[photo not recovered: 29835.jpg]
For the past couple of weeks I have been having countless problems with engine runs, most of which resulted in at least some screaming lean laps, and sometimes a full flight screaming lean. Some flights would begin normally, then run lean for a couple of minutes before dropping back into a normal four cycle. Some flights had sudden engine stoppages at odd points in the pattern. A couple of times the engine just refused to start ending the session. Amidst this series of sorry engine runs there was further confusion until I positively identified a bad glow plug. I finally attributed most of theses bad runs to a series of new tanks. Some, but all, of these bad runs were showing up in flights with each of the new tanks. The tanks were of relatively conventional design and the construction was adequately competent. I could see no reason for the problems. Some of the lousy runs seemed to be similar from tank to tank, others were not. The problems were very hard to diagnose and I'll admit to having, for awhile, suspicions of engine, rather than tank, problems.
[photo not recovered: 29836.jpg]
Finally I remembered the sage words of my engine guru, Dubby Jett. The master says "80% of all engine problems are the tank. The rest are the tank". Taking his wise words to heart, I finally figured it out. When pressure testing these new tanks, I would hold them under watter and apply pressure to the tank with a fuel bulb using the full weight of my upper body. Any leaks would show up as streams of bubbles. These leaks show up in some new tanks and simply indicate a pinhole in a solder lap joint, easily fixed with a little more heat and solder.
[photo not recovered: 29868.jpg]
While testing, I discovered a new problem. My fuel forty year old fuel bulb was disintegrating. To get the tank tested and into the airplane without further delay, I taped the top of the bulb where most of the leaks were and held a finger over a leak at the bottom of the bulb to complete the tank test. While testing the two tanks that leaked, pressure was lost by the bulb while the tank was still under water. Water was sucked in unnoticed where bubbles had come out. The leaks were repaired, the tanks tested OK and seemed normally empty when installed.
OK, once again I demonstrated that engines won't run on water and run very poorly on water contaminated fuel.
Over the last 40 years or so, I've built a large number of tanks and never had this problem. I attribute this human failure to the distraction of attempting to pressure test tanks with a deteriorated bulb (which has since been replaced, $6.75+).
Once again, the sun shines, the birds sing and so does my Ro-Jett .76. The last couple of nights, engine runs and lap times have been perfect. I am amazed, considering the amount of screaming lean that I have flown during this episode, the Ro-Jett still seems happy. It has a really strong seal, hot and cold, and turns the same prop as before at the same RPM. I guess the new metal technology saved the engine, which is a very comforting thought.
[photo not recovered: 29869.jpg]
I got tired watching Snaggletooth roar around in endless laps of screaming two stroke. I started doing full patterns with sub 4 sec laps. Last night, after another happy practice session, while wiping Snaggletooth, I discovered stress cracks on the bottom of the fuselage under the flap hinge line from the violent high speed maneuvering. Well, what the heck. Its windy today anyway. Tomorrow I'll be able to practice again with a strong 2 - 4 breaking Ro-Jett turning a wide blade 15" PROP singing happily and well fed from its new tank.
MORAL: "80% of all engine problems are the tank. The rest are the tank", Or maybe, fuel contamination. Tanks are easy to build. Its a handy skill for most serious flyers.
Al
For the past couple of weeks I have been having countless problems with engine runs, most of which resulted in at least some screaming lean laps, and sometimes a full flight screaming lean. Some flights would begin normally, then run lean for a couple of minutes before dropping back into a normal four cycle. Some flights had sudden engine stoppages at odd points in the pattern. A couple of times the engine just refused to start ending the session. Amidst this series of sorry engine runs there was further confusion until I positively identified a bad glow plug. I finally attributed most of theses bad runs to a series of new tanks. Some, but all, of these bad runs were showing up in flights with each of the new tanks. The tanks were of relatively conventional design and the construction was adequately competent. I could see no reason for the problems. Some of the lousy runs seemed to be similar from tank to tank, others were not. The problems were very hard to diagnose and I'll admit to having, for awhile, suspicions of engine, rather than tank, problems.
[photo not recovered: 29836.jpg]
Finally I remembered the sage words of my engine guru, Dubby Jett. The master says "80% of all engine problems are the tank. The rest are the tank". Taking his wise words to heart, I finally figured it out. When pressure testing these new tanks, I would hold them under watter and apply pressure to the tank with a fuel bulb using the full weight of my upper body. Any leaks would show up as streams of bubbles. These leaks show up in some new tanks and simply indicate a pinhole in a solder lap joint, easily fixed with a little more heat and solder.
[photo not recovered: 29868.jpg]
While testing, I discovered a new problem. My fuel forty year old fuel bulb was disintegrating. To get the tank tested and into the airplane without further delay, I taped the top of the bulb where most of the leaks were and held a finger over a leak at the bottom of the bulb to complete the tank test. While testing the two tanks that leaked, pressure was lost by the bulb while the tank was still under water. Water was sucked in unnoticed where bubbles had come out. The leaks were repaired, the tanks tested OK and seemed normally empty when installed.
OK, once again I demonstrated that engines won't run on water and run very poorly on water contaminated fuel.
Over the last 40 years or so, I've built a large number of tanks and never had this problem. I attribute this human failure to the distraction of attempting to pressure test tanks with a deteriorated bulb (which has since been replaced, $6.75+).
Once again, the sun shines, the birds sing and so does my Ro-Jett .76. The last couple of nights, engine runs and lap times have been perfect. I am amazed, considering the amount of screaming lean that I have flown during this episode, the Ro-Jett still seems happy. It has a really strong seal, hot and cold, and turns the same prop as before at the same RPM. I guess the new metal technology saved the engine, which is a very comforting thought.
[photo not recovered: 29869.jpg]
I got tired watching Snaggletooth roar around in endless laps of screaming two stroke. I started doing full patterns with sub 4 sec laps. Last night, after another happy practice session, while wiping Snaggletooth, I discovered stress cracks on the bottom of the fuselage under the flap hinge line from the violent high speed maneuvering. Well, what the heck. Its windy today anyway. Tomorrow I'll be able to practice again with a strong 2 - 4 breaking Ro-Jett turning a wide blade 15" PROP singing happily and well fed from its new tank.
MORAL: "80% of all engine problems are the tank. The rest are the tank", Or maybe, fuel contamination. Tanks are easy to build. Its a handy skill for most serious flyers.
Al
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