Sealing around Carb Bowl Drain

f14av8r · #1 12-19-2015, 11:26 AM

Hi Everyone,
I need some help figuring out how to best seal around the area of the carb bowl drain boss - see arrow. The photo is of the underside of my updraft carb with the filtered air box plate attached.

I guess the first question is, do I even need to worry about it? It's not a large gap but I'm concerned about it because any air coming through this area will come into the interior circle of the round K&N filter and have an unfettered access to the carb intake (bypass the filter). The builder had some rubber baffle material RTV'd to the top of the plate in an attempt to seal the gap. It came loose and that sent me down the path to finding an alternative. I want to be careful with the solution to make sure I don't use anything that can get sucked into the intake should it come loose or deteriorate. In the photo, you can see a gasket between the plate and the carb body. I won't be reusing that - going metal to metal - but not doing so will leave a little bigger gap around the forward area of the carb drain boss.

Thanks in advance!
Randy

Vlad · #2 12-19-2015, 04:46 PM

Hi Randy,
In Vans building instructions there is a method to seal that spot. You have to cut out a piece off your round filter and form a seal with RTV. I don't have the instructions close by but do a search you will find how to do it. It will look something like this. Old filter to the right I replaced it after 1,000 hours.

rvbuilder2002 · #3 12-19-2015, 08:20 PM

Quote:

Originally Posted by Vlad

Hi Randy,
In Vans building instructions there is a method to seal that spot. You have to cut out a piece off your round filter and form a seal with RTV. I don't have the instructions close by but do a search you will find how to do it. It will look something like this. Old filter to the right I replaced it after 1,000 hours.

Actually the OP is asking about a different issue.

The air filter interference that requires trimming and remolding the filter is only on O-320's using an MA4-SPA carb. (the pump is from the accelerator pump). An O-360 with a MA-5 carb. does not have the same filter interference.
The need for sealing the hole needed for the float bowl drain bump in the carb. body exists with both model carbs and the install manual specifies sealing this with a bead of tank sealant around the perimeter on the top.

Remolding the cutaway portion of the air filter for an SPA carb. is also specified to be done with tank sealant.
Red RTV is not fuel resistant. It says right on the package "not to be used in/on fuel systems".
There have been forced landings in RV's that were caused by engine stoppages resulting from red RTV being used in the induction system.

Vlad · #4 12-19-2015, 09:02 PM

Quote:

Originally Posted by rvbuilder2002

....

Remolding the cutaway portion of the air filter for an SPA carb. is also specified to be done with tank sealant.
Red RTV is not fuel resistant. It says right on the package "not to be used in/on fuel systems".
There have been forced landings in RV's that were caused by engine stoppages resulting from red RTV being used in the induction system.

In FAB instruction written some 12 years ago it suggested to fill all the gaps with silicone rubber (RTV) or tank sealant. Little I knew that RTV was solvable in petroleum live and learn.

az_gila · #5 12-19-2015, 09:31 PM

Quote:

Originally Posted by Vlad

In FAB instruction written some 12 years ago it suggested to fill all the gaps with silicone rubber (RTV) or tank sealant. Little I knew that RTV was solvable in petroleum live and learn.

Also in my undated "Installing the FAB-360 Filtered Airbox" instructions.

Not even a mention of tank sealant as an option in those instructions.

Page 2 - paragraph 2 -

"...When using a marvel carburetor, the drain will produce interference with the mount plate. A drain cutout is outlined on the template for those using the carburetor. During final assembly fill all gaps that would allow unfiltered air to enter between the carburetor and mount plate (especially around the drain plug) with silicone rubber sealant...."

There should be enough RVs flying with builders who followed the instructions to determine if there is a problem.

Where are the later install manuals that Scott refers to?

rvbuilder2002 · #6 12-19-2015, 10:40 PM

Quote:

Originally Posted by az_gila

There should be enough RVs flying with builders who followed the instructions to determine if there is a problem.

Really Gil?

I guess you think I am lying about forced landings.
HERE IS ONE If you do some searching in the forums on Brian Denk, you can probably find some info regarding the details. I know of others but can't recall any details at the moment to point you to specific reports.

Will using a sealant not compatible with fuel automatically kill you? No.
Not any more than a lot of other things as long as specific circumstances do not happen. Maybe you will never have a carb. leak that gets bad enough before getting detected that the sealant turns to mush and gets sucked into the carb venturi..... after witch the engine stops.

So even if this is the only one and 100's of others are flying this way with no further accidents, does that mean it should be promoted as a good practice?

rvbuilder2002 · #7 12-19-2015, 11:23 PM

Quote:

Originally Posted by az_gila

Any sealant used should be able to be removed and replaced yearly.

I take the fuel tank sealant off of mine every year with no trouble at all.....

az_gila · #8 12-19-2015, 11:28 PM

Quote:

Originally Posted by rvbuilder2002

Really Gil?

I guess you think I am lying about forced landings.
HERE IS ONE If you do some searching in the forums on Brian Denk, you can probably find some info regarding the details. I know of others but can't recall any details at the moment to point you to specific reports.

Will using a sealant not compatible with fuel automatically kill you? No.
Not any more than a lot of other things as long as specific circumstances do not happen. Maybe you will never have a carb. leak that gets bad enough before getting detected that the sealant turns to mush and gets sucked into the carb venturi..... after witch the engine stops.

So even if this is the only one and 100's of others are flying this way with no further accidents, does that mean it should be promoted as a good practice?

Really Scott? I'm actually referring to builders who follow the instructions given - that sort of defines a "good practice" to most builders.

If they are all wrong will there be an update to the FAB Airbox instruction?

Will there be a SB issued suggesting removal and replacement of existing RTV sealants (installed according to the plans) at that location?

It's either a big deal or it isn't - what is the correct solution going forward for existing RV builders who followed the plans?

Whatever material is used there should not be too permanent, draining the carb bowl using the plug that protrudes into the FAB airbox is usually specified as a 100 hr/annual inspection item. Any sealant used should be able to be removed and replaced yearly.

az_gila · #9 12-19-2015, 11:31 PM

Somehow my editing is screwing up the posting sequence - the above post was originally made before Scotts reply about sealant removal.

The other questions are not yet answered though...

rvbuilder2002 · #10 12-19-2015, 11:57 PM

"Silicone sealant" is a generic statement that requires.

There are large performance differences between many "purely" silicone sealants and silicone "RTV's"

If a specific sealant like High Temp red RTV says not for use on parts in contact with gasoline, it is probably not a good choice of sealant when trying to find something to use based on the suggestion of silicone sealant.

Tank sealant is a no brainer...... it is meant for contact with fuel and is used as a sealant for many fuel related situations.