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O.S. .46AX Tuning

I recently purchased a slightly used O.S. .46AX. I installed it inverted in a Seagull PC-9. The centerline of the fuel tank is approximately level with the carburetor. I also installed a Great Planes quick refueling jack in the feedline. The jack is approximately 3/4" above the centerline of the tank/carburetor.

The engine cranks very easily and has a low and steady idle. However, I am running in to problems when attempting to adjust the needle valve according to the owner's manual. I am finding that the engine reaches maximum RPM with the needle valve within a few degress of fully closed (clockwise)! Otherwise it stumbles and hesitates when the throttle is opened fully. It revs fairly smoothly after adjusting the mixture control valve slightly clockwise from the factory setting.

Is there something I am doing wrong here? I have used other O.S. motors in the past and was able to tune them according to the owner's manual.

Thanks!

Reply to
Patrick Johnson
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It sounds like the needle valve is faulty or dirty. It'll probably run OK the way it is, but you may want to try cleaning it, or just getting a new one.

Reply to
Tim Wescott

Sounds like someone forced the needle into the seat in an earlier life. This can distort the spraybar (or needle seat on remotes) allowing more fuel to pass through the enlarged opening. If the engine tunes and runs fine at that setting then you should fly it. If you are having problems at other RPM ranges, then it may need to be replaced.

Reply to
Paul McIntosh

What you need to remember is the functions of the needle valve and the mixture control (you do have the manual?). The needle valve handles only the high speed setting while the mixture takes care of the idle and the intermediate range.

OS says to quickly run up the engine. If it should stumble, then adjust the mixture. What I have found is OS engine need to be run up slowly a couple of times (sort of like clearing the carb out), before the throttle can be rapidly opened or closed.

A very good idea is to leave the mixture alone. I have always found the factory setting is good enough. If you screw with it, you'll be screwing with it for a long time. Hopefully, the previous owner didn't touch it.

According to what you have said, you haven't adjusted the needle valve correctly. Back it out 1.5 turns from a seated position. Start the engine and run it up. Each time turn it in a couple of clicks and detect the change in RPM. Repeat this until you reach the point of maximum RPM (or past it where the engine just begins to stumble), then back the NV out about five clicks.

Now check that the engine transitions smoothly from idle to full bore. If it doesn't, then you'll have to adjust the mixture. OS has a flow chart in the manual of this proceedure, and I pity anyone who attempts to follow it. It does work, but it takes a lot of patience and an afternoon. Don't forget to run it up slowly a couple of times before you try a quick throttle change.

Ciao,

Mr Akimoto

Reply to
Mr Akimoto

I'm on my second season on a .46 AX, and have put enough fuel through it to be on my second set of bearings. To me it sounds like your idle mixture is too rich and someone has tried to compensate by leaning the high speed mixture. First, I would follow the instructions in the manual

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for re-setting the idle mixture screw - see Realignment of Mixture Control Valve section. Then I would set the high speed needle to 1.5 turns CCW from closed and give it a try. In my experience resetting per the manual instructions will result in the idle mixture being too rich by about 1/8 turn and the high speed being too rich by 1/4 - 1/2 turns, but it's a good place to start.

Good luck - BTW, my favorite prop on that engine is a 10.5 x 6 APC (on a Sig Somethin' Extra).

Reply to
Usenetboy

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