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07-06-2016, 08:11 AM
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Join Date: Jun 2005
Location: Cedar Park, TX
Posts: 3,156
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After sleeping on it, I AM going to pull it apart today for a look-see and check the e-gap. Luckily I'm not one of those afraid to dig into a mag. I even have a spare on the shelf if needed.
Oh, and it was advanced.
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Scott Card
CQ Headset by Card Machine Works
CMW E-Lift
RV-9A N4822C flying 2200+hrs. / Cedar Park, TX
RV8 Building - fuselage / showplanes canopy (Done!)
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07-06-2016, 08:22 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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Quote:
Originally Posted by scard
So, recently we noticed it becoming very difficult to manage CHTs, mainly #3 & #4. The change was so slow that we barely noticed. With summer upon us it is a glaring issue. 3 and 4 passing 415deg by downwind on the first takeoff of the day. Wow, that just isn't normal. Cruise CHTs seem higher too. Are we sure this isn't just our imagination or an instrumentation problem? We finally declared that it isn't our imagination. Now what?
... I'll let that dwell for a few minutes then followup with findings  .
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I believe CHT's are directly related to OAT change. If you see a 70F change in OAT, you will see a 70F change in CHT.
You may not have a problem.
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07-06-2016, 09:41 AM
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Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
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As the cam wears down it breaks the points late. Same for worn points, the breaker half that the cam pushes gets farther from the cam. Retiming the mag makes it pop at the proper time, but internally the point break is moving away from e-gap which is of course the magic position where the rotating magnet has maximum flux relative to the core lams. It's physical, you can feel it. Moving point break away from that spot rapidly reduces spark output. Pretty much all the wear and the rotational forces push the mag towards retard. If your points grow a stalactite, which they do because one point deposits material onto the other point you have a pit on one side and a spike on the other. If the breaker halves shift then the spike rests on an unworn spot making them taller. This will cause advance. Also as the cam wears in it leaves a tall nub at the top and/or bottom of the cam. You retime a couple degrees. Couple years later you retime a couple degrees. Then the cam moves in it's slot and/or the breaker halves shift and a tall nub is suddenly breaking the points early (where they used to break). That's the only advancing mechanism I know of (unless you jumped an accessory case gear or something in your impulse coupling is not snapping all the way back).
Anyone else think of another advance mechanism?
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Actual repeat offender.
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07-06-2016, 11:12 AM
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Join Date: Jun 2005
Location: Cedar Park, TX
Posts: 3,156
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Ok, just back from the hangar. I opened it up and there are no internal issues. It looks great inside and the e-gap was pretty much spot on. I'm comfortable that it must have loosened just enough to slip on the accessory case. We're waiting on a prop cable (tomorrow) for a test flight. I'll bet we can stick a fork in this one. I'm looking forward to CHTs being back in line without all that advance.
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Scott Card
CQ Headset by Card Machine Works
CMW E-Lift
RV-9A N4822C flying 2200+hrs. / Cedar Park, TX
RV8 Building - fuselage / showplanes canopy (Done!)
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07-17-2016, 07:21 AM
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Join Date: Jun 2005
Location: Cedar Park, TX
Posts: 3,156
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I'll reopen this for the peanut gallery. We ended up putting a new mag on it. Still high CHTs on all but #1 cyl, also corresponding higher oil temp. I'm relating these temps to what we know they have always been in the last 1600 hrs. About 30+ deg hotter cht and +10 deg oil temp. Remember this is a left impulse coupled mag, a right Lightspeed, O-320. All intake tubes are tight and clean.
Tanya went out and flew around mostly straight and level in our normal cruise profile at 4500' doing 145 TAS for a half hour. We would expect to see CHTs at around 350, which is where #1 was, but 2,3,4 were 386+ and very difficult to keep under 400 in any climb. Mag drop checks were fine. She flew around on only the lightspeed for 10min and all temps were exactly in line with our normal expectations. Are we just making less power with the mag off, thereby cooler? I would think the lightspeed can support near complete combustion based on its very low mag drop during runup.
What gives? I'm running out of ideas. Is there a failure mode in the mag harness that could produce advance (I don't think so). I'm thinking about hanging a timing light off of each plug to see if I can find anything interesting (doubt it). I'm also thinking about experimenting with making the mag "late to the party".
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Scott Card
CQ Headset by Card Machine Works
CMW E-Lift
RV-9A N4822C flying 2200+hrs. / Cedar Park, TX
RV8 Building - fuselage / showplanes canopy (Done!)
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07-17-2016, 07:41 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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Perhaps looking at it from the LightSpeed side might be worthwhile.
If you assume the mag is timed correctly, the results you find with running just the LightSpeed is opposite of expected (the Lightspeed should be 30+ degrees of advance at that power level and would show little drop when turning off the mag). Things to explore:
- Is the Lightspeed timed correctly (timing set when at TDC - NOT 25 degrees BTDC)? Use a timing light with the Manifold Pressure line disconnected to verify. Do this again with the Manifold line attached and see if you get any advance (may not depending on what RPM you are at)
- If this is a LightSpeed Plasma II have you connected the external pot to control timing?
- Don't assume the LightSpeed is functioning correctly.
After you have eliminated the LightSpeed then go back and figure out why the mag timing is off.
Carl
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07-17-2016, 07:43 AM
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Join Date: Jul 2007
Location: Keller, TX
Posts: 1,553
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Quote:
Originally Posted by Lycosaurus
Time to look at intakes to the cylinder, and possibly replace gaskets and sump fittings. Those two cylinders are likely running lean.
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Minor intake leaks only affect the engine at reduced throttle. At wide open throttle there is no differential pressure to force air in downstream of the carb or FI body. Only when the throttle is mostly closed or closed does it have an effect. It is easy to test by recording egts at 23" then at 13" or so. If egts go up there is probably a leak.
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"Cum omni alio deficiente, ludere mortuis."
(When all else fails, play dead.)
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07-17-2016, 07:48 AM
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Join Date: Jul 2007
Location: Keller, TX
Posts: 1,553
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Baffles?
You may have already mentioned it, but are the baffles still in good shape? Seals fitting good and laying flat against the cowl? What do the plugs look like? Any evidence of lean running?
If fuel injected, have you cleaned the injectors? How about the fuel distribution device, I forgot what it is called. A little piece of dirt can affect chts a lot. It would be unusual for three cylinders to see it at the same time.
You mentioned it has been creeping up. Perhaps a gradual buildup in the injectors of some gunk?
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RV-8 180 hp IO-360 N247TD with 10" SkyView!
VAF Donations Made 8/2019 and 12/2019
"Cum omni alio deficiente, ludere mortuis."
(When all else fails, play dead.)
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07-17-2016, 07:53 AM
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Join Date: Jun 2005
Location: Cedar Park, TX
Posts: 3,156
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Quote:
Originally Posted by Bubblehead
You may have already mentioned it, but are the baffles still in good shape? Seals fitting good and laying flat against the cowl? What do the plugs look like? Any evidence of lean running?
If fuel injected, have you cleaned the injectors? How about the fuel distribution device, I forgot what it is called. A little piece of dirt can affect chts a lot. It would be unusual for three cylinders to see it at the same time.
You mentioned it has been creeping up. Perhaps a gradual buildup in the injectors of some gunk?
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O-320 No change in baffles. Plugs look normal.
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Scott Card
CQ Headset by Card Machine Works
CMW E-Lift
RV-9A N4822C flying 2200+hrs. / Cedar Park, TX
RV8 Building - fuselage / showplanes canopy (Done!)
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07-17-2016, 07:56 AM
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Join Date: Aug 2005
Location: Atlanta, GA
Posts: 4,218
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Have you checked your mixture? Perhaps a linkage has worn or slipped or some other issue has mucked with it?
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Kyle Boatright
Marietta, GA
2001 RV-6 N46KB
2019(?) RV-10
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