A little history; in the late 80s and early 90s, I raced a 1967 Austin-Healey Sprite Mk IV (same as an MG Midget Mk III) in Sports Car Club of America's F Production class. We used Super 300 to seal head gaskets. So, I was familiar with the product, albeit in a different application.
When the carburetors were returned to me, I was a little surprised to discover a greenish-brown sealant on both the float bowl o-rings as well as the slide diaphragms. I queried the re-builder and learned that the sealant was indeed, Super 300. OK, fine. I was a little surprised that the float bowl o-rings needed this because not only were they brand new, they also sat nicely in their recesses in the float bowls. There was really no reason to 'glue' them in place.
I re-assembled everything and started the engine. It idled pretty nicely. I took it for a short ride to get it up to temperature, and then tried to accelerate up to the speed limit. At 5000 rpm, the engine stopped pulling and pretty much "fell on its face". Not good. I checked to see if I had an electrical issue (only one cylinder working). Nope, both exhaust pipes were hot. Then, I wondered if the petcock was some how restricting fuel flow. Nope, after removing the petcock from the equation, the engine still ran poorly. At this point, the only thing left was to remove the carburetors and inspect them. After popping off one of the float bowls, this is what I saw:
The Super 300 had clearly come in contact with the gasoline and dissolved into it. Needless to say, I gave both carburetors a good cleaning. It took a small piece of Scotch-Brite, soaked in carburetor cleaner, to get the o-ring sealing surfaces clean. Other places required a toothbrush to remove that residue. The obvious conclusion was that Super 300 is not compatible with gasoline. Not so fast my friend.
I had to understand what was going on here. So, I called Permatex and spoke with a tech. He told me that Super 300 was compatible with gas. Huh? So, why did this occur? It would seem that Super 300, like other Permatex products, has a shelf life. And, the shelf life of Super 300 is two (2) years. I contacted the re-builder and discovered that his container of Super 300 was somewhere around twenty (20) years in age. !!! So, boys and girls, don't assume that your sealants are still good after they've been sitting in your toolbox or on your garage shelf for years and years and years. Most likely, they're not.
As a follow-up, the Permatex tech suggested I use a newer product; PermaShield. Not only does this product have the same capabilities as Super 300, it supposedly will handle dis-assembly and subsequent re-assembly without requiring another application. That's pretty nice. The shelf life of PermaShield is also two (2) years but Permatex is thinking of extending that. I also found that Super 300 isn't as readily available as it was when I was racing. Every auto parts store I visited indicated it was "special order". PermaShield is supposedly stocked by all of the major auto parts store chains with the possible exception of NAPA (why, I do not know).
So, got a container of Super 300 on the shelf? Don't have any idea how old it is? I would suggest you discard it and go get something fresh. Even if it's another container of (now special order) Super 300, at least you know it'll do what you expect it to do (especially if fuel is involved). Perhaps writing a date on the container isn't a bad idea either.
After the cleaning, the carburetors work as they should and the engine pulls all the way past 10,000 rpm. Nice.