High EGT

I agree with RW that one plug missing should not make it have high EGT. Losing one plug doesn't make it run lean which is usually caused by less than adequate fuel / air mixture.

Food for thought:

I have seen higher EGT's with poor air flow through the air filter. I had this happen a couple of times with paper air filters. Owners complained they were at max temp. I put in a K&N air filter and on this one engine they dropped 150F across the board.

Hey Sean,

Is it possible you have a small exhaust leak in the reworked header pipe?  This can draw air into the header pipe during the low pressure portion of the exhaust pulse, leading to ignition of unburned fuel in the exhaust. 

Jeff Blakeslee wrote:

Hey Sean,

Is it possible you have a small exhaust leak in the reworked header pipe?  This can draw air into the header pipe during the low pressure portion of the exhaust pulse, leading to ignition of unburned fuel in the exhaust. 

Hmmm! How would I know???  - What would be the symptoms??

You may or may not have small popping sounds at certain throttle positions, but don’t count on that.  I would just do a careful visual inspection of the header pipes to make they seat well at joints and at the cylinders. Also look for cracks around the heat affected zone of all welds, and that your EGT probes are properly sealed.  It’s along shot, but also easy to rule out.  I just thought the timing of this problem seemed to coincide with your recent exhaust work.  

Hi Jeff,

Thanks for that.

I have suspected the modified header/exhaust pipe since its installation. Have had it out twice. Not knowing what to look for, I found no anomalies.

Yesterday did an in situ clean of carburettor. Removed float bowl - no contaminants. Removed piston & blew carby cleaner down needle jet assembly.

Checked all 8 plugs. Bottom plugs heavily contaminated. Cleaned & reinstalled.

Opened idle mixture screw to 1.75 turns (is this leaning or enriching?) , in the hope of less rich slow speed running.

Flew aircraft - no improvement to EGT.

Expecting rain over eastern Australia. When weather permits, will, yet gain, do another carby balance.

If no improvement, will swop EGT probe location  (not so easy in my set up) to see if high temp follows.