Shop recommends replacing my carburettor

[M20 Ogler]

What are the symptoms of a carburettor that needs servicing or replacement?

Started happening about 2 weeks ago on my 1966 C.
 

Engine died on landing rollout last four flights even after shop has been adjusting the idle up. Min Idle had dropped to 500 with no explanation. It now at 600 rpm. Still having issues and A&P both said there is no room to adjust it more. Idle mixture seems to be set correctly because I get the 50 rpm rise before shutdown when I pull the mixture. 
 

Also lots of coughing and missing at low power on ground when engine is hot. I agreed to get an exchange carburettor coming because I think that is the safest bet here. 
 

Seems to run fine at takeoff and cruise power but I think I should keep it in the hangar until the new carb is on. 

[kortopates]

how many years since last overhaul? The seals/gaskets tend to be be shot and leak after 10-12 years, depending on the environment.


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[M20 Ogler]

2 minutes ago, kortopates said:

how many years since last overhaul? The seals/gaskets tend to be be shot and leak after 10-12 years, depending on the environment.


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It’s been a few decades so I’m inclined to believe it’s time to give that carb some TLC

[kortopates]

It’s been a few decades so I’m inclined to believe it’s time to give that carb some TLC

I’d also make sure it’s the richer of the approved part numbers. They’re available OH exchange from Spruce.


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[A64Pilot]

I’m sure they checked for intake leaks first?

[Shadrach]

8 minutes ago, A64Pilot said:

I’m sure they checked for intake leaks first?

Weird set of symptoms. I would think intake leaks would be most prevalent at idle. I don't think he'd see the 50rpm rise going to ICO if unmetered air was entering the system. Nevertheless the coughing and sputtering during ground ops reads like an intake leak.

Seems like starting at the air box and filter and working back would be a cheap first step. As would doing a leak test.

[Shadrach]

On 8/26/2023 at 1:07 PM, kortopates said:

I’d also make sure it’s the richer of the approved part numbers. They’re available OH exchange from Spruce.


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I have it on good authority that these engines don't need more fuel flow...

[kortopates]

I have it on good authority that these engines don't need more fuel flow...

So i’ve read above, lol!
Yet we’ve also read from one owner how a richer carb greatly improved ability to keep CHT’s down.
Truth is we can tell an engine isn’t getting enough fuel at WOT when EGT’s are hovering around 1500F.


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[Shadrach]

5 minutes ago, kortopates said:

So i’ve read above, lol!
Yet we’ve also read from one owner how a richer carb greatly improved ability to keep CHT’s down.
Truth is we can tell an engine isn’t getting enough fuel at WOT when EGT’s are hovering around 1500F.


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I also have it on good authority that raw EGT numbers are meaningless. Apparently some of these NA engines peak in the high 1600 to 1700 range...

[Hank]

4 hours ago, Shadrach said:

I also have it on good authority that raw EGT numbers are meaningless. Apparently some of these NA engines peak in the high 1600 to 1700 range...

It all depends on exactly where the EGT thermocouple is mounted. Move one 1/4" and see how the number changes.

[M20 Ogler]

Wouldn’t an intake leak make it run too lean? 

[Shadrach]

On 8/30/2023 at 7:11 PM, Hank said:

It all depends on exactly where the EGT thermocouple is mounted. Move one 1/4" and see how the number changes.

I don’t care where the thermocouple is mounted; if your take off EGTs are 1450, your engine is not rich enough. Have you ever seen an O360 that would generate a peak EGT of 1700? Regardless of thermocouple position, certain things are obvious from raw numbers.

[Shadrach]

1 hour ago, M20 Ogler said:

Wouldn’t an intake leak make it run too lean? 

Affirm.

[kortopates]

Wouldn’t an intake leak make it run too lean? 

Mostly true, but carbureted engines are trickier than that since the fuel is delivered by the carburetor a leak can be affecting fuel charge air and not just air like in an injected engine.


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[kortopates]

It all depends on exactly where the EGT thermocouple is mounted. Move one 1/4" and see how the number changes.

Having seen literally thousands of these you’d be amazed just how much they all have in common.


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[A64Pilot]

On 8/30/2023 at 7:34 PM, M20 Ogler said:

Wouldn’t an intake leak make it run too lean? 

Coughing and missing at low power can be indicative of an intake leak, intake leaks often only lean out one cylinder where the leak is and that one over lean cylinder can make it run rough, often intake leaks don’t really do much at high power because there is very little vacuum to leak, it’s low power when they show up. But yes un-metered air, that’s the fancy name for a leak, leans out the mixture.

But primary reason to check is that it’s a common problem, very easy check that costs no money just a few minutes to check.

I troubleshoot starting with easy and cheap stuff, even if it’s not the most likely problem, but sometimes it is, and when it is the fix was simple and relatively inexpensive, and it saves the spending big bucks to change a part to then find that the fix was easy and cheap.

Sort of like buying new mags then finding out you had a bad spark plug(s).

[A64Pilot]

On 8/30/2023 at 7:47 PM, Shadrach said:

I don’t care where the thermocouple is mounted; if your take off EGTs are 1450, you’re engine is not rich enough. Have you ever seen an O360 that would generate a peak EGT of 1700? Regardless of thermocouple position, certain things are obvious from raw numbers.

EGT’s really depend a lot on compression, high compression ratio engines like our angle valves run much lower EGT’s

My little C-85 with its 6 to 1 compression ratio normally runs EGT’s that would be scary on my IO-360, and the Wife’s Model-T with 4 to 1 will have its exhaust manifold glowing bright red with its 4 to 1 compression. But the C-85 will run on 73 Octane fuel and the Ford likely can handle 40’s Octane.

So take an old low compression motor that has high blow by (my C-85) and it’s EGT really gets up there, how high I don’t know it’s not instrumented, be interesting to know but not enough to instrument it. 

While you might can infer something from EGT for a normally aspirated engine the EGT number is irrelevant, only relevance to it is how much rich or lean of peak are you, the temp of peak isn’t relevant.

You guys like Savvy, read this 

https://resources.savvyaviation.com/understanding-cht-and-egt-2/

[M20 Ogler]

I’ll ask them to check for intake leaks. The last time I had an intake leak it was caused by a warp in the in flange on the intake pipe on Cyl no 2 where it lands on the cylinder. It was backfiring at idle. Its not doing that now 

What it’s doing now is the idle was creeping down lower and lower and I’m being told there is no more room to adjust it back up. Also the engine is dying on my landing roll outs. 

[Shadrach]

2 hours ago, A64Pilot said:

EGT’s really depend a lot on compression, high compression ratio engines like our angle valves run much lower EGT’s

My little C-85 with its 6 to 1 compression ratio normally runs EGT’s that would be scary on my IO-360, and the Wife’s Model-T with 4 to 1 will have its exhaust manifold glowing bright red with its 4 to 1 compression. But the C-85 will run on 73 Octane fuel and the Ford likely can handle 40’s Octane.

So take an old low compression motor that has high blow by (my C-85) and it’s EGT really gets up there, how high I don’t know it’s not instrumented, be interesting to know but not enough to instrument it. 

While you might can infer something from EGT for a normally aspirated engine the EGT number is irrelevant, only relevance to it is how much rich or lean of peak are you, the temp of peak isn’t relevant.

You guys like Savvy, read this 

https://resources.savvyaviation.com/understanding-cht-and-egt-2/

Agree that EGTs track compression ratio. The less thermally efficient the engine, he more energy that leaves via the exhaust.

I have to disagree that EGTs track low compression, though I am open to being educated. I’ve seen no evidence that diminished cylinder compression elevates EGT as long as it’s not from a pronounced exhaust leak. Low compression has little to no effect on power or thermal efficiency. There’s an aviation urban legend that Continental ran an engine without rings and it still made rated horsepower, albeit briefly…

At 2500rpm a four cylinder Lycoming has 83 power pulses per second. Variances in compression from cylinder to cylinder don’t really matter in terms of combustion. Maybe at the extreme edge, but then you have a bigger problem on you hands

[A64Pilot]

Yeah, I’m not saying your carb doesn’t need replacing, but that hopefully the intake was leak checked first or if not that it will be when the carb is replaced.

Vacuum leaks often are the cause of drivability issues to use an automotive term.

Idle not being able to be adjusted does point to idle circuit, but getting the proper RPM increase upon slowly pulling the mixture seems to indicate it’s good.

 

[Shadrach]

55 minutes ago, M20 Ogler said:

I’ll ask them to check for intake leaks. The last time I had an intake leak it was caused by a warp in the in flange on the intake pipe on Cyl no 2 where it lands on the cylinder. It was backfiring at idle. Its not doing that now 

What it’s doing now is the idle was creeping down lower and lower and I’m being told there is no more room to adjust it back up. Also the engine is dying on my landing roll outs. 

I thought it was running rough during ground ops?

[A64Pilot]

15 minutes ago, Shadrach said:

Agree that EGTs track compression ratio. The less thermally efficient the engine, he more energy that leaves via the exhaust.

Disagree that EGTs track low compression. I’ve seen no evidence that diminished cylinder compression elevates EGT as long as it’s not from a pronounced exhaust leak. Low compression has little to no effect on power or thermal efficiency. Have you not heard the urban legend that Continental ran an engine without rings and it still made rated horsepower, albeit briefly.

Any engine, whether it be a lawnmower, auto or aircraft motor loses power when they are “worn out”, power wise worn out refers to lost compression not excessive bearing clearances etc.

Most that have had an engine overhauled or replaced if the old engine was worn out notice an increase in power.

Yes I’ve heard of engines with low compression making good power, but take it with a grain of salt, and more to the point I don’t know what happens to their EGT.

I bumped into the lower compression having markedly higher EGT quite by accident flying a friends low compression carbureted 235 HP 540 after flying my high compression IO-540, both were fresh motors making good power but his EGT was so much higher it scared me when I saw it, wanted to land right away thinking something must be wrong, but of course nothing was.

On edit, after thinking about it I realized tgat a low compression motor is simply less efficient and this decrease in efficiency is reflected by higher EGT, a more efficient motor of course converts more heat to HP and has lower EGT

[M20 Ogler]

New carburettor is on now. Had to cancel test flight today because it’s failing mag check. When I switch to left mag I get a 300 rpm drop, rough running, and popping sounds from the exhaust. Oh the joys of owning an airplane

[jetdriven]

It could also mean the mixture is it way too rich