Ign
 

I'd place my bets on ignition issue also. You mentioned fouled plugs which is meaning that plug is probably not firing but being dosed in fuel. The problem is real as you mention weak takeoff and climb power.

EGT is a tough problem to solve because the number doesn't really mean much. These EGT wires going to the engine monitor can also be very sensitive. I had one wire with a very minor pinch and it made the EGT number decrease by 400 deg.

You need to start by making sure the sensors are giving you valid data. Remember these probs can vary a lot based on position and they're giving you a value (to 1 deg accuracy which is useless) based on exhaust valve open 25% of the engine cycle. What you really care about is are they reasonable close to each other and how they react to leaning.

It has not.

Also check the inlet manifold gasket
 

I have seen a case where #3 had a high EGT. In the end it was broken inlet manifold gasket. Hard to see.

+1

If the EGT on #3 didn't fall off during the high power mag check, that takes ignition off the table. If all other cylinders are peaking around 1400, it is still not mixture. An exhaust valve that is either not fully closing (typically due to the common oxidizd oil build up in the valve guide seen on lyc) or a cracked or bent valve will cause higher EGT readings. When the exhaust valve doesn't fully close, the probe sees more consistent heat (both the power cycle and exh cycle, not just the exhaust cycle) and therefore a higher reading.

checking sensors is prudent, but they typically fail reading low, not high.

Larry

Broken valve spring??

Looks like ignition is ruled out as the cause of the #3 EGT, since your ignition checks are coming out good and the one EGT is still high then that's doesn't seem to be the issue.

When you got the results with carb heat application were you lean of peak EGT on all cylinders? Carb heat should enrich the mixture because the air is less dense.

1. Compression and borescope to see how that exhaust valve is seating.
2. Look for induction leaks...start with a quick check of fuel staining like this http://airplaneownermaintenance.com/...2/IMG_7372.jpg
3. Swap EGT probs between cylinders to see if the indication follows the swap.

I was not lean of peak, it would be considered rich of peak or like 1/2" forward from stumble . Then I applied carb heat and saw the drop in egt.

So my mechanic determined it was running to rich. He had enriched it before, and now they leaned it out by "a turn and a half". I test flew it yesterday and is running much smoother and my climb egt reading decreased by about 20 degrees on #3. However no matter what I do in flight besides leaning to stumble, #3 is constantly in the 1540s and # 2 still showing about 1280s. That spread is still concerning. He did called Lycoming on the matter and they said that spread wasn't to be worried about as long as CHTs where under 450. I dont know my CHT, maybe I should upgrade my gauge to a unit that does.

What are the EGTs on 1 and 4? The issue isnt the egt sprea across cylinders, but whether or not there has been a change in readings now vs before at the same power configuration. If #3 used to run at 1300 in cruise when WOT and "1/2" from stumble" and now runs at 1540, I don't care what lyc specs are and your mechanic is "ok with it," something has happened and you should figure out what it was that happened / changed. It may me something minor that you can live with and it could also be a pre-cursor to a serious failure event. You do not want to swallow a valve one day. Not an enjoyable or inexpensive event.

My suggestion is not to accept this change as a simple matter of coincidence. EGTs do not magically rise 200 from one day to the next for no reason.

You still haven't told us where #3 used to peak. It seems you are familiar with leaning and peak EGT. We need to know this becuase any EGT higher than your previous peak EGT is not a simple mixture issue (too rich, too lean, intake leak, etc.), unless the sensor has gone bad and failing high. For example, if you used to peak at 1350 and now peak at 1540, the reason cannot be that you are leaner or richer than before, with all else being the same. Those conditions would net a new EGT below 1350. Many different reasons why you can have a higher peak EGT than yesterday, but mixture is not one of them.

Larry

mixture
 

The mixture adjustment your mechanic made is only for idle mixture and has no effect at cruise power.
The actual number for EGT is meaningless for normally aspirated engines. The 0 320 is know for terrible mixture distribution and that will result in the wide variation in EGT. Wrap some exhaust wrap around the intake pipe on #3 and see if that drops the EGT.
You need to install CHT and if the CHT stays below 425 in climb and 400 in cruise your are ok. I think Lycoming calls for 1800 r/m or even 2000 rpm for mag check. 100 rpm drop at 1500 rpm would be ok but 100 is not good at 1800.