loss! I dumped my Venus too and ordered a new one immediately. I use the stock muffler on the 120AX, mounted using the OS .90 template in the manual and the extension that comes with the engine (double check the angle so the muffler doesn't rub in the engine compartment). The vertical performance with this setup makes me laugh, every time! It's totally remarkable... Oh, got side tracked My point was that the muffler coming out the bottom looks totally fine, and is practically hidden behind the prop anyway.

Some great advice given to me was to run the engine in with the tank at the firewall. Once its broken in and the needles set, you can use a pump and move the tank back to the CG - the plane flies much nicer that way, and I'm eager to get back to flying the plane that way.

I have a Venus II still in the box and an OS 120 AX. Would it be sacreligious and totally uncool to mount the engine upright? I know this ruins the looks, but I like upright engines. I have tried inverted engines a couple of times but had to flip them over to start the engine.
Regards
JC

I would mount it sideways about 110 degree angel - then the muffler will be on the underside. I had my OS 91Fx mounted that way and have no trouble with starting or running the engine.

And the best of all very little cut out on the cowling, just for the engine head, throttle and the muffler.

I belive you will have a lot of mess on your left ving and left body side if you use the engine mounted upright.

I have used 3 AX engines and had them all mounted at the 135 degree angle, so the muffler exits the bottom center of the plane; they all start on the first flip (yes, I use a chicken stick with my 120AX!) with no problems. This angle keeps the fuel from completely soaking the glow plug during ignition, which some say is a cause of engines not starting when inverted. The instruction manual for the 120AX comes with a template to mount the engine at this angle, and it works and looks good.

If you WANT to install right side up, you can do whatever you want to do; it would require just as much cutting, and possibly be even more complicated. Let me point out that the muffler exiting the bottom (and with the use of an exhaust deflector) the plane requires VERY LITTLE clean up at the end of the day.