colder #6 CHT

[tx_swordguy]

I replaced #1 cylinder due to scoring and in removing the other heads to check I cleaned the heads/pistons and lapped the valves. I now have #6 CHT colder than the rest by 60 or more degrees F. The OAT is around 50F at 845' msl (30F colder than when I last flew) I would expect the EGT to go up and CHT to go down which they appear to have done generally across the board. On ground run (breaking in the cyl) with a shroud the #6 wouldn't get above 185F while the others got up and above 300. I attributed that to the shroud forcing more air on that one cyl. With the cowling on and flying the #6 gets warmer but only to about 230F. That would be great except that it is different than the rest and not like it was before I did the head work. cruise CHT 1-5 are 270-290 #6 is about 220-230. rpm is 2700 and EGT is leaned to 1170-1250 spread. WOT at full rich brings CHTs up to about 290-308 on 1-5 and #6 is about 250. WOT straight/level full rich gets 1 and 6 up to about 370 EGT in about 30 sec when I back off the throttle at which point it cools down rapidly. I am planning on playing with the aerocarb needle and working on the EGT but my concern is the cold #6 CHT and what that might indicate. I am leaning toward the exhaust valve not seating well enough either because I didn't lap it smooth enough or the spring is worn a little and not pushing it closed hard enough. I do not know what temp I would be looking at for that hypothesis but my internet research seems to point in that direction. I have a cheap Harbor freight leak down tester that I plan on trying out and hopefully that will point me in a direction. Thinking I may just replace the exhaust valve, spring and relap it to see what happens but all ideas are welcome. Especially if it involves less work.
Mark- ps I placed this ? on the jabiru/cammit email forum as well

[wlarson861]

I would try switching the probes between two cylinders and see if #6 is still cold or if the problem follows the probe.

[builderflyer]

Mark,

If you had said that the lower CHTs that you experience on cylinder #6 you actually experience on cylinder #1, I'd say that sounds about right. Any possibility you have reversed the CHT sensors between 1 and 6?

Art,,,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[tx_swordguy]

don't see how each side was laid out and marked. I swapped 4 and 6 CHT probe and planning to go flying tomorrow so we will see what happens. If it still looks funny tomorrow I will try the harbor freight leak down tester. I did notice that #6 plug was leaner by a bit than the #4 plug was when I swapped them. #4 has always been the hottest cyl. so that surprised me I pulled the #6 intake tube from the manifold and resealed it after feeling a bit of a wobble with it. May just watch it over the next 10+ hrs and see what happens I am not getting the feeling a lower temp is an immediate concern so I will continue playing with it, with out taking the head off for now. It is running well and I can control the EGTs as long as I am not WOT indefinately. Might also do a mag check at altitude to see if I get a cold cyl from the ignition side.