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Permalink Reply by John Burns on December 6, 2021 at 1:03pm

What airspeed and rate of climb?  What RPM are you seeing in climb and cruise?  What is your oil temperature?  What generation Jabiru 3300 do you have? 

You are correct air flow is the solution to high CHT readings.  High EGT usually indicates a lean mixture.  Nick at Arion Aircraft (the USA Jabiru engine dealer) recommended increasing the main jet size in the carburetor to decrease high EGT's.  He is a good reference for these problems and will likely ask the same questions that I have if you contact him for technical support.

I am having problems with EGT, but not CHT on a Generation 4 Jabiru 3300.

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Permalink Reply by John Austin on December 6, 2021 at 1:34pm

Tuning the Bing Carburetor on Jaibiru Engines is a good resource for tuning/jetting advice.  It is not unusual at all to have to do some initial experimenting with the jetting - after all, you're stuffing an air-cooled, 6 cylinder engine into a low speed, high drag airframe.

I quite successfully got stable EGT's/CHT's in my Jab 3300/STOL 750 with carefully sealing any cooling airflow leaks and re-jetting as necessary.  I love to tinker and experiment, so I eventually installed an AeroInjector TBI carb which provides a mixture control - then, it was easy to adjust the mixture and put the EGT's anywhere I want them.  It's nice to be able to deliberately richen the mixture in climb to keep the engine cooler.  I routinely run CHT's in the upper 200'sF with a rare cylinder barely over 300F on the hottest summer days, and as I said, I can adjust the mixture to whatever EGT I desire.  However, installing the AeroInjector was definitely an experimental undertaking and it'll be much easier to re-jet your Bing carb.

John

N750A

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Permalink Reply by Bob Simmons on December 6, 2021 at 7:51pm

Your prop and pitch can affect this too because it changes the load on the engine. When you call Nick, give a good history. Is this a new install or aircraft? Is this a setup that previously performed fine and just started getting hotter? Is this an old aircraft with a new engine? New prop? Changed pitch? Different cowl? If this is a new install, it's probably running too lean. The Bing carburetor on new 3300 engines come from the factory set up for the Jabiru J250, which is too rich for a draggy aircraft like the 701. So the Johns here are probably right, but it doesn't hurt to check with Nick.

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Permalink Reply by RandyC on February 11, 2022 at 6:32pm

Props

Both the 85-hp and the 120-hp Jabiru engines have a maximum speed of 3300 rpm, some 600 rpm higher than the typical Lycoming engine. These engines are not happy swinging big props or turning at lower rpm. Jabiru has a maximum prop length of 70 inches that must be respected; a longer prop can over-stress the crankshaft and lead to a catastrophic failure. If most builders accept the maximum-length limitation, many do not quite embrace the idea that this engine likes to run at higher rpm. A propeller for the Jabiru should be pitched so that the engine will turn 3200-3300 rpm at full power in level flight. If it will not, it has too much pitch. While many people like to cruise their Lycoming engine at 2300-2400 rpm, the Jabiru is much happier at 2800-2900. Jabiru owners must simply recalibrate their comfort level up by about 500 rpm. It is important to find a throttle setting that brings EGTs and CHTs into the factory-recommended temperature limits and still makes good power.

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