Hey all, I've got a older model OS 61SF 2 stroke that is on a model (Kadet LT-40) that sat for a while after being blown off the work table at the field and losing it's tail feathers. I put it back together last week and took it out to see how it would fly. Ground checks seemed to go fine Radio check was good at >150ft and the motor (after tuning) seemed fine on a 5-10sec Nose up test. Just a slight increase in RPM but no gagging or such. Well, I took it off and at about 50ft of altitude and climbing the engine ran WAY down and back up again adn started to cycle like this. I almost freaked, but called an "engine trouble coming around to land!" I lowered the nose and the engine came right back to full throttle. I was able to successfully bring it around and land it, but I never got much more than my original 50 ft because the engine would run down and sound like it wanted to die if I lifted the nose above level. When I got it back to the pits, I re-ran my ground checks and noticed tiny bubbles in the line between the needle valve (it's attached/part of the back cover of the motor) and the carb. If I held it up long enough abotu 15 sec, the bubbles would get to be bad enough to almost stall the engine. if the engine waas running slow, there was no problem. When I run the motor up the bubbles appear. There are no bubbles that I could see in the fuel line from the tank to the needle valve, but there they were/are when I run the engine up and if I lift the nose they get even worse. I checked the needle valve o-ring and it seems fine.I replaced all of the fuel lineschecked the tightness of the little cover on top of the needle valve assembly and gooped it on the threads to make sure it was sealed. I also removed the needle valve and tightened the needle valve assembly and the inlet nipple that is associated with it. Still bubbles. in an offhand conversation, someone mentioned replacing the fuel tank, so I did that AND replaced ALL of the lines again to no avail. still bubbles and it still runs down and wants to quit. The fuel tank does sit slightly below the carb opening (1/4 in. MAYBE as much as 1/2 inch) but I don't see how that would cause bubbles in only the line after the needle valve. the only other piece of info that I can think to add is that one of the cowl cheeks vibrates at various throttle positions because of a previous wreck, but not at full throttle. I've epoxied a piece of triangle stock in the corner to brace that up in case it's vibration and I will know in the AM when the epoxy is set and I run it again. The mount is tight on the firewall as is the engine to the mount. The firewall does not rattle that I can tell. I am out of ideas here. This engine ran fine the last time the palne was flyable. I don't understand what could be wrong. I would rather not demount the motor and put it on another mount and stick it on the run-in bench to see if it does it there, but I'm about ready to try that too.I look forward to any responses/wisdom/suggestions about what might be wrong and how I can fix it. thanx Mark
- posted
20 years ago