Bing 64 tuning on a 4 stroke

[WurlyBird]

I have gotten to be pretty fluent in jet tuning with Bing carbs on 2 strokes and I don’t want to apply the wrong info to my 4 stroke. I have an HKS 700E (don’s top reading if you’re a 912 person) and I am having EGT issues. I only have 6 hours on the engine and the highest OAT I have operated in so far was mid 40s. At that temp I had a hot spot between 4200-4900 where EGT would hit limits if allowed. Today I went to fly and it was 28f, the coldest yet by about 10 degrees. At full throttle my take off run only produced 5600 RPM and 1390 EGT (limit is 1400), normally I hit 5900-6000. The engine did not produce proper power and almost exceeded limits.

In my head it all points to higher air density leading to a leaner mixture and I need a bigger main jet. But I can not find info on tuning Bing carbs on 4 strokes. It seems to be set it and forget it because of the constant velocity system. Anyone else braving the cold temps and experience anything similar? Am I misdiagnosing the issue? Does prop loading affect 4 strokes the way it does 2 strokes, similar, different, at all? On my Lycoming loading the prop leads to higher CHTs which ultimately lead to higher EGTs, but it takes a minute.

input and thoughts, please.

[avidflyer]

The bing 64 has three circuits, just as the bing 54 does. Idle circuit, mid range, and full throttle. The mid range uses a jet needle that you can raise or lower, much like you would on the two stroke carb. Main jet can be changed like the 54 as well. JImChuk

[WurlyBird]

Sorry, not trying to be dismissive, but I’m trying to get past the simplest explanation.

So I get how the 64 differs from the 54, and I have seen the exploded diagram. I also understand the operation of all three circuits and the purpose of all three jets. But is there any information on the tuning of a Bing when attached to a 4 stroke? Or is there any difference? It seems to me there should be a difference since there is actual valves instead of ports, they are not the same engines. Do other people think my operational description points to jets or might there be another issue with the CV carb? Why is there a thousand articles on tuning carbs for 2 strokes and not a one for the same Bing carb on a 4 stroke. I have spent hours searching for this and it seems illusive.

As it stands I will try and tune it similar to a 2 stroke since there is no better information. I expect marginal results.

[avidflyer]

On my 912 UL, I set the jet needles richer in the winter. Didn't need to change the main jets though. But if I had to I would. Not quite as cold as Alaska, but I'm in northern Mn. JImChuk

[WurlyBird]

Thank you. At least now I know I won’t be the only one tinkering with jets. Tonight I am going to pull the main jets to see what their size is and order the next two sizes. I was leaning toward raising the jet needle until it got hot at full throttle, so I’ll adjust the main first. Alaska can keep aviation interesting, we are only a couple months away from -2000’ DA.