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Permalink Reply by Dave Geiling on January 26, 2024 at 6:53am

Sam,

I'm not a chemist and I didn't stay at a Holiday Inn Express last night, but from all I've read what you show is a lead based compound that forms when combustion temperatures are too low.  I think Mike Busch has some articles mentioning this being an issue with valve sticking,  My 3300 looks the same way.  I believe that the cause of this is the Bing carb.  I have experimented as you are doing with little positive results.  I also replaced the Bing with the new, currently installed carb from Jabiru, which has proved to be more consistent, but no better to tune and more difficult to acquire different size jets.  My next project will include a Marvel Schebler carb with a manual mixture control and if the budget will allow, I will add an air-fuel ratio instrument to allow additional info relative to the actual fuel mixture the engine is working with.

I know I might be criticized by some who think the Bing is an acceptable carb, but my opinion is that until Jabiru replaces it with a carb that has some manual means of leaning in flight they are not providing an acceptable aircraft engine!  Don't get me wrong, I really like my Jabiru, the power per pound, the smoothness, and of course the exhaust note that the folks on the ground get to hear are all great attributes.

It would be informative if John Austin could offer some borescope pictures of his piston tops after changing to the Rotec (I think) throttle body carb.

Dave G

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Permalink Reply by John Austin on January 26, 2024 at 8:13am

Sam,

First thing, you are apparently way over-oiling the filter - that may even be slightly obstructing the filter and making you run rich.  I always just lightly oil K&N filters and don't saturate them.  That 210F CHT is unbelievably low - are you running a 6 probe system and it's that low on all cylinders?   If you're using probes under the plugs, the temp will be indicating lower than actual due to air flow cooling - it's much more accurate to have the probe screwed to the hole between the plugs and shielded from air flow - there's a Jabiru service bulletin on the mod and you can shield the probe head with a short piece of tubing or fiberglass wire loom, etc. I never see the pinkish deposits you show, perhaps that is due to the over-oiled filter?

Dave,

I don't have any pictures on file, but my pistons will accumulate some whitish deposits and I assume it is from the rare times I run 100LL (I usually run 93 UL/no ethanol).  I have a throttle body carb with a mixture control, but it is the Sonex/AeroConversions "AeroInjector" carb.  People either love or hate this carb - it has to be installed very carefully with a continuously rising fuel line from firewall to carb so that there are no downward turns to form a trap for fuel vapor, causing "burps" or hesitation of the engine when fuel vapor hits the carb.  The mixture control works great and I can dial the fuel flow to as precise as 0.1 gal/hr.  I've got a new Gen 4 on-order and Nick at Arion said I could continue to run the AeroInjector on the Gen 4 and that it would not void my warranty as long as there wasn't a claim related to carb issues.

John

N750A

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Permalink Reply by Sam Cyphers on January 26, 2024 at 12:11pm

Hi John, the wilder part of the 210 CHT is thats the hottest cylinder. I have all cylinders connected to a 6-position switch and single gauge. As I go through my positions 1-6, I get different EGT read-outs. And 210 is my most rear 6 cylinder, meaning my 1,2,3,4 postion cylinders are far less. My CHT probes are installed correctly in the hole between the plugs and shielded from airflow. So yes, that is a scary low CHT and I agree. So it looks like I need a smaller jet size, higher EGTs meaning higher CHTs. 

If this doesn't work I might experiment with additional baffles in my ram air intakes

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Permalink Reply by Sam Cyphers on January 26, 2024 at 12:02pm

Dave, this is incredibly helpful! I have another thread on this story, but what happened to me is upon annual inspection I discovered the scat tub between filter and carb had come off, meaning I was drawing raw air into the carb. I put the connect together again, and all my temps went way low. Which is to say my mixture was really far off. To compensate I have been brining down the jet sizes to allow less fuel/more oxygen and a cleaner burn. I agree with you and Mike Busch on leaning, and I can believe there is not a function on a Bing carb for this. Until I can get a new carb and go through that process I am going to bring down the jet sizes, increase the EGT and hopefully get a clean fuel burn. 

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Permalink Reply by Michael radtke on January 29, 2024 at 9:53am

Hi Sam,

    all the commenters are spot on. We run a jab 3300 w a rotec tbi so we can manuely adjust the mixture.

One thing you didnt mention is what type fuel you are using.

We always use 100LL which actually has alot of lead in it.

We add TCP or Decalin a lead scavaging liquid to ea tank of gas.

You can order this from aircraft spruce, get the optional siringe to extract and put in tank.

I hope this helps.

michael.

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