Question Marvel-Schebler MA-4-5 Carburetor Setup

  • Arthur Tiller
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09 Aug 2023 06:24 #4064 by Arthur Tiller
Replied by Arthur Tiller on topic Marvel-Schebler MA-4-5 Carburetor Setup
I was still referring to the mixture an adjustment screw on the carb. Sorry about the confusion.

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07 Aug 2023 11:00 #4059 by STEVE ELLS
Replied by STEVE ELLS on topic Marvel-Schebler MA-4-5 Carburetor Setup
Hi Arthur;
Yes, altitude makes a difference in the rise seen as the mixture control knob in the cockpit it pulled aft to shut the engine off. Reason? There's less atmospheric pressure at 5,300 feet than there is a an airport that's at 1000 feet or sea level.
 
Lower atmospheric pressure translates into fewer oxygen molecules entering the carburetor at higher altitudes than will enter at lower altitudes. Less oxygen means that less fuel is required at idle rpm to get the desired 50 rpm rise.

The 1 1/2 turns I mentioned in an earlier reply are a starting point. The engine should run at that adjustment at both altitudes. But the idle mixture adjustment screw in the carburetor must be either screwed in by turning it clockwise (to lessen the amount of fuel if the rpm is greater than 50 rpm at shutdown) or screwed out by turning it counterclockwise (to increase the amount of fuel if the rpm drops straight off at shutdown).

Since atmospheric pressure has an effect, the correct idle mixture setting at 5,300 feet will not be correct at 1,000 feet or sea level.

I'm just slightly confused. You ask in the last sentence "what would you think the usual position on the mixture knob be from full in after the setup is complete?"  

The position of the mixture knob on the panel when the mixture is full rich must have some "cushion" That means the mixture arm on the carburetor (I'm not talking about the idle mixture screw) is against the full stop on the carburetor, the knob on the cockpit mixture control should not be bottomed out against the panel. This "cushion" should be at least 1/4 inch. This is important to make sure the mixture setting on the carburetor hits full rich before the mixture control in the cabin hits full in.

I hope that's clear.

Best,
Steve

 
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07 Aug 2023 06:34 - 07 Aug 2023 06:47 #4058 by Arthur Tiller
Replied by Arthur Tiller on topic Marvel-Schebler MA-4-5 Carburetor Setup
I would be wary of tweaking the mixture with the engine running. Especially if I were the only A&P working on the job. I assume that one and a half turns would be the close to the middle of the range between too lean and too rich. A high altitude adjustment would be towards lean and perhaps a tweak towards rich would be good for a sea level airport. I wonder if a setting of two and a half turns from full in would be a safe place to start for a 1000 ft airport. Perhaps a little on the rich side.

My most recent experience with these carburetors dates back to the mid 80s and at high altitudes. On a high DA day back then I remember a very noticeable increase in RPM (+150 RPM) when leaning during a run up.

Now that I am operating near sea level I am a little curious about the lack of any noticeable rise in RPM during shut down for an aircraft that came from a 5300 MSL airport. An A&P has told me that some times at our local airfield (+1000 Ft) it is some times hard to see any increase in RPM during a shut down. In the aircraft I fly the RPM needle at idle is bouncing 50 RPM at idle.. Sometimes I can imagine that I see an increase at shut down. But it is hard to be sure.

For an airport at 1000 MSL: In your experience, what would you think the usual position on the mixture knob be from full in after the setup is complete?
Last edit: 07 Aug 2023 06:47 by Arthur Tiller. Reason: Format and af few words.

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29 Jul 2023 13:32 #4035 by STEVE ELLS
Hi Arthur;
The original Notice of Proposed Rule Making (NPRM) did list the PA 28-180. After that issue, the FAA issued a Supplemental NPRM. The PA-28-180 was not listed in the SNPRM or the AD.

The SNPRM did say that the FAA has done further testing between the NPRM (Dec 2018) and the issuance of the SNPRM (June 2020) that added some airplanes and subtracted some airplanes from the original AD.

The further testing much have concluded that the PA 28-180 wing/structure was not in the crack protocol limits. 

Steve
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29 Jul 2023 05:47 #4031 by Arthur Tiller
I am curious as to why the PA-28 180 is not listed in the applicability table of the wing spar inspection AD. The table lists the PA-28R 180 but not the PA-28 180. What is so special about the PA-28 180?  Thanks

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