Cylinder leaning itself while climbing?

Fearless Tower

Fearless Tower

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Fearless Tower
Curious what would cause a cylinder to start running leaner while climbing without changing the mixture?

Left engine has single CHT/EGT. While climbing above 6500', the left engine EGT starts to rise. Shortly after that the CHT also starts to rise. Then the engine starts to run rough, but rough like other cylinders are too rich - confirmed by leaning which smooths the overall engine out, but obviously doesn't change the rise in EGT/CHT on the cylinder with the single probe.

This is a relatively new development before the left engine behaved identically to the right.
 
intake leak? depending on where it is....it should be noticeable when idling.
 
Fearless Tower said:
This is a relatively new development before the left engine behaved identically to the right.
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Its been awhile, but my guess is the barometric bellows/linkage has an issue. Most pressure carbs I'm familiar with will "auto-lean" via the bellows with increase in altitude. The fact things change when you move the mixture and you report no other fuel schedule issues points toward the bellows. Sometimes you could get lucky and find a blocked/dirty orifice on the carb but usually its an internal issue. However, I never really had a chance to fully understand how those type carbs worked. Any chance you know the carb model number?
 
Bendix Pressure Carbs function somewhat like the Bendix RS Fuel Injection

Systems. The ones I familiar with disperse fuel into the “eye “ of the Impeller.

I’m thinking one plug cutting out would raise EGT and possibly detonate causing

high CHT. Smoother with Leaner doesn’t make sense though.

Do you have Auto-Rich, Auto- Lean?
 
Magman said:
Bendix Pressure Carbs function somewhat like the Bendix RS Fuel Injection

Systems. The ones I familiar with disperse fuel into the “eye “ of the Impeller.

I’m thinking one plug cutting out would raise EGT and possibly detonate causing

high CHT. Smoother with Leaner doesn’t make sense though.

Do you have Auto-Rich, Auto- Lean?
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This engine has a Bendix injection unit. Not a carburetor.

No auto-rich/lean
 
Bell206 said:
Define what you mean by "Bendix injection unit." Are you talking about the original Bendix-Strombergs? Or something new?
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It’s an early fuel Injection system for radial engines. It was an STC in the early 60s.

Basically bolts on in place of the carb, but consists of a single nozzle and some internal diaphragms. No moving parts.

It’s referred to as fuel injection, but doesn’t direct fuel to each cylinder individually like a flat fuel injected engine
 
Fearless Tower said:
Basically bolts on in place of the carb, but consists of a single nozzle and some internal diaphragms. No moving parts.
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Not being snarky, trying to learn.

Wouldn’t a diaphragm be considered a moving part?
 
Fearless Tower said:
It’s an early fuel Injection system for radial engines. It was an STC in the early 60s.
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Do you have any system diagrams/system description you can post? STC #? Did they add an electric fuel pump to the system? Fuel flow indicator? Where is the single injection nozzle installed? Without any references tough to make an educated guess as there could be slight differences with your specific single nozzle system. Regardless, since its leaning on its own, my first guess would be to one of the metering diaphragms leaking at altitude or an orifice/jet has some issue.
 
Early Bonanzas had a Bendix pressure carb with auto mixture. The auto mixture portion might have an issue..cannot say for sure, but that would be my guess. Like Bell and Magman posted, these systems are somewhat complicated and there are relationships between different portions of the unit that come into play. Could be something else, but I would look here first.
 
Could the increase in EGT be the result of incomplete burning in the Combustion

Chamber? This could be caused by single ignition. As you gain altitude the air is

less effective as an insulator and there is a greater likelihood of “ flashover” in

the Leads or even the Mag Distributor. Hence you may not be lean at all.

New plugs and checking Leads with a high voltage tester would not be time

wasted.

FYI- Some of the larger radials had on-board Ignition Analyzers

( aka “Chinese Television”) that allowed you to observe firing of any one or all

plugs. One cylinder CHT and no EGT though. And this from aircraft built in the

‘40’s!
 
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