What causes this????

Brantel · #1 03-22-2010, 05:29 AM

I think I found another symptom to why I had a vibration or roughness on my second flight...maybe the first as well but I did not notice it then.

I downloaded my data from the D120 and trended my cht and egt values. All seem to track well with each other until full power for takeoff is applied. Then the #2 egt drops down fast to over 200* less than the others. When reducing power to land, it comes back in line with the rest.

I will post some graphs as soon as I can.

The engine is a carbed O-360 with two slick mags. The mags were uses and the engine is a fresh rebuild with chrome cylinders. FP prop.

I did take the mags apart and the brush is good, the cam ok, the distributor and rotor are well used. The points have some small buggers on them.
The caps test fine.

The plugs looked good on all but #1 the bottom was a little wet.

G3i Ignition · #2 03-22-2010, 06:21 AM

When you pulled the plugs ex. #1. did you look in the upper bore of the plug where the harrness attaches? Was it wet/fuel in there? Rotate the plugs to different cylinders and make note and see if your EGT temps change to that cylinder. I have seen bad and/or cracked insulators in the plug that act up only under higher power settings.

Thomas S.
www.g3ignition.com

schristo@mac.com · #3 03-22-2010, 06:53 AM

sounds like it might be a bad plug in that cylinder? fires good under low pressure / power but not under high so the egt goes way down. did you check the plugs in a tester?

OceanBob · #4 03-22-2010, 08:38 AM

I agree with Stephen. The first thing to check is the plug.

1. Replace both plugs in #2 (don't waste time moving plugs around)

2. If the problem persists then borrow an ignition harness tester and check for a short in the plug wires. This is a portable box that shoves 100,000 volts into the unhooked plug wire and if you have a breakdown in the insulation the spark will jump thru that weak spot. It will also jump the gap if you have a hairline crack in the plastic rotor cap. You can hear it crackle.

3. Chrome cylinders have problems (cast iron rings) breaking in sometimes and you might check the cylinder walls in #2 to see if you have any scratches (broken ring?) that are fouling the plug with oil and preventing ignition. (this is a long shot however)

4. A real long shot would be a valve that 'sticks' and lags preventing full compression in that cylinder. Or it sticks because you have a broken valve spring which allows the engine to run at low rpm but simply can't dance fast enough at higher power. It's hard to see this problem but if you run out of options above you will have to remove the rocker cover, both plugs, move the cylinder to bottom dead center, shove a nylon rope into the cylinder, slowly bring the cylinder up until the rope is crushed against the valves; using a lycoming valve spring depressor compress the cap around the valve, remove the keeper keys and lift off the spring assembly to check for a broken spring (or weak spring). *as a side bar, in the old days if we had a valve that leaked air during a compression check on a lower time engine, we would chuck the loose valve up in a drill motor, lift it off the seat, spin it up and pull it back to clean the garbage off the valve and seat....usually this worked..hahaha....Political correctness prevents me from naming this sort of Valve Job. :-)

Good luck..! Let us know what you find.

Bob

OceanBob · #5 03-22-2010, 09:10 AM

Lower temps happen for three reasons. A plug that doesn't fire a cylinder that has low compression and a rich mixture. I've had a PRIMER leak that sucked extra fuel into the manifold causing a rich mixture on one cylinder. like I said LONG SHOT. It was a bad O ring on the primer plunger. Just tossing ideas around.

Bob

Brantel · #6 03-22-2010, 09:18 AM

I should also mention that the CHT appears normal on #2. I will post trend charts in a few...

Brand new harnesses but the plugs came with the engine.

Brantel · #7 03-22-2010, 01:10 PM

Here are the trends for the 1st flight and a shorter 2nd flight.

Notice how everything tracks together untill high power is applied. Then the #2 EGT drops off sharp.

1st flight:

2nd flight:

molson309 · #8 03-22-2010, 03:11 PM

I would think a misfiring/nonfiring plug would cause EGT values to climb rather than fall.
If I turn off one ignition during cruise on my -7A I see the EGTs climb at lease couple of hundred degrees; I have been told that this is because the fuel is still burning during the exhaust cycle.

Mark Olson N407V RV-7A ~550 hours

Radomir · #9 03-22-2010, 03:31 PM

I wouldn't expect EGTs in the 1400s at full rich... 1200s would be numbers I'd expect to see.. (maybe even high 1100s, depending on probe location etc).. All this is just a general thought.... you could nit-pick a bit.. so at best this is just a guideline..

Brantel · #10 03-22-2010, 04:43 PM

The documents and papers I have read have stated that if you turn off a mag, you should see the EGT values climb but if you only see the EGT drop on one cylinder with both mags on, that it indicates a fouled plug, mag or harness issue....

That seems to be constant across all the info sources.

Thanks!!!

Quote:

Originally Posted by molson309

I would think a misfiring/nonfiring plug would cause EGT values to climb rather than fall.
If I turn off one ignition during cruise on my -7A I see the EGTs climb at lease couple of hundred degrees; I have been told that this is because the fuel is still burning during the exhaust cycle.

Mark Olson N407V RV-7A ~550 hours