Mystery solved.... At least I hope so.

I called the shop today to get an update. Neither Alex nor Preston was available at the time, so the person who answered the phone said they'd have one or the other call me back later in the day. Later in the day rolled around and Alex called and gave me an update.

This is what happened, based on my understanding of what Alex told me. I think I have it down pretty well. First of all, for the sake of brevity and clarity, I'm going to use the following references:

• "Carb 1": The original carburetor that was on my engine when I bought the plane, and had probably been on there at least since 1997.
• "Carb 2": The carburetor that the carb shop in Florida sent back to One Stop Aviation instead of carb 1.
• "Carb 3": The carburetor that will hopefully be arriving at the shop at Harvey Field before long. 
• "Fuel pump 1": The original fuel pump that was on my engine when I bought the plane, and had been installed about a year before I bought it.
• "Fuel pump 2": The new/overhauled fuel pump that One Stop Aviation sent to the shop at Harvey Field when they realized the did not overhaul fuel pump 1 like they said they would in the work quote.

There are two reasons that the carb shop did not overhaul carb 1 and return it, and instead swapped it for carb 2. The first is that there is a service bulletin (I'm guessing it's from Lycoming, though it could be from Beechcraft) that said not to use that model of carburetor on that engine anymore. The second is that carb 1 had, at some point in the past, been "modified in the field". A small portion of the carb housing had been ground or filed away to provide clearance for the fuel pump.

The issue was that carb 2 is the wrong model for this engine. I don't know why the carb shop thought it was the correct model, but it is, in fact, the wrong one. Although it works, it touches the fuel pump. The reason that the shop at Harvey was able to run the engine anyway with carb 2 and fuel pump 1 is that they still mount correctly and properly flush, they just touch each other. You can run the engine with them like that, but you don't want to leave it that way for long.

As far as I know, fuel pumps 1 and 2 are both the correct model and are identical in form and function. As I noted in my previous post, the part number for fuel pump 2 appears to be a match for the part number I was able to determine from the IPC. So the fuel pump was never the issue.

Once carb 2 was sent back to the carb shop and they did some further checking, they determined what the actual correct carburetor is. As it turns out, it was manufactured with the clearance for the fuel pump. Although I didn't ask, I suspect that the service bulletin was issued because the carb model in question does contact the fuel pump and an updated carburetor was put into production.

The carb shop did not have a carburetor of the correct model ready to go, so they are overhauling one now and will ship it overnight when it's ready. This is carb 3. I asked Alex if he could get me the part number for carb 3 and he sent it to me via email a short time later. It's 10-5219. I looked it up, and it's the part number used by the manufacturer, Marvel Schebler. The product page on Aircraft Spruce for that carb lists the equivalent Lycoming part number, LW-15957. This is the part number that my friend and I determined is the correct one based on the IPC, which increases confidence that it's the right one.

Alex said that it will probably take about a week for the carb shop to finish the overhaul and ship the carb out. Once it arrives and, assuming it really is the right one, it's installed, that should be the last thing that needs to be done. At that time, the plane should be ready to fly once again. I sure hope so.