jindoguy(at)gmail.com
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 Posted: Mon Aug 06, 2007 8:23 pm Post subject: Hirth 2703 Update |
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Finished putting everything back together on the Challenger and the Hirth 2703 this afternoon and got in a quick test flight. The highest CHT I saw on taxi was 300. It rose to 400 during the climb to pattern altitude at 6200 RPM, but quickly settled back to 280 when I leveled out and throttled back to 5700. All this on the first 100 degree day of the year.
So what's changed other than the fan belt?
I stripped the exhaust system to check the pistons for signs of seizure. No marks or signs of metal transfer on either. I did find that the thermocouples for the CHT gauge were installed without removing the spark plug insulator per the engine manual. I also spent a lot of time removing every last shred of the old belt until the fan turned like a pinwheel when I blew on it. There was a small crack in the fan shroud that required welding. The entire fuel system was rebuilt. The exhaust system was a mess held together with grade 8 bolts and insulators that were installed without clearance or compression limiters.
Were any or all of these enough to cause the high temps? Partial fuel starvation from collapsed lines, a small exhaust leak or two, a small leak in the shroud. I suspect that when the owner replaced the fan belt he was less than thorough in removing belt debris from the fan bearings, but it's just a suspicion. I know he didn't clean the mating surfaces of the drive pulley and the flywheel, so there may have been some wobble in the pulley that accelerated the fan belt's destruction in 25 hours. The fan belt may have been loose, too. I can only use the information the owner gave me about seeing CHT's of 490 after a long taxi, and in the 450+ range while climbing. I know I saw 540+ on one head and 510+ on the other, but all this was done with the thermocouples installed improperly.
I must say that overall I'm pretty impressed with how robust this engine is. If the readings from the CHT gauge were anywhere near accurate, it received some severe abuse and seems to have shrugged it off. Don't know how it will work out in the long run. For now, I just need it to go another 20 hours or so.
Rick
--
Rick Girard
"Ya'll drop on in"
takes on a whole new meaning
when you live at the airport. [quote][b]
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