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-   -   Increasing CHTs (https://vansairforce.net/community/showthread.php?t=139588)

scard 07-05-2016 04:18 PM

Increasing CHTs
 
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 :).

istrumit 07-05-2016 06:31 PM

My 10 has 9.5:1 compression and does the same thing on hot days....worse when fully loaded. I depart WOT and Max RPM, I have to keep the nose over at 125 kts else the CHTs go wild.

Somewhere on this board I read that backing off the RPM to 2400 shortly after take off is helpful. So, I do that now and it makes a big difference. I can keep the climb rate up for longer wo having to also reduce throttle.

Lycosaurus 07-05-2016 07:32 PM

My guess...
 
Time to look at intakes to the cylinder, and possibly replace gaskets and sump fittings. Those two cylinders are likely running lean.

scard 07-05-2016 07:56 PM

I like the intake idea. Yeah, I look at those often and have a stack of gaskets that I use often.

However, the rest of the story... So, OSH is coming AND we have an airshow performance coming up so it is time to make some kind of declaration that fixes any doubt. Time for a targeted flight test. Yep, up and out on downwind coming through 400deg, grrrrr.... Cool it off and setup a 130kt climb at 300fpm. CHTs climbing again. Yep, I call BS. Level off at 5k' in 70deg air doing 140kt in low cruise. Still way higher CHT than we know. (pilot tuned to the airframe with 1800hrs) What causes sneaking / no major change high CHT?... As I orbit the airfield at 5500'.

Hmm, Right lightspeed ignition off. No change. Ok back on. Left side Slick mag off. CHT immediately drops. Wow, still falling. Pull up into a 90kt climb and CHT still falling. Hold what you've got and turn it back on. Yep, here come the CHTs at high speed.

Yep, Houston we have a problem, and a diagnosis. The timing was off on the Slick mag by 3.5 teeth. Wow, that is huge and a first for this airframe. I haven't pulled it to check the e-gap yet, nor has it been test flown yet due to the shop build. I'm still thinking about this thing that is accepted as mag drift. It is either vibration in the mounting clamp or internal wear.

rvbuilder2002 07-05-2016 08:20 PM

Internal mag.timing is the most likely answer.

N941WR 07-05-2016 08:21 PM

Scott, just a data point.

The P-mag's "A" Curve starts at 26.2* BTC.

I have found that by dialing them back 1.4 degrees to 24.8* BTC, my CHT's came right down in line. (This was done some time ago.)

I have been playing with the max advance feature on the P-mags and can dial in my CHT's with timing in flight rather than fuel.

Can you play with the max advance setting on the Light Speed? It might help.

alpinelakespilot2000 07-05-2016 08:39 PM

Quote:

Originally Posted by scard (Post 1092910)
The timing was off on the Slick mag by 3.5 teeth.

Is your Slick subject to the Service Bulletin that requires action if the timing drifts x# of degrees? Sounds like one possibility is that yours is doing what the SB suggest has happened to a fair number of others.

BillL 07-06-2016 07:11 AM

Good job on diagnostics Team Card.
 
Very interesting - Mahlon said (somewhere) that the lower plugs had more influence than the top in some installations/conditions. One might conclude in this case the top plugs are not doing much, even advanced. Just observed a friends 540 with large mag advance in only 20hrs on a new Lycoming. Within the SB limits. Odd you did not see this in the static mag drop test, he did. Is that because it is not WOT? (and carbed)

At what point is a mag internal inspection in order?

So many questions.

aerhed 07-06-2016 07:41 AM

Bill, I would say an internal mag inspection is a good thing to do any time you get an uneven drop that can't be fixed at the spark plug end. People seem to be scared to open magnetos. There's only like six parts inside. Sure makes me feel better than just asking for opinions and "wondering" about it. I say, rip em apart and make em sparky.

aerhed 07-06-2016 07:48 AM

Scott, you were almost ten degrees off. If you didn't find that the mag was loose at the base you should have that apart before you go anywhere. I'm guessing the breaker cam is shot. If so, pretty soon the points won't be able to break period. That's if your timing was late/retarded. If it was advanced you may have a distibutor gear coming apart. I've seen the "bad" cams wear to nubs in just a couple hundred hours.


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