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02-21-2016, 04:21 PM
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Join Date: Nov 2015
Location: DFW
Posts: 55
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Quote:
Originally Posted by aerhed
. . . A carb with a rattly shaft will suck air from the sides, that's air that doesn't go past the idle ports.
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Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.
What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.
I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.
Do you think this is the problem?
Anyone know a good carburetor shop in the DFW area?

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02-21-2016, 09:07 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Jbon
Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.
What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.
I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.
Do you think this is the problem?
Anyone know a good carburetor shop in the DFW area?

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The shaft bushing interface is not designed to be air tight and contains no seal. Air leak there is expect and is not causing a problem. the shaft has no mechanism to stop side to side movement other than the butterfly valve in the throat. You should have side to side play.
Do some more research on this before you get too concerned. Many mechanics tune idle by sound and performance. If your engine is running well don't concern yourself too much. Go research mixture adjustment procedures. I have seen some good articles with more detailed advice. It may just be the testing procedure that you are using.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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02-21-2016, 10:26 PM
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Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
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Yah, they're not airtight. If its worn bad you will see it most at like 3/4 throttle. Not talking slight movement, but actual rattle the shaft with your fingertips worn. The bubbles around the screw are from your air supply blowing back through the idle ports (little tiny holes by the throttle plate edge at idle).
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Actual repeat offender.
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02-22-2016, 06:59 AM
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Join Date: Nov 2015
Location: DFW
Posts: 55
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Thanks guys, I'll do some more research on mixture. Good to know that leakage there is normal. The engine seems to run okay. No stumble on acceleration, or fouled plugs.
What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag. The guy at Electroair indicated that because of the higher efficiency of the EIS, I shouldn't see much of any drop. When I mentioned my mixture issue, he said to get that squared away first. Again, plugs look good. The leads look pretty good too, but they are nearly 20 years old. Any thoughts there?
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02-22-2016, 09:33 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Jbon
What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag.
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Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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02-22-2016, 04:35 PM
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Join Date: Nov 2015
Location: DFW
Posts: 55
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Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.
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02-22-2016, 08:55 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Jbon
Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.
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Unlike choosing a wife, looks mean nothing here. By test, I mean with an Ohmmeter.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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02-22-2016, 08:59 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by lr172
Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.
Larry
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Just thought about this more. Are these mag drops done at 1600-1800 RPM? Idle mixture has zero impact after about 1200 RPM. I would consider not taking too much advice from the EI guy who suggested you sort out your idle mixture for behavior at 1800 RPM.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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02-23-2016, 10:50 AM
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Join Date: Nov 2015
Location: DFW
Posts: 55
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Agreed. That makes sense. Next step is to break out the ohm meter. These leads are "Autolite". Carbon, I assume. What resistance per foot should I be looking for?
Thanks
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