Carb Overhaul/Rebuild vs. New?

farmrjohn

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farmrjohn
Looking for thoughts on MS SPA3A carbs. This is for an O-200 installed in 1994 with over 1800 hours on it. There is oil/fuel that drips from the air box both during operation and after shutdown. The engine starts easily and seems to run fine with good power and fuel economy. Questions: 1. when to tell if it's time to overhaul or rebuild (either chronological age and/or operating characteristics) and 2. if suggested, which route to take for best value, rebuild, overhaul, or new?
 
Throttle Bushings still tight? Sloppy?
 
I'll jump in...
Mine seeps some fuel from time to time around the gasket to the airbox.
The throttle bushings have some play, but not ridiculously so.
Also runs fine, and seemingly normal EGT on climb out, so I don't expect too much air leakage, yet.
Same questions as OP...
 
If it’s leaking… something is wrong.

A simple in shop tear down will tell if so bad it needs refurb/replace.

That being said, if it’s running fine, no real reason to suspect throttle shaft bushings are so bad it’s obvious it needs that, so a simple in shop rebuild should be fine.
 
If it's leaking it likely needs overhaul.

Sometimes they last 100 hours sometimes they last 3000 hours. We personally do not open them ourselves, just send them out and have spares on hand, although it can be hard to find some of the old MS stuff. Newer fuel servos or injector pumps are just as sporadic.
 
MAKE SURE that it's the carb leaking. Don't assume it. The O-200 typically has its primer nozzle in the "spider" manifold right above the carb, and if the primer has issues, it will let fuel get sucked into the carb and might even allow flow when sitting. Is the primer even fully "In and Locked?" There is a spring-loaded needle in that plunger that closes off the outlet in the primer cylinder, but the primer plunger has to be all the way in and locked there.
The MA3 carb will slop fuel out of its bowl vent into the carb bore. That vent is behind the venturi, where the still air gives pretty close to ambient air pressure. The O-200 overhaul manual has a few paragraphs that are often overlooked by mechanics:

1707423222793.png

See that? Using washers that have rubber in them, and tightening the stud nuts finger tight. I have found too many with those washers missing altogether, and the carb gets shaken because the spider can't flex on those studs, the carb gets violently shaken at certain RPMs, and fuel gets sloshed out of the bowl vent into the carb. Makes rough running and fuel spillage. A better picture:

1707423490227.png
 
How is your carb leaking oil?.....that means you have oil leaking past the intake valve guides and running down the intake tubes into the carb. That's not a carb issue.
 
MAKE SURE that it's the carb leaking. Don't assume it. The O-200 typically has its primer nozzle in the "spider" manifold right above the carb, and if the primer has issues, it will let fuel get sucked into the carb and might even allow flow when sitting. Is the primer even fully "In and Locked?" There is a spring-loaded needle in that plunger that closes off the outlet in the primer cylinder, but the primer plunger has to be all the way in and locked there.
The MA3 carb will slop fuel out of its bowl vent into the carb bore. That vent is behind the venturi, where the still air gives pretty close to ambient air pressure. The O-200 overhaul manual has a few paragraphs that are often overlooked by mechanics:

View attachment 125197

See that? Using washers that have rubber in them, and tightening the stud nuts finger tight. I have found too many with those washers missing altogether, and the carb gets shaken because the spider can't flex on those studs, the carb gets violently shaken at certain RPMs, and fuel gets sloshed out of the bowl vent into the carb. Makes rough running and fuel spillage. A better picture:

View attachment 125198
I can confirm the primer is fully in and locked during operation except when using it prior to first start. The carb is mounted with the lock-o-seals finger tight. Two annuals ago the shop said that the carb was loose and tightened them down, crushing the lock-o-seals. That was corrected with the help of a different AP/IA thanks to your previous comments here and referencing the manual.
How is your carb leaking oil?.....that means you have oil leaking past the intake valve guides and running down the intake tubes into the carb. That's not a carb issue.
The leaked fluid residue on the cowling below the airbox has a slight blue tint to it as seen against the cowl's white background. However the fluid that drips out afterward into a catch cup looks more like regular oil. I'll be changing the intake tube rubbers this annual which should reveal any leaking past the intake valve guides in the tubes to the spider.
 
The leaked fluid residue on the cowling below the airbox has a slight blue tint to it as seen against the cowl's white background. However the fluid that drips out afterward into a catch cup looks more like regular oil.
Most likely it's all gasoline residue. It leaves blue oily stuff when it evaporates, and can turn brown, too.
 
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