I just removed my carburetor due to a fuel leak that occurred immediately after shutdown. About a shot glass or two worth of fuel would seep down the intake and leak onto the top of the airbox. I installed a newly rebuilt carb and the same thing is happening. My IA says the replacement carb is bad and to get another one. He recommended QAA but after this last rebuild from spruce (rebuilt by MS) failed I'm thinking a brand new one might save me time...?

Interestingly since this happened I've heard of other reports of 180s and 182s leaking fuel immediately after shutdown. Unfortunately I only found my leak after a shutdown / restart cycle where I had a backfire on the restart leading to a quickly extinguished fire.

Any ideas what causes the carb to leak only after shutdown?

A carburetor issue similar to yours came up a while back,

Hman442 wrote:A carburetor issue similar to yours came up a while back,

viewtopic.php?t=2275

Wow, I don't know how that happened ? Anyway, search carb leak, and a thread from February talks about an issue similar to what you're describing

Ahh sorry I missed that topic when I first searched! What's the protocol? Delete this thread and repost on the old one?

Wondering if anyone has found a smoking gun or if it's just two bad carbs in my case.

Could it be related to the updraft carb and intake air speed vs gravity?? I know in my '58 182, if I pull the mixture knob at an idle (~650rpm), the motor will just lope and chug along for a while before it shuts down. And when it finally does die, there is a slight fuel smell. However, if I am taxiing up to the hangar at 1000rpm and pull mixture, I get a quick little raise in rpm and then it just quits, and no fuel smell.

I don't think I get any discernible amount of fuel "leaking" out, but I've never really jumped out and looked after shutdown.

ebag wrote:Could it be related to the updraft carb and intake air speed vs gravity?? I know in my '58 182, if I pull the mixture knob at an idle (~650rpm), the motor will just lope and chug along for a while before it shuts down. And when it finally does die, there is a slight fuel smell. However, if I am taxiing up to the hangar at 1000rpm and pull mixture, I get a quick little raise in rpm and then it just quits, and no fuel smell.

I don't think I get any discernible amount of fuel "leaking" out, but I've never really jumped out and looked after shutdown.

I agree. If there are no other "symptoms", no continued leakage, etc., I would consider this & try leaning aggressively while taxiing back to your hanger, then bring the idle up to 1000 & then pull mixture to kill engine, check & see if you still have an issue. Taxiing along slowly at full rich & then killing the engine - could be gas just not pulled into the cylinders yet.

The leak still occured after we pulled the primer.

Next I drained the mogas/avgas blend I've been running and filled a few gallons of avgas into the tank. No leak using pure avgas. I'm going to play around with this a little more tomorrow with one wing mogas, the other avgas. A friend will be doing the same in his 180 in which he recently started running partial mogas (ethanol free/tested). May also give MS carbs a call.

That said, in retrospect, I don't believe this issue alone caused the flare up a week ago. Long story there including a separated crossover tube, operator (throttle) error and a backfire.

Happy to be troubleshooting at my hangar instead of in the middle of nowhere!

It sounds like you're on your way to finding the answers. Keep us informed on what you find.

This is a common problem with mogas in the O-470. It's not the carb, it's unburned fuel in the horribly inefficient manifold.
Anytime on the ground the mixture should be leaned as much as possible. Shut down the engine at 1000rpm.
Otherwise just deal with it or stop using mogas. I hope your getting your mogas from the airport and not a gas station. Unless mogas comes from an ag supplier or airport it's probably got ethanol.

Yaup - it is indeed the mogas. Tested with mogas in one wing, avgas in the other. Every time I shut down with mogas there is a small leak. It does seem to be coming from the manifold. In fact, I could hear the gas gurgling down to the intake before it started seeping out.

This was with a warm engine and happened no matter what shutdown procedure I used.

I do use gas station gas - ethanol free. Test every batch for ethanol before filling up (https://www.pure-gas.org/). Usually run a blend of avgas and mogas.

Now what to do with the extra perfectly good MA-4-5...

Man, I wished I had seen this at the start of your journey. I just went thru the EXACT same thing 3 months ago, exactly as you, fuel on air box, swapped carb, sent new carb back to manufacture with myself and my mechanic (I live 30 min from place that rebuilds them for Spruce) put it back on, checked for manifold leaks, and on and on. In the end I finally found a letter from precision airmotive. SIL-MS-12 labeled poor idle cutoff andfuel leak after shutdown. It even calls out SPECIFICALLY the O-470-R that if you run autogas, esp. in the cold, it will leak. So, put carb back on, flew out the autogas, replaced with 100LL and after a few weeks the issue went away. total PITA, conferred with many people. Only one old mechanic, early on said “470’s crappy induction system always leaked if your running Mogas” would have saved me about $1200.00 if I had listened to him.....
BTY, one thing we found out. If you crank a 470, run it for 10-15 min or so, 100LL, Mogas, whatever, they will drool after shutdown, but if you fly, not so much.

This happened to mine, I was using Mo Gas, for 30 hrs at the time, and after a flight was leaking lots of fuel, mechanic overhauled the carb, same problem.
Bought a new one and stopped using Mo gas and no more problems.
All the money I saved at the time using Mo Gas (saved$5.00 per gallon, when 100LL was almost $10 in Costa Rica) I had to spend in a new carb.
Broke even.