OS 25FP with Venturi running poor

Is the OS Venturi Part #22312000 for the 20,25FP-S the right size for the engine?
I've tried a OS 25 FP engine, stock OS muffler, with a venturi mounted in a Flite Streak ARF. I've had a few so so runs but I can never get good consistant runs. I've modified the front fuse when it broke off with a solid core and hardwood mounts. With the engine running OK I can get just under 5 second laps with 60' lines. I've tried several different types of fuel tanks; pressured one supplied with ARF, Pressurized 4 oz Sullivan, Uniflow. I've tried several mixtures of fuel and some with 5 % Nitro and 25 % Castor but the motor doesn't run much different. I've tried several sizes of props, MAS & APC; 9x6, 10X5 but the 9X4 MAS works about the best for the higher RPM. The field elevation is about 3200 ft.

The engine usually never hits the peak RPM when the needle valve is adjusted leaner because as soon as the engine gets near the peak RPM the engine dies. So I run it quite rich and I have found that the engine speed seems to change. Even running rich and in flight the engine quite often dies or looses power.
I've tried another OS 25 FP engine and removed the Carb and installed a new OS Venturi #2231200 and new OS NVA #22311000. But this engine does not perform any better.
Another flyer at the field has the FS ARF, modified front fuse, and uses a Magnum GP 25 with the carb and it flies great with about 4.5 second laps on the same 60' lines.
I've tried running the OS engines with the carb on the test bench and they seems to run fine but I've not tried them flying in the air with the planes fuel tanks.
Any helpful information will be appreciated. Marvin.

LAST EDITED ON Nov-07-04 AT 10:55 PM (CDT)
 
Do you know what the venturi size is? I've got a Streak with an FP25(no muffler) running a 250 venturi with the green air filter on it and using an OS NVA and it's doing well. The fuel is Powermaster 10/22 and 5/22 and the tank is a 3.25 OZ chicken hopper. There are a couple of flyers here running FP20's and I believe they are running a 228 venturi with an ST needle valve assembly. Can you tell if your problem is with fuel draw or something else?

Hi Elwyn. I've checked the venturi and it is a .250. I've tried the regular tank and a 4 oz Sullivan tank on the inborad side of the fuse as some viewers are using. None provide constant flow to the engine. Is there a better tank to use that you suggest? Should I try a bladder tank? I'm not sure how to mount one as I have several that build for another plane and I could try it.

I guess I'm not sure how to tell what my fuel problem is. I thought I had tried the normal checks on the different fuel tanks with no improvements. So I questioned the venturi size. Leonard had stated on an earlier thread that he had found the Magnum venturi was over sized. But since your OS 25 FP has the same venturi and you have no problems then I need to look at other fuel problems.
Cheers, Marvin.

Hmmm? Strange. Should be a good combo but lap times should be closer to 4.0.
Ever try an APC 9-4 and 10-22 Powermaster fuel?
Is the needle responsive on the ground to settings? Should go from dead lean to blubbering rich in about one turn or maybe a little more. If not suspect trash.
Is is possible that all the fuel you have tried is somehow contaminated, full of crud, etc.

I've had good luck with my 25FP on an ARF Streak. I have the .250 OS venturi, the "stock" muffler, and run a 9X4 APC prop. The tank is a 3 oz. GRW uniflow made for short nose profile airplanes. I use muffler pressure to the uniflow vent, with the overflow plugged. The reason I put stock in quotation marks relative to the muffler is that there are 2 different ones that OS shipped. The one I'm using is a longer, skinny one. There is also a short fat version that was shipped with some engines that I haven't tried.

I use 10% nitro, 22% oil (half castor) and set ground rpm as 11,800. The run is a fast 2 cycle that is very steady. On 60 ft. lines, lap time is consistantly around 4.8 seconds. Running this lean, I usually fuel with only 2 oz. The full 3 oz tank runs 8 minutes, 35 seconds.

We are at 2500 ft altitude.

Steve
The older I get, the better I used to fly!

LAST EDITED ON Nov-07-04 AT 08:54 PM (CDT)
 
With a .25 in venturi and a 9/4 prop the motor should run and needle fairly easily. On the ground you should be able to gradually turn the needle valve in and the motor gradually pick up speed, start burping between 4 stroke and 2 stroke, and go into a solid two stroke. Then go a bit further and it will either start to sag or quit from being to lean. If it will not do that, it either isn't broken in(maybe an iron piston version?) or it has other problems.

If the motor is well broken in and still wants to die instead of speed up when leaned out it may have too tight a main bearing.(not likely with OS). Or it may have a bind from crash damage or improper assembly.

You also may be getting fuel feed problems due to vibration. Any bubbles in the fuel line at any time? There should be none.

You can also try and 8/4 wood prop. this will be plenty to fly a Flite Streak and puts a lot less load on the engine, making it easier to set.

Also, try sealing the O ring area on the venturi with silicon when putting it in. I got burned with this just lately.

Are you sure the backplate is well-sealed? Screws tight, gasket intact? Any leak here can cause several strange symptoms.

I agree with Ralph. I've recently had this problem with a 35FP and tried everything you all have talked about, then I took off the back plate and found the gasket was crushed to nothing. I replaced the gasket and that was it. The engine responded to needle adjustments and full power was right there when it was needed.