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Recently, when Oscar, (owner of a Mooney,) and his son stopped in at Classic Aviation, he mentioned that he had been having some trouble with high CHT’s on takeoff and climb, (maybe even in the 420-450 degree range at times.) Everything was fine in cruise.
So we talked about the normal things you would think of to reduce cylinder head temps… check the baffles and baffle seals, verify correct mag timing, verify a good ground connection between the engine and airframe, verify correct fuel flow, etc. Oscar indicated he had considered all those options. He did say he thought maybe the fuel flow should be a little higher, but with a carbureted engine, full power fuel flow is not field adjustable.
So one question would be: should he replace the carburetor, or send it out for bench testing?
This is what we’re talking about in today’s episode, and we’re looking for your ideas.
If you have any ideas for how Oscar can bring his CHT’s down (Lycoming O-360 engine,) please comment on this episode, send me an email, or leave a voice message here on the website.
In the next episode, we’ll be talking about a story Jeremy Zawodny sent me about his partial power loss after takeoff in his Glastar. It’s definitely a story worth learning from!
Dean,
One of the things I suggest checking for when dealing with high CHTS (on Lycoming engines), after fixing obvious things like baffle leaks is cylinder “flashing.” Flashing is the metal that squeezes out between the mold halves when the cylinders are cast during production. On the outside most of it is removed, but there is a particularly difficult spot to reach just behind the spark plug wells. If you look straight down by the plug you can see the area in question. If there is any debris or flashing blocking the airflow there, this will cause a high CHT reading as either the gasket style CHT probe under the spark plug or the bayonet style threaded into the hole on the bottom of the cylinder are both in this area.
Some cylinders apparently left the factory with poor or no flashing removal. I had a Grumman Cheetah at the time that had cylinders with this problem. Removal is simple but time consuming. Its done by hand and it takes about 30-45 min with a needle file to clean all that metal out of there, but I had an immediate and significant drop in measured CHTs.
There used to be a website with pictures and information on this subject but I cannot find the link to share. The individual who created it had data and did testing expressing that even small protrusions in this area from casting flashing created big turbulence in the airflow and resulted in cooling efficiency reductions more than you would think for the small reduction in area caused by the small bumps themselves. The point is, it is important to have the fins in this area completely smooth to get the best chances for improvement. Even if you have some opening in that area it should be wide open for best cooling. There person that made the website I was referring to had at least one cylinder completely blocked off save a 1/8″ hole apparently drilled at the factory to open the area for cleaning that was never accomplished.
I am an A&P/IA but this is a job an owner could do in my opinion. I actually prefer an owner do this as its simple, hard to mess up and very time consuming. If it works
its great, but if it doesn’t and you bill an owner 2-3 hours for doing the work as an A&P then you might have an unhappy customer.
Thanks Matt! You’ve taught me something new on this one, and I’ll definitely be looking for this situation. I appreciate your detailed response, and I will cover this in a future episode.
Dean
Norm Robinson, I fly my SuperCub right up the road from you from Leesburg, VA
Love your podcast
When I owned an M20 E and on one occasion experienced high CHTs it was do to the cowl flaps sticking and not opening entirely. Similarly mine only occurred on takeoff. The Mooney manual has a very specific measurement from the bottom of the cowl to the fully open cowl flap. A little LPS, working it back and forth and never had a high CHT issue again.
Norm
Thank you Norm! That is really good information and certainly worth checking out. I’ll mention this idea in a future episode. Stop in and see me sometime at SHD.
Mike Ellis – owner of a Comanche that is having the same problems with his 250. He posts on YouTube and his latest couple of videos talk about it. He is sending his carb to Marvel-Schebler as I type this to have it benched and possibly have the jets replaced (drilled out? can that be done?)
Al Greene
Thanks Allan! That’s good to know. Let us know how that turns out for Mike on his Comanche.