Carb Airbox Installation Cleanup

[dnickerson46]

-9A with an O-360-A1A.
The plate on the top of the airbox has a cutout for the bowl drain. Is there a better way to attach the airbox top plate so the bowl drain is accessible? I attached a picture. The builder filled in all around the cutout with RTV. Is this necessary? According to Marvel Schebler, good practice is to drain the carb bowl once a year during condition inspection.

I removed the carburetor for overhaul. It's been 20 years. The vent bowl gasket failed and caused a very poorly running engine.

 

[Ralph Inkster]

The plate on top of the FAB isn’t elegant for sure, but it works for the most part.
The cutout is necessary to minimize the overall height of the FAB unit for cowling clearance.
RTV is cheap & easy to remove if you want access to the drain plug, a few minutes effort is all that’s needed (& a new tube of RTV).
To not seal around the carb flange is to negate the usefulness of the filter. Wise? Not so much.

 

[blaplante]

Having worked on a couple carb'd RV's,
Not just the drain notch, but also the filter notch on the carb for the accel pump.
I have a new appreciation for my RSA-5 fuel injection servo. Flat bottom! What a concept!
Yes, just some RTV... just don't start the engine before it sets up!

 

[dnickerson46]

Thanks for the input on this question. When a carburetor is overhauled, does the overhauler send both the upper and lower gaskets? There is a rubber gasket between the carburetor and the engine intake manifold and the cork gasket between the carb and the airbox.

 

[Scott Hersha]

I’ve had to take mine off twice. Once in March when I did my condition inspection to drain the bowl, and then again about a month ago when I sent my carb out for modification. I resealed that area both times with RTV. I tried to come up with a better solution, but nothing I tried would work. I thought rope caulk would work and it looked great when I got it on - kind of like clay if you’ve never used it. But then I thought, how would it react to gasoline? Answer - not well, it dissolves into a slimy mess. Of course, I didn’t fly with it, it was just an experiment. So, until I can come up with something better, RTV is what I will use.

 

[Scott Hersha]

Thanks for the input on this question. When a carburetor is overhauled, does the overhauler send both the upper and lower gaskets? There is a rubber gasket between the carburetor and the engine intake manifold and the cork gasket between the carb and the airbox.

Mine came back with two gaskets. One for the top of the carb, and one for the bottom between the carb and the airbox top plate. I didn’t have that one on the bottom before, but since he sent me one, I used it. I also have a gasket just like the top gasket that fits between the engine and the powder coated steel cable bracket. The carb bolts on to bottom of the cable bracket with the gasket between, so there are actually two gaskets on top. This is the way my engine came to me new. I have several of those carb gaskets in my parts bin - not sure where I got them all.

 

[71459]

I wouldn't slop a bunch of RTV on there - it will just create a mess. There is an infinitesimal amount of leakage around there - sealing it wont make any difference at all. I'd suggest just leave it alone a keep the installation clean.

 

[dnickerson46]

I wouldn't slop a bunch of RTV on there - it will just create a mess. There is an infinitesimal amount of leakage around there - sealing it wont make any difference at all. I'd suggest just leave it alone a keep the installation clean.

It was quite a mess and I wondered the same thing. How would sealing around the bowl drain with RTV make a difference. Yes the snorkel probably does provide some "ram air" but there are so many other places for air leakage.

 

[jabarr]

I’m surprised no one has mentioned that Vans specifies that any gaps in this area are to be filled with “fuel tank sealant “. That certainly excludes RTV!
RTV should not be used near any fuel component.

Filtered Air Box (FAB) Instructions - Van's Aircraft Total Performance RV Kit Planes

Changes made to Filtered Air Box kit for vertical induction engines. Includes new kit parts and assembly methods for improved alignment and carb heat air door changes.

www.vansaircraft.com

 

[Ralph Inkster]

I wouldn't slop a bunch of RTV on there - it will just create a mess. There is an infinitesimal amount of leakage around there - sealing it wont make any difference at all. I'd suggest just leave it alone a keep the installation clean.

Maybe your installation exhibits “infinitesimal” air gaps (good on you for filing an accurate cut out to fit your carb!). I have worked on MANY dozens of RV’s & seen some pretty crude cutouts on some top plates. RTV is a friend here & I had never had a bad reaction in that location.

 

[dnickerson46]

Thanks to everyone for the advice. I received a call from the carb shop and Dave said, "you had one of those $55M carbs." He was referring to the $55M lawsuit against Precision Airmotive for the float design. I've learned in aviation that often a small thing can lead to a big thing. In this case the bad bowl vent gasket lead to finding floats that would have failed. Another thing I've learned in diagnosing problems whether in aviation or computers. Often there is this small voice that says, "check that" even though there is not necessarily a logical reason. I can't explain it other than there some type of pattern recognition going on in our brains that we cannot explain. The moral of the story is, listen to and heed the small voice.

On a slightly different topic, what is the recommended sealant for the fuel line NPT connectors?

Is there a recommended thread lock or other suggestion for holding the star washers against the plate while the fitting the carb on to thje engine?

 

[Walt]

On a slightly different topic, what is the recommended sealant for the fuel line NPT connectors?

Is there a recommended thread lock or other suggestion for holding the star washers against the plate while the fitting the carb on to thje engine?

Loctite 567.
Star washers should never seat against aluminum if that is what you mean "the plate".
PS: My rule for carb/FI overhaul is 12yrs is good, 15 you're living on the edge.

 

[dnickerson46]

Loctite 567.
Star washers should never seat against aluminum if that is what you mean "the plate".
PS: My rule for carb/FI overhaul is 12yrs is good, 15 you're living on the edge.

FYI - I found the Lycoming carburetor installation manual and it says Loctite 569 for the fuel connectors, it's probably very similar to Loctite 567. 569 is more expensive of course.

 

[dnickerson46]

If I do decide to use RTV to seal around the carburetor fuel drain, what version RTV should I use?

 

[paulsmeds]

I'm in the same boat with a new to me carb'd 6A with a significant gap between the back of the carb and the airbox flange. Sounds like the thought is use RTV -- but with it being near the carb+gas the worry about fuel breaking it down and sucking goo into the carb. What about a thick layer of Permatex #2 (fuel resistant - however never really dries) let set up for a couple days then a layer of high temp RTV over it to sort of seal it in? Short of this, it seems like a cut out plate of aluminum on top of the airbox flange and then a thin layer of RTV. I can't stand being able to see into the filtered air chamber but at the same time don't want to suck up some fuel degraded RTV in the carb.

 

[dnickerson46]

I'm in the same boat with a new to me carb'd 6A with a significant gap between the back of the carb and the airbox flange. Sounds like the thought is use RTV -- but with it being near the carb+gas the worry about fuel breaking it down and sucking goo into the carb. What about a thick layer of Permatex #2 (fuel resistant - however never really dries) let set up for a couple days then a layer of high temp RTV over it to sort of seal it in? Short of this, it seems like a cut out plate of aluminum on top of the airbox flange and then a thin layer of RTV. I can't stand being able to see into the filtered air chamber but at the same time don't want to suck up some fuel degraded RTV in the carb.

I have been flying without the RTV around the back of the carb and airbox flange and I really don't notice a difference. Yes, some unfiltered air could be sucked into the carb and perhaps some hot air from the inside of the cowling but there is a lot more filtered surface area. I know guys who operate 2 stroke 582s with no filters for 30 years with no adverse effects. The benefit is being able to remove the float bowl drain bolt as recommended by the carb manufacturer.

 

[paulsmeds]

Mine looks almost identical but a little bigger gap on one side. I'm not so worried about pulling in hot air it is more the unfiltered air and gunk that always accumulates. The thought of pulling in dust and grim/grit straight in is unsettling to a newly overhauled engine! I was thinking about a removable plate on the top side that more closely meets up with the carb held in by a couple small bolts or screws just to close the gap and then a very small amount of Permatex #2 or RTV. That would allow easy removal and access to the drain bolt. Maybe I'm over thinking this just don't like that I can see right into the "clean" side of the air filter from the outside.

 

[blaplante]

I have been flying without the RTV around the back of the carb and airbox flange and I really don't notice a difference. Yes, some unfiltered air could be sucked into the carb and perhaps some hot air from the inside of the cowling but there is a lot more filtered surface area. I know guys who operate 2 stroke 582s with no filters for 30 years with no adverse effects. The benefit is being able to remove the float bowl drain bolt as recommended by the carb manufacturer.

Are you doing oil analysis?
When my alt air door was leaking the Blackstone guys picked up on high levels of Silicon in my oil.
Maybe your airport is cleaner...

 

[paulsmeds]

It is a new to me plane and definitely will do oil analysis when it is oil change time. I'm trying to clear up as many issues as we can and this is one of those. There was gunk and grime all around that flange on the top leading right into the gap. If we weren't careful cleaning we could have pushed an entire layer of grime into the clean portion of the airbox.