What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Sealing around Carb Bowl Drain

f14av8r

Well Known Member
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

Air%20Box%20Plate_zpscu6ye85d.jpg
 
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.


 
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.
 
....

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.
 
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.:rolleyes:


Where are the later install manuals that Scott refers to?
 
Last edited:
There should be enough RVs flying with builders who followed the instructions to determine if there is a problem.:rolleyes:

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 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.
 
Sequence

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...
 
"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.
 
What to use vs. what has been used

"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.

Generic or not, I can't find any "silicone sealant" that is rated for contact with fuel.

The closest is a Dow Corning 730 at around $100 for 3 oz, and that is listed as a "fluorosilicone".

Can you reference a specific "silicone sealer" that is actually fuel resistant? This is what the instructions call out.

Perhaps using a "generic statement" for products to use in the factory instructions is a poor idea? :rolleyes:

Two respondents to this thread have already used the red automotive silicone stuff, making a Vans SB a good idea.


I agree 'tank sealer' is a good idea, and that is what the instructions should really call out. Mine, and many others apparently, did not.
 
Last edited:
I am surprised to see two of the very best posters on VAF engaging in a bit of a "flame" war! :eek:
 
I am surprised to see two of the very best posters on VAF engaging in a bit of a "flame" war! :eek:

Pete, not sure that's what going on. Like you, knowing both of them as valuable contributors to VAF, I think it's more a semantics issue. I know that sometimes when I text (or write responses to VAF posts) it doesn't come out exactly like I planned it too.

Anyway, It's an important issue that we all learn from.

Keep up the good work guys and thank you both for your contributions on VAF. I know personally, my aircraft is flying because of guys like you who contribute so much to VAF.
 
Thanks

I really appreciate all the responses. I've been puzzling over this one for days. You are all concerned about the same things I am - avoiding ingestion and ease of maintenance. Based on all the excellent discussion, I'm going to use RTV sealant.

Thanks for your reply too Vlad. I should have specified the type of carburetor. Mine's a MA-5.

Thanks again!
Randy
 
Last edited:
Just to add my 2c, I don't think using RTV to seal around the air cleaner (like on the 320 filter or the base plate) presents any hazard, if a chunk breaks loose the engine will easily injest it and spit it out, kinda like it does to loose hardware in the airbox.

There is a BIG difference in application here, using RTV as a thread or gasket sealer is definitely a major problem, using it around the air cleaner.. well I just don't see it as an issue in that application.

Quoting accidents caused by "RTV" used as a tread or gasket sealer is not the same thing as what the OP asked about (but a good reminder nonetheless), unless someone can show me a case of where RTV being ingested caused a problem.
 
Last edited:
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.

Quoting accidents caused by "RTV" used as a tread or gasket sealer is not the same thing as what the OP asked about (but a good reminder nonetheless), unless someone can show me a case of where RTV being ingested caused a problem.

Actually Walt, the incident (wont call it an accident because it resulted in a safe forced landing on an airport) that I linked in my above post was an RV-8 where the builder used High Temp RTV to seal portions of the induction airbox. I fully realize that RTV on pipe threads is a totally different situation...

If I remember correctly, a loose piece of RTV that had turned to mush because of exposure from a fuel leak on the carb, lodged in a portion of the venturi in the throat of the carb. Caused a complete power loss.
 
In the post Scott linked above, the author talks of his "stuck float" experience. How on earth does a piece of RTV on the airbox make it into the float bowl of the carb? Scott can you please shed some light here?
 
In the post Scott linked above, the author talks of his "stuck float" experience. How on earth does a piece of RTV on the airbox make it into the float bowl of the carb? Scott can you please shed some light here?

I haven't gone back and dug for the details.... like I said , just going on memory....

I think this was a partially stuck open (or sinking?) float that cause over flow of the float bowl and fuel wetting the airbox with the eventual engine stoppage when some RTV got loose. I believe there was an ongoing problem for a while that he was trying to diagnose (interm. float problem?) that eventually ended in a forced landing.

As we know, it often takes a number of factors to align to make something become a problem........
 
Re-open

So, what has been the general consensus here for the o360 carb plate sealant? I’ve never put anything in the gap because of the ingestion worry.
 
I've had RTV there since 2006, never have seen any softening or loosening due to fuel leaks. Nevertheless, upon re-reading this thread I believe I'll follow Scott's encouragement to use Proseal.
 
Back
Top