HIGH EGT
in General Tech Discussion
11 years ago
Older 912UL (1997) 81 hp with no air box and 170 hours total time on a Zenith CH701. The prop is a warp drive 68” set for 5500 on take off (hits 5500 about when I break ground).
I’ve had an high EGT issue with this engine for the two years I’ve had this aircraft. I fly out of a 2100’ elevation strip. Here’s the history:
Prior to overhaul of the carbs it was barfing fuel, the float pivot shaft pulled out and had fallen down on one side in the float bowl and was missing the spring on one idle adjustment screw. It ran rough, wouldn’t idle below 2200 RPM and the right EGT would run above 1650 F unless you kept it at 5500 RPM or above.
I overhauled both Bing 64 CV carbs (gaskets, O rings, Viton needle, float arms). Needle jet is changed to 2.74 (it was running lean with a 2.76 even at position #4) and needle position at position #1. They appeared to be functioning correctly and idled smoothly at 1800 RPM running smoothly through the RPM range up to 5500. Idle air mixture screw set at 1 1/4 turns. Idle jet is a 35.
The #2 EGT (cyl. 3) ran high with more than 100 degree split between right and left. Usually the EGT is below the red line on takeoff and climb, but shortly after level off I can see temps over 1650 on the #2.
I did do an intake leak check by spraying starter fluid around the intake while it was running. No apparent leak. Plugs are gapped at .26.
Pulled carbs again and changed the main jet to 158 (I got the airplane with a 160 main jet) and the needle jet to 2.72, set needle at position #3 and adjusted idle air mixture screw to 1.5 turns.
At 8500’ altitude the EGT’s were perfect, when reducing power down low (3000’) the right EGT runs very high and unless you add cruise throttle or pull back to less than 50% power it gets out of limits).
Replaced main jets in both carbs. Went from 158’s to 155’s to see if that helped.
The left choke had been hanging and not closing completely intermittently so I built new choke cables and that appeared to cure the hanging choke issue.
Takeoff and climb out was fine but the EGT problems seem to be on both sides now. When I pull the choke circuit I get a slight RPM rise and a temperature drop so it had to be too lean.
Between 4200 and 5400 RPM the EGT still went out of limits (gets as high as 1700) on both sides now.
I replaced the main jets again with the 158’s and the EGT at 5500 RPM would run right at 1600 on both sides. Between 4200 and 5400 the EGT would rise to 1650 and above unless I changed power setting or pulled the choke.
I pulled the needles and placed them in position #4 (was #3). It helped, with the over temp now only between 4800 and 5400 RPM.
I have a good pneumatic sync at 3500 RPM (and I have checked the gauges for instrument error). The float arms are exactly level and the floats appear to be in good condition and floating level. The throttle cables appear to be hitting the stops at approximately the same times as well.
In a ground run up at 2200 RPM I can pull the choke circuit and get at 5-600 RPM rise before it starts to run rough. Mag drop at 4000 RPM is 100.
I’m lost and don’t know what to try next.
I’ve had an high EGT issue with this engine for the two years I’ve had this aircraft. I fly out of a 2100’ elevation strip. Here’s the history:
Prior to overhaul of the carbs it was barfing fuel, the float pivot shaft pulled out and had fallen down on one side in the float bowl and was missing the spring on one idle adjustment screw. It ran rough, wouldn’t idle below 2200 RPM and the right EGT would run above 1650 F unless you kept it at 5500 RPM or above.
I overhauled both Bing 64 CV carbs (gaskets, O rings, Viton needle, float arms). Needle jet is changed to 2.74 (it was running lean with a 2.76 even at position #4) and needle position at position #1. They appeared to be functioning correctly and idled smoothly at 1800 RPM running smoothly through the RPM range up to 5500. Idle air mixture screw set at 1 1/4 turns. Idle jet is a 35.
The #2 EGT (cyl. 3) ran high with more than 100 degree split between right and left. Usually the EGT is below the red line on takeoff and climb, but shortly after level off I can see temps over 1650 on the #2.
I did do an intake leak check by spraying starter fluid around the intake while it was running. No apparent leak. Plugs are gapped at .26.
Pulled carbs again and changed the main jet to 158 (I got the airplane with a 160 main jet) and the needle jet to 2.72, set needle at position #3 and adjusted idle air mixture screw to 1.5 turns.
At 8500’ altitude the EGT’s were perfect, when reducing power down low (3000’) the right EGT runs very high and unless you add cruise throttle or pull back to less than 50% power it gets out of limits).
Replaced main jets in both carbs. Went from 158’s to 155’s to see if that helped.
The left choke had been hanging and not closing completely intermittently so I built new choke cables and that appeared to cure the hanging choke issue.
Takeoff and climb out was fine but the EGT problems seem to be on both sides now. When I pull the choke circuit I get a slight RPM rise and a temperature drop so it had to be too lean.
Between 4200 and 5400 RPM the EGT still went out of limits (gets as high as 1700) on both sides now.
I replaced the main jets again with the 158’s and the EGT at 5500 RPM would run right at 1600 on both sides. Between 4200 and 5400 the EGT would rise to 1650 and above unless I changed power setting or pulled the choke.
I pulled the needles and placed them in position #4 (was #3). It helped, with the over temp now only between 4800 and 5400 RPM.
I have a good pneumatic sync at 3500 RPM (and I have checked the gauges for instrument error). The float arms are exactly level and the floats appear to be in good condition and floating level. The throttle cables appear to be hitting the stops at approximately the same times as well.
In a ground run up at 2200 RPM I can pull the choke circuit and get at 5-600 RPM rise before it starts to run rough. Mag drop at 4000 RPM is 100.
I’m lost and don’t know what to try next.
