its been a good spring so far for figuring out the JPX engine. I just solved another gremlin today and i'm super happy. i also discovered another but will hopefully be easily fixed.
i'll post a pic tonight, but when i moved the fuel pump to adapt it to the rotax recommended way of mounting the mikuni fuel pump, it left two holes in the crankcase where the bolts went for the old mounting brackets. three of us thought the holes werent' all the way through to the crankcase. as it turned out, a problem cropped up after i reinstalled the pumps the new way - it wouldn't idle lower than about 2200rpm and if it did drop, it just stalled. much tuning and tweaking was done to no avail.
at last, today i said "i've checked the fuel, i've checked the spark, therefore it must be air". i pulled the air filter off - everything looked fine and no changes there. then i started spinning the prop through while i was thinking - hearing the familiar HISS out of what i thought were the decompression valves...... then i focused on it and thought "sounds like its coming from the front crank seal" so i put my ear right up to the seal and turned the prop through several times, and just like a girlfriend from my teens i felt nice soft air blowing into my ear - FROM THE STUPID HOLES LEFTOVER FROM THE OLD PUMP MOUNT!!!!!!!!!!
i was so angry with myself for not checking them further - sure enough, with fingers on the holes, the hissing went away and compression was higher......... swearing for the next 10 minutes while plugging the holes with 4 bolts and loctite, i started the engines up and they ran perfectly! went up for a great half our flight before work, smooth air with lots of thermals. was climbing at 800ft/min in my 2-seater in nice smooth lift with 2 hawks - what a way to start the day!
moral of the story - suspect everything, focus on nothing when troubleshooting!
Tyler
PS: the last problem that popped up was that with the throttle at full on the ground (i do a lot of full power testing on the ground), i started searching for vapour bubbles in the gas lines (daffy and i have been chasing this demon for months). the line is perfect from the tank, all the way until the line mounts on the nacelle to begin routing it for the fuel pump (on the JPX). every so often, the engine vibrates in a mode that makes the line shake about 1/4" in one place, and it produces the odd bubble (a bubble about every 10 seconds or so). so i watched the outlet of the fuel pump, and it was nice solid gas (no bubbles) but then i would see a bubble enter the fuel pump as one big bubble, and it would exit the pump as millions of tiny bubbles and then go into the carb.... the engine was able to handle this reduced amount of bubbly gas fine (float bowl carb), but i need to dampen the fuel line so that these point vibration loads on the fuel line disappear. once i do this i think my fuel system will be mint and trouble free. but how do you mount a fuel line without clamping it to the nacelle where vibration is transmitted from the engine?????? (sorry for the long post!)Statistics: Posted by JPXman — Thu Jun 07, 2007 4:45 pm
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