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rickofudall
Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA
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 Posted: Sun Jul 21, 2013 6:51 pm Post subject: Starting carburetors |
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The airplane has ~500 hours on it and the 912 ULS. Except for one logbook entry for carburetor cleaning it appears that this was the first carb rebuild. The needle valves in both carbs failed and both would overflow within three to four blade cold cranking. The starting carburetors, which had worked flawlessly seemed to have failed, the engine was hard starting. I rebuilt one carb at a time to always have a complete carb for reference and to avoid mixing parts. I started with the right carb (overflow tube pointed outboard. When I pulled the starting carb housing I found the inboard end of the shaft was stamped "L". Staining on the disk seemed to indicate that the slot was properly positioned so that when it was rotated the slot was moved over the port connected to the float bowl. The rebuild was a complete overhaul, all gaskets, o rings, diaphragms and floats were replaced, as was the needle and needle jet. I located the dot on the outboard end of the starting carb shaft so that it pointed toward the cable and lightly tightened the four screws of the housing. Disassembly of the left carb (overflow tube pointing inboard) revealed that the starting carburetor shaft was stamped on the inboard end "R". Again, staining on the disk showed that the slot uncovered the port from the float bowl when it was rotated.
At this point I was beginning to doubt my sanity. I considered that German for Right starts with and "L". It doesn't. I thought perhaps Rotax directed right and left as seen with from the prop end of the engine. From what I could find in the Installation Manual, right and left are relative to the pilot looking forward with the engine mounted in a tractor installation.
Since the history of the engine seemed to show that the starting carbs were working until recently, but had stopped working (I should add that there was no sign that the starting carbs had been opened before I did) I decided to put the disk assembly with the shaft stamped "R" in the right carb and put the "L" disk in the left carb.
Because the airplane's owner wants to overhaul the fuel system to include a fuel return line to control fuel pressure, we did not attempt to start the engine when I re-installed the carbs today.
The possibilities seem to be;
A. Both starting carbs were miss assembled with a right shaft mated to a left disk and vice versa.
B. I have misinterpreted the Installation manual and right and left orientation are relative to the prop end of the engine.
C. These carbs were originally installed in the Twilight Zone.
Comments? Advise?
Rick Girard
--
Zulu Delta
Mk IIIC
Thanks, Homer GBYM
It isn't necessary to have relatives in Kansas City in order to be unhappy.
- Groucho Marx
[quote][b]
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Mon Jul 22, 2013 5:23 am Post subject: Re: Starting carburetors |
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Hi Rick,
The chokes were marked and on the correct carb. If you switched them then switch them back or they won't work. The center punch dot on the end of the choke shaft should be facing the tube that looks like it is molded in the choke assembly. Tighten the 4 screws well so there I are no leaks and they are supposed to have blue Loctite. (Medium strength)
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
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